Mild and Selective Deprotection of tert-Butyl(dimethyl)silyl Ethers with Catalytic­ Amounts of Sodium Tetrachloroaurate(III) Dihydrate

Abstract

A simple and mild method for the removal of tert-butyl(dimethyl)silyl (TBS) protecting groups with catalytic amounts of sodium tetrachloroaurate(III) dihydrate is described. The procedure permits selective deprotection of aliphatic TBS ethers in good to excellent yields in the presence of aromatic TBS ethers, aliphatic triisopropylsilyl ethers, aliphatic tert-butyl(diphenyl)silyl ethers, or sterically hindered aliphatic TBS ethers. Additionally, TBS ethers can also be transformed into 4-methoxybenzyl ethers or methyl ethers in one pot by using larger quantities of the catalyst and a higher reaction temperature.

Key words

deprotection - silyl ethers - alcohols - ethers - catalysis - gold

Protection/deprotection strategies play important roles in modern organic synthesis.[1] [2] The tert-butyl(dimethyl)silyl (TBS) group is one of the most widely used protecting groups for alcohols because of its easy installation, its stability to various reaction conditions, and the selectivity of its cleavage reaction. Numerous methods are available for removal of TBS groups,[3–9] including the use of acidic,[4] basic,[5] reducing,[6] oxidizing,[7] or fluoride-based reagents[8] among others.[9] However, new mild and selective protocols for the deprotection of TBS ethers are still in great demand for use in syntheses of multifunctional compounds, particularly complex natural products.

Commercially available sodium tetrachloroaurate(III) dihydrate (NaAuCl₄·2H₂O) is the least expensive gold catalyst and has been used in several types of reaction, including nucleophilic addition to multiple bonds,[10] [11] [12] [13] nucleophilic substitution of propargylic alcohols,[14,15] nonsymmetrical etherization,[16] and others.[17] [18] In the course of an ongoing total-synthesis project, we serendipitously found that the TBS protecting group was cleanly removed in the presence of a small amount of NaAuCl₄·2H₂O. Inspired by this observation, we explored the possibility of using NaAuCl₄·2H₂O as an effective catalyst for the deprotection of TBS ethers.

First, we evaluated the effects of the solvent and the catalyst loading on the gold(III)-catalyzed desilylation of the TBS ether of 6-(benzyloxy)hexan-1-ol (1) (Table [1]). In the presence of 0.01 equivalents of sodium tetrachloro­aurate(III) in methanol, deprotection of the TBS ether 1 proceeded smoothly to give the corresponding alcohol 1′ in 95% yield after 3.5 hours at room temperature (Table [1], entry 1). The cleavage also proceeded in other polar solvents, but yields were much lower (entries 2–4). Alcohol 1′ was still obtained in excellent yields and in reasonable reaction times when the amount of catalyst was reduced to 0.005, 0.001, or even 0.0005 equivalents, (entries 5–7), but the desilylation became sluggish when 0.0001 equivalents of the catalyst were used (entry 8).

Table 1 Optimization of Conditions for the Sodium Tetrachloro­aurate(III) Catalyzed Deprotection of TBS Ether 1

Entry	Catalyst (equiva)	Solvent	Time (h)	Yieldb (%)
1	0.01	MeOH	3.5	95
2	0.01	THF	12	73
3	0.01	MeCN	12	53
4	0.01	acetone–H2O (1:1 v/v)	12	40
5	0.005	MeOH	7	96
6	0.001	MeOH	12	95
7	0.0005	MeOH	36	93
8	0.0001	MeOH	36	25 (72c)

^a With respect to the substrate 1.

^b Isolated yield of pure product.

^c Recovery of substrate 1.

Next, we synthesized various aliphatic and aromatic TBS ethers to examine the substrate scope of our deprotection method. The deprotection reaction of the primary TBS ethers was conducted in the presence of 0.001–0.005 equivalents of sodium tetrachloroaurate(IIII) dihydrate in methanol at room temperature. Primary TBS ethers containing electron-donating or electron-withdrawing groups were readily deprotected to give the corresponding alcohols in high yields (Table [2], entries 1–5). Secondary and tertiary TBS ethers were also desilylated smoothly, although longer reaction times or greater catalyst loadings were required (entries 6 and 7). Although sodium tetrachloroaurate(IIII) dihydrate catalyzes addition reactions of alkenes or alkynes,[10] [11] [12] [13] the TBS protecting group of compound 9 was removed successfully under the current condition without any effect on the double bond (entry 8). Aromatic TBS ethers are usually deprotected by treatment with basic reagents or fluoride-based reagents that frequently produce unwanted side reactions, such as silyl migration.[19] Gratifyingly, the cleavage of aromatic TBS ethers proceeded well in the presence of 0.05–0.1 equivalents of the catalyst in methanol at room temperature. Aromatic TBS ethers bearing electron-donating groups were much more reactive than those bearing electron-withdrawing groups (entries 9–12).

Table 2 Deprotection of Primary, Secondary, Tertiary and Aryl TBS Ethers Catalyzed by Sodium Tetrachloroaurate(III) in Methanol

Entry	Substrate	Catalyst (equiva)	Time (h)	Product	Yieldb (%)
1	R = H (2)	0.005	8	R = H (2′)	95
2	R = OMe (3)	0.001	4	R = OMe (3′)	94
3	R = CF3 (4)	0.005	15	R = CF3 (4′)	91
4	5	0.005	8	5′	97
5	6	0.005	2	6′	96
6	7	0.01	24	7′	92
7	8	0.05	40	8′	93
8	9	0.005	12	9′	86
9	R1 = All; R 2 = OMe (10)	0.05	7	R1 = All; R 2 = OMe (10′)	87
10	R1 = t-Bu; R 2 = OMe (11)	0.05	24	R1 = t-Bu; R 2 = OMe (11′)	92
11	R1 = NHAc; R 2 = OMe (12)	0.05	48	R1 = NHAc; R 2 = OMe (12′)	86
12	R1 = Ac; R 2 = H (13)	0.1	48	R1 = Ac; R 2 = H (13′)	21 (67c)

^a With respect to the substrate.

^b Isolated yield of pure product.

^c Recovery of substrate 13.

Next, we examined the selective deprotection of TBS ethers containing various sensitive functional groups under the optimized conditions (Table [3]). On treatment with 0.005 equivalents of sodium tetrachloroaurate(III) dihydrate in methanol at room temperature, the TBS group in substrates 14–18 was selectively removed, whereas other common acid-labile protecting groups such as allyl, acetyl, methoxymethyl, (2-methoxyethoxy)methyl, and isopropylidene were unaffected (Table [3], entries 1–5). The catalyst also showed good selectivity to various silyl protecting groups. Preferential cleavage of TBS ether groups in the presence of tert-butyl(diphenyl)silyl (TBDPS) ether groups gave the monodeprotected products in high yields, and the corresponding diols were not obtained (entries 6 and 7). When a less-bulky triisopropylsilyl group (compared with TBDPS) and a TBS groups were present together, the deprotection was less selective, but the desired monoalcohols were still obtained as the major products (≥83% yield) (entries 8 and 9). Moderate selectivity between triethylsilyl and TBS protecting groups was achieved by using 0.001 equivalents of the catalyst (entries 10 and 11). An aliphatic TBS ether group was selectively removed in the presence of an aromatic TBS ether group in 92% yield (entry 12). We also examined the possibility of selectively deprotecting TBS diethers (entries 13–15). In all cases, the less-hindered TBS ether group was cleaved in preference to the more-hindered one in good to excellent yield; this makes our method very useful in total syntheses of complicated compounds such as natural products or their analogues.

Table 3 Selective Deprotection of Various Silyl Ethers Catalyzed by Sodium Tetrachloroaurate(III) in Methanol

Entry	Substrate	Catalyst (equiva)	Time (h)	Product	Yieldb (%)
1	R = All (14)	0.005	5	R = All (14′)	95
2	R = Ac (15)	0.005	2	R = Ac (15′)	94
3	R = MOM (16)	0.005	4	R = MOM (16′)	97
4	R = MEM (17)	0.005	4	R = MEM (17′)	95
5	18	0.005	11	18′	96
6	19	0.005	3	19′	96
7	20	0.005	7	20′	97
8	21	0.005	2	21′	83 (7)c
9	22	0.005	7	22′	86 (5)c
10	23	0.001	0.5	23′	86 (8)c
11	24	0.001	1	6	77 (18)c
12	25	0.005	5	25′	92
13d	26	0.005	40	26′	93
14	27	0.005	7	8	82 (9c)
15	28	0.005	5	28′	86 (5c)

^a With respect to the substrate.

^b Isolated yield of pure product.

^c Yield of diol.

^d EtOAc–MeOH (1:1, v/v) was used as the solvent.

Because nonsymmetrical ethers can be prepared from alcohols by using sodium tetrachloroaurate(III) dihydrate as catalyst,[16] we decided to examine the one-pot transformation of TBS ethers into other frequently used ethers. Under relatively harsh condition [NaAuCl₄·2H₂O (0.05 equiv), THF, reflux], TBS ether 1 reacted with 4-meth­oxybenzyl alcohol to give the desired ether 29 directly (Scheme [1]). More interestingly, substrate 30, which readily forms the corresponding carbocation, gave either the deprotected product 30′ or the methyl ether 31, depending on the amount of the catalyst used and the reaction temperature (Scheme [1]).

Scheme 1 Transformation of TBS ethers into 4-methoxybenzyl or methyl ethers

In conclusion, we have developed an effective protocol for the deprotection of TBS ethers by using a very small amount of sodium tetrachloroaurate(III) dihydrate as catalyst. Notable features of the protocol include mild conditions, low cost, easy operations, good functional group compatibility, and high selectivity. The method should therefore have widespread applications in syntheses of complex, multifunctional, or sensitive molecules. In addition, by using larger amounts of catalyst and higher reaction temperatures, TBS ethers can also be transformed into 4-methoxybenzyl or methyl ethers in a one-pot process.

¹H NMR and ¹³C NMR spectra were recorded in CDCl₃ or CD₃OD on a Bruker Avance 300 or Bruker Avance 400 instrument. Chemical shifts (δ) are referenced to internal TMS or CDCl₃. High-resolution mass spectra were recorded on a Bruker maXis Impact mass spectrometer. Melting points were determined by using a Stuart Scientific SMP10 instrument and are uncorrected. IR spectra were recorded in the ATR mode on a Nicolet 6700 FT-IR Thermo Scientific spectrometer; only the more significant peaks are reported. All reagents and solvents obtained commercially and were used as received without further purification. Reactions were monitored by TLC on glass-backed plates coated with a 0.2 mm thickness of silica gel 60 F254; chromatograms were visualized by UV radiation (254 nm) or by staining with phosphomolybdic acid and H₂SO₄. Column chromatography was performed on 300–400 mesh silica gel.

Except for compound 8, 28, and 30, all TBS ethers were prepared according to the procedures reported in the literature.

3-{[tert-Butyl(dimethyl)silyl]oxy}-3-methylbutan-1-ol (8)

NaAuCl₄·2 H₂O (4.0 mg, 0.01 mmol, 0.005 equiv) was added to a solution of disilyl ether 27 [20] (665 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 7 h. The mixture was then diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] to give a colorless oil; yield: 358 mg (82%).

IR (KBr): 3355, 2945, 2857, 1468, 1250, 1041 cm^–1.

¹H NMR (300 MHz, CDCl₃): δ = 3.82 (t, J = 5.9 Hz, 2 H), 1.72 (t, J = 5.9 Hz, 2 H), 1.31 (s, 6 H), 0.87 (s, 9 H), 0.13 (s, 6 H).

¹³C NMR (75 MHz, CDCl₃): δ = 75.31, 60.00, 45.70, 29.86, 25.79, 17.90, –2.01.

MS (ESI, MeOH): m/z = 241 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ Calcd for C₁₁H₂₆NaO₂Si: 241.1600; found: 241.1612.

tert-Butyl[1-((1S*,5R*)-5-{[tert-butyl(dimethyl)silyl]oxy}-4-methylcyclohex-3-en-1-yl)-1-methylethoxy]dimethylsilane (28)

TBSOTf (1.52 mL, 6.6 mmol) was added dropwise to a solution of diol 9′ [21] (511 mg, 3 mmol) and 2,6-lutidine (0.76 mL, 6.6 mmol) in dry CH₂Cl₂ (6 mL) at 0 °C, and the mixture was stirred for 5 h. H₂O (5 mL) and CH₂Cl₂ (20 mL) were added, and the organic layer was separated and washed successively with sat. aq NaHCO₃ (5 mL), H₂O (2 × 5 mL), and brine (5 mL), then dried (MgSO₄) and filtered. The filtrate was concentrated in vacuo and the resulting residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:25)] to give a colorless oil; yield: 1.065 g (89%).

IR (KBr): 2966, 2920, 2861, 1475, 1368, 1256 cm^–1.

¹H NMR (400 MHz, CDCl₃): δ = 5.52 (dd, J = 3.6, 1.6 Hz, 1 H), 4.01 (br s, 1 H), 2.11 (dt, J = 16.8, 5.6 Hz, 1 H), 1.88–1.78 (m, 2 H), 1.75–1.67 (m, 4 H), 1.37 (dt, J = 3.6, 13.2 Hz, 1 H), 1.21 (s, 3 H), 1.18 (s, 3 H), 0.91 (s, 9 H), 0.85 (s, 9 H), 0.10 (s, 6 H), 0.07 (s, 6 H).

¹³C NMR (75 MHz, CDCl₃): δ = 134.45, 125.05, 74.73, 69.46, 39.62, 33.58, 28.37, 27.57, 26.86, 25.95, 25.95, 21.25, 18.29, 18.15, –2.02 (2 C).

MS (ESI, MeOH): m/z = 421 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₂₂H₄₆NaO₂Si₂: 421.2934; found: 421.2940.

tert-Butyl(dimethyl)[(3,4,5-trimethoxybenzyl)oxy]silane (30)

A solution of alcohol 30′ [22] (396 mg, 2 mmol), imidazole (300 mg, 4.4 mmol), and TBSCl (332 mg, 2.2 mmol) in anhyd CH₂Cl₂ (4 mL) was stirred at overnight at r.t. H₂O (5 mL) and CH₂Cl₂ (20 mL) were added to the mixture, and the organic layer was separated, washed successively with sat. aq NaHCO₃ (5 mL), H₂O (2 × 5 mL), and brine (5 mL), then dried (MgSO₄) and filtered. The filtrate was concentrated in vacuo and the residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:25)] to give a colorless oil; yield: 569 mg (91%).

IR (KBr): 2962, 2928, 2857, 1596, 1499, 1459, 1378, 1017, 832, 775 cm^–1.

¹H NMR (300 MHz, CDCl₃): δ = 6.57 (s, 2 H), 4.69 (s, 2 H), 3.85 (s, 6 H), 3.83 (s, 3 H), 0.96 (s, 9 H), 0.11 (s, 6 H).

¹³C NMR (75 MHz, CDCl₃): δ = 153.16, 137.14, 136.78, 102.80, 64.86, 60.78, 55.96, 25.89, 18.36, –5.28.

MS (ESI, MeOH): m/z = 335 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₆H₂₈NaO₄Si: 335.1655; found: 335.1663.

Deprotection of TBS Ethers; General Procedure

A solution of the TBS ether (2 mmol) in MeOH (4 mL) was treated with NaAuCl₄·2H₂O (4.0 mg, 0.01 mmol, 0.005 equiv) at r.t. When the starting material had disappeared (TLC), mixture was diluted with EtOAc (10 mL) and filtered through activated alumina. The solution was then concentrated in vacuo and the resulting residue was purified by flash column chromatography.

6-(Benzyloxy)hexan-1-ol (1′)[4n]

Prepared as a colorless oil from silyl ether 1 [4n] according to the general procedure using 0.01–0.0001 equiv of NaAuCl₄·2H₂O in various solvents; for yields, see Table [1].

¹H NMR (300 MHz, CDCl₃): δ = 7.34–7.33 (m, 5 H), 4.50 (s, 2 H), 3.63 (t, J = 6.5 Hz, 2 H), 3.47 (t, J = 6.5 Hz, 2 H), 1.66–1.55 (m, 4 H), 1.40–1.38 (m, 4 H), 1.26 (br s, 1 H).

¹³C NMR (75 MHz, CDCl₃): δ = 138.64, 128.31, 127.60, 127.46, 72.86, 70.31, 62.87, 32.68, 29.68, 25.98, 25.55.

MS (ESI, MeOH): m/z = 231 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₃H₂₀NaO₂: 231.1361; found: 231.1351.

Benzyl Alcohol (2′)[4b]

Prepared according to the general procedure from 2 [4b] (445 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:3)] as a colorless oil; yield: 206 mg (95%).

¹H NMR (300 MHz, CDCl₃): δ = 7.37–7.28 (m, 5 H), 4.69 (s, 2 H), 1.69 (br s, 1 H).

¹³C NMR (75 MHz, CDCl₃): δ = 140.83, 128.46, 127.52, 126.91, 65.13.

MS (ESI, MeOH): m/z = 131 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₇H₈NaO: 131.0473; found: 131.0478.

(4-Methoxyphenyl)methanol (3′)[4j]

NaAuCl₄·2 H₂O (0.8 mg, 0.002 mmol, 0.001 equiv) was added to a solution of silyl ether 3 [4j] (505 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 4 h. The mixture was then diluted with EtOAc (10 mL), and filtered through activated alumina. The solution was concentrated in vacuo and resulting residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:3)] to give a colorless oil; yield: 260 mg (94%).

¹H NMR (300 MHz, CDCl₃): δ = 7.27 (d, J = 8.7 Hz, 2 H), 6.88 (d, J = 8.7 Hz, 2 H), 4.46 (s, 2 H), 3.80 (s, 3 H).

¹³C NMR (75 MHz, CDCl₃): δ = 159.11, 130.42, 129.27, 113.70, 71.37, 55.12.

MS (ESI, MeOH): m/z = 161 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₈H₁₀NaO₂: 161.0578; found: 161.0597.

[4-(Trifluoromethyl)phenyl]methanol (4′)[23]

Prepared according to the general procedure from silyl ether 4 [24] (581 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] as a colorless oil; yield: 321 mg (91%).

¹H NMR (300 MHz, CDCl₃): δ = 7.62 (d, J = 8.1 Hz, 2 H), 7.47 (d, J = 8.1 Hz, 2 H), 4.77 (s, 2 H), 1.91 (br s, 1 H).

¹³C NMR (75 MHz, CDCl₃): δ = 145.67, 129.76 (q), 126.80, 125.42 (q), 122.78, 64.40.

MS (ESI, MeOH): m/z = 199 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₈H₇F₃NaO: 199.0347; found: 199.0342.

2-Phenylethanol (5′)[8a]

Prepared according to the general procedure from silyl ether 5 [8a] (473 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:3)] as a colorless oil; yield: 237 mg (97%).

¹H NMR (300 MHz, CDCl₃): δ = 7.33–7.30 (m, 2 H), 7.24–7.22 (m, 3 H), 3.86 (t, J = 5.0 Hz, 2 H), 2.87 (t, J = 5.0 Hz, 2 H), 1.52 (br s, 1 H).

¹³C NMR (75 MHz, CDCl₃): δ = 138.48, 128.97, 128.51, 126.40, 63.58, 39.14.

MS (ESI, MeOH): m/z = 145 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₈H₁₀NaO: 145.0629; found: 145.0630.

(±)-trans-Cyclohexane-1,4-diyldimethanol (6′)[25]

Prepared according to the general procedure from 6 [26] (517 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (3:1)] as a white solid; yield: 277 mg (96%); mp 63–65 °C.

¹H NMR (300 MHz, CDCl₃): δ = 3.47 (d, J = 4.8 Hz, 4 H), 1.86–1.84 (m, 4 H), 1.46–1.44 [m, 4 H, including 1.46 (br s, 2 H)], 1.01–0.96 (m, 4 H).

¹³C NMR (75 MHz, CDCl₃): δ = 68.51, 40.57, 28.87.

MS (ESI, MeOH): m/z = 167 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₈H₁₆NaO₂: 167.1048; found: 167.1039.

(R)-(–)-Menthol (7′)[8b]

NaAuCl₄·2 H₂O (8.0 mg, 0.02 mmol, 0.01 equiv) was added to a solution of silyl ether 7 [8b] (541 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 24 h. The mixture was then diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] to give a white solid; yield: 288 mg (92%); mp 42–43 °C.

¹H NMR (300 MHz, CDCl₃): δ = 3.41 (br s, 1 H), 2.21–2.13 (m, 1 H), 1.99–1.95 (m, 1 H), 1.64–1.58 (m, 2 H), 1.43–1.38 (m, 1 H), 1.28–1.25 (m, 2 H), 1.11–1.08 (m, 1 H), 1.00–0.85 (m, 1 H), 0.93 (d, J = 5.1 Hz, 3 H), 0.92 (d, J = 4.8 Hz, 3 H), 0.81 (d, J = 5.1 Hz, 3 H).

¹³C NMR (75 MHz, CDCl₃): δ = 71.53, 50.16, 45.08, 34.55, 31.63, 25.85, 23.19, 22.17, 20.96, 16.11.

MS (ESI, MeOH): m/z = 179 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₀H₂₀NaO: 179.1412; found: 179.1421.

3-Methylbutane-1,3-diol (8′)[27]

NaAuCl₄·2 H₂O (39.8 mg, 0.1 mmol, 0.05 equiv) was added to a solution of silyl ether 8 (437 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 40 h. The mixture was then diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (2:1)] to give a colorless oil; yield: 194 mg (93%).

¹H NMR (300 MHz, CDCl₃): δ = 3.92–3.87 (m, 2 H), 2.77 (br s, 1 H), 2.50 (br s, 1 H), 1.75 (t, J = 5.5 Hz, 2 H), 1.30 (s, 6 H).

¹³C NMR (75 MHz, CDCl₃): δ = 71.82, 59.94, 43.23, 29.51.

MS (ESI, MeOH): m/z = 127 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₅H₁₂NaO₂: 127.0735; found: 127.0731.

(1S*,5R*)-5-(1-Hydroxy-1-methylethyl)-2-methylcyclohex-2-en-1-ol (9′)[21]

Prepared according to the general procedure from 9 (569 mg) and purified by flash column chromatography [silica gel, EtOAc–PE (2:1)] as a colorless oil; yield: 293 mg (86%).

¹H NMR (300 MHz, CDCl₃): δ = 5.58 (d, J = 3.6 Hz, 1 H), 4.04 (br s, 1 H), 2.15–2.11 (m, 1 H), 2.04–2.01 (m, 1 H), 1.79 (s, 3 H), 1.76–1.69 (m, 2 H), 1.42 (dt, J = 9.9, 2.7 Hz, 1 H), 1.22 (s, 3 H), 1.89 (s, 3 H).

¹³C NMR (75 MHz, CD₃OD): δ = 135.45, 126.17, 72.79, 69.29, 39.81, 34.22, 28.09, 27.21, 27.01, 21.21.

MS (ESI, MeOH): m/z = 193 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₀H₁₈NaO₂: 193.1204; found: 193.1206.

4-Allyl-2-methoxyphenol (10′)[29]

NaAuCl₄·2 H₂O (39.8 mg, 0.1 mmol, 0.05 equiv) was added to a solution of silyl ether 10 [30] (557 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 7 h. The mixture was then diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] to give a pale-yellow oil; yield: 286 mg (87%).

¹H NMR (300 MHz, CDCl₃): δ = 6.86–6.83 (m, 1 H), 6.69–6.67 (m, 2 H), 5.99–5.88 (m, 1 H), 5.49 (s, 1 H), 5.10–5.04 (m, 2 H), 3.87 (s, 3 H), 3.32 (d, J = 6.6 Hz, 2 H).

¹³C NMR (75 MHz, CDCl₃): δ = 146.44, 143.92, 137.80, 131.89, 121.17, 115.45, 114.26, 111.14, 55.83, 39.84.

MS (ESI, MeOH): m/z = 187 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₀H₁₂NaO₂: 187.0735; found: 187.0735.

4-tert-Butylphenol (11′)[4i]

NaAuCl₄·2 H₂O (39.8 mg, 0.1 mmol, 0.05 equiv) was added to a solution of silyl ether 11 [4i] (529 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 24 h. The mixture was then diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] to give a white solid; yield: 277 mg (92%); mp 97–99 °C.

¹H NMR (300 MHz, CDCl₃): δ = 7.26 (d, J = 4.5 Hz, 2 H), 6.77 (d, J = 4.5 Hz, 2 H), 4.65 (s, 1 H), 1.29 (s, 9 H).

¹³C NMR (75 MHz, CDCl₃): δ = 153.05, 143.58, 126.42, 114.80, 34.05, 31.51.

MS (ESI, MeOH): m/z = 173 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₀H₁₄NaO: 173.0942; found: 173.0354.

N-(4-Hydroxyphenyl)acetamide (12′)[31]

NaAuCl₄·2 H₂O (39.8 mg, 0.1 mmol, 0.05 equiv) was added to a solution of acetamide 12 [32] (531 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 48 h. The mixture was then diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (2:1)] to give a white solid; yield: 260 mg (86%); mp 166–168 °C.

¹H NMR (300 MHz, CD₃OD): δ = 7.30 (d, J = 6.6 Hz, 2 H), 6.73 (d, J = 6.6 Hz, 2 H), 2.07 (s, 3 H).

¹³C NMR (75 MHz, CD₃OD): δ = 171.46, 155.40, 131.69, 123.54, 116.28, 23.57.

MS (ESI, MeOH): m/z = 174 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₈H₉NNaO₂: 174.0531; found: 174.0524.

1-(4-Hydroxyphenyl)ethanone (13′)[8b]

NaAuCl₄·2 H₂O (79.6 mg, 0.2 mmol, 0.1 equiv) was added to a solution of hydroxy ketone 13 [8b] (501 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 48 h. The mixture was then diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] to give a white solid; yield: 57 mg (21%); mp 109–110 °C.

¹H NMR (300 MHz, CDCl₃): δ = 7.91 (d, J = 8.7 Hz, 2 H), 6.93 (d, J = 8.7 Hz, 2 H), 2.58 (s, 3 H).

¹³C NMR (75 MHz, CD₃OD): δ = 198.89, 161.62, 131.24, 129.41, 115.59, 26.21.

MS (ESI, MeOH): m/z = 159 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₈H₈NaO₂: 159.0422; found: 159.0415.

6-(Allyloxy)hexan-1-ol (14′)[33]

Prepared according to the general procedure from 14 [34] (545 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (3:1)] as a colorless oil; yield: 301 mg (95%).

¹H NMR (300 MHz, CDCl₃): δ = 5.98–5.85 (m, 1 H), 5.27 (dd, J = 1.7, 17.3 Hz, 1 H), 5.17 (dd, J = 1.1, 10.4 Hz, 1 H), 3.97 (d, J = 4.2 Hz, 2 H), 3.65 (t, J = 4.1 Hz, 2 H), 3.44 (t, J = 5.0 Hz, 2 H), 1.62–1.56 (m, 4 H), 1.41–1.38 (m, 5 H).

¹³C NMR (75 MHz, CDCl₃): δ = 134.93, 116.64, 71.69, 70.24, 62.64, 32.56, 29.58, 25.89, 25.51.

MS (ESI, MeOH): m/z = 181 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₉H₁₈NaO₂: 181.1204; found: 181.1198.

6-Hydroxyhexyl Acetate (15′)[4n]

Prepared according to the general procedure from 15 [4n] (549 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:1)] as a colorless oil; yield: 301 mg (94%).

¹H NMR (300 MHz, CDCl₃): δ = 4.06 (t, J = 5.0 Hz, 2 H), 3.64 (t, J = 4.8 Hz, 2 H), 2.05 (s, 3 H), 1.64–1.58 (m, 4 H), 1.40–1.38 (m, 4 H).

¹³C NMR (75 MHz, CDCl₃): δ = 171.20, 64.40, 62.59, 32.49, 28.49, 25.63, 25.31, 20.87.

MS (ESI, MeOH): m/z = 183 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₈H₁₆NaO₃: 183.0997; found: 183.0990.

6-(Methoxymethoxy)hexan-1-ol (16′)[4j]

Prepared according to the general procedure from 16 [4j] (553 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:2)] as a colorless oil; yield: 315 mg (97%).

¹H NMR (300 MHz, CDCl₃): δ = 4.62 (s, 2 H), 3.65 (t, J = 6.5 Hz, 2 H), 3.53 (t, J = 6.5 Hz, 2 H), 3.36 (s, 3 H), 1.75–1.52 (m, 4 H), 1.42–1.39 (m, 4 H).

¹³C NMR (75 MHz, CDCl₃): δ = 96.38, 67.72, 62.84, 55.06, 32.67, 29.66, 25.99, 25.53.

MS (ESI, MeOH): m/z = 185 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₈H₁₈NaO₃: 185.1154; found: 185.1148.

6-[(2-Methoxyethoxy)methoxy]hexan-1-ol (17′)[4j]

Prepared according to the general procedure from 17 [4j] (641 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:3)] as a colorless oil; yield: 392 mg (95%).

¹H NMR (300 MHz, CDCl₃): δ = 4.71 (s, 2 H), 3.75–3.62 (m, 4 H), 3.60–3.51 (m, 4 H), 3.40 (s, 3 H), 1.75–1.53 (m, 4 H), 1.46–1.39 (m, 4 H).

¹³C NMR (75 MHz, CDCl₃): δ = 95.43, 71.80, 67.77, 66.68, 62.79, 58.95, 32.65, 29.59, 25.95, 25.49.

MS (ESI, MeOH): m/z = 229 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₀H₂₂NaO₄: 229.1416; found: 229.1413.

1,2:3,4-Bis-O-(1-Methylethylidene)-α-d-galactopyranose (18′)[4k]

Prepared according to the general procedure from 18 [4k] (749 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (2:1)] as a colorless oil; yield: 500 mg (96%).

¹H NMR (300 MHz, CDCl₃): δ = 5.57 (d, J = 5.0 Hz, 1 H), 4.62 (dd, J = 7.9, 2.2 Hz, 1 H), 4.34 (m, 1 H), 4.29 (t, J = 6.3 Hz, 1 H), 3.95–3.83 (m, 2 H), 3.79–3.72 (m, 1 H), 2.10 (dd, J = 9.6, 3.0 Hz, 1 H), 1.54 (s, 3 H), 1.46 (s, 3 H), 1.34 (s, 6 H).

¹³C NMR (75 MHz, CDCl₃): δ = 109.20, 108.45, 96.10, 71.22, 70.57, 70.43, 68.17, 61.79, 25.82, 25.75, 24.76, 24.18.

MS (ESI, MeOH): m/z = 283 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₂H₂₀NaO₆: 283.1158; found: 283.1153.

6-{[tert-Butyl(diphenyl)silyl]oxy}hexan-1-ol (19′)[4j]

Prepared according to the general procedure from 19 [4j] (942 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:4)] as a colorless oil; yield: 685 mg (96%).

¹H NMR (300 MHz, CDCl₃): δ = 7.67 (dd, J = 7.6, 1.7 Hz, 4 H), 7.45–7.35 (m, 6 H), 3.68–3.60 (m, 4 H), 1.60–1.53 (m, 4 H), 1.36–1.33 (m, 4 H), 1.05 (s, 9 H).

¹³C NMR (75 MHz, CDCl₃): δ = 135.58, 134.16, 129.49, 127.57, 63.85, 62.98, 32.76, 32.49, 26.88, 25.58, 25.46, 19.22.

MS (ESI, MeOH): m/z = 379 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₂₂H₃₂NaO₂Si: 379.2069; found: 379.2070.

[4-({[tert-Butyl(diphenyl)silyl]oxy}methyl)phenyl]methanol (20′)[35]

Prepared according to the general procedure from 20 [35] (982 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:4)] as a colorless oil; yield: 731 mg (97%).

¹H NMR (300 MHz, CDCl₃): δ = 7.72–7.69 (m, 4 H), 7.44–7.35 (m, 10 H), 4.78 (s, 2 H), 4.70 (s, 2 H), 1.10 (s, 9 H).

¹³C NMR (75 MHz, CDCl₃): δ = 140.61, 139.50, 135.57, 133.53, 129.69, 127.71, 126.99, 126.26, 65.35, 65.29, 26.84, 19.31.

MS (ESI, MeOH): m/z = 399 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₂₄H₂₈NaO₂Si: 399.1756; found: 399.1753.

6-[(Triisopropylsilyl)oxy]hexan-1-ol (21′)[4j]

Prepared according to the general procedure from 21 [4j] (777 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:3)] as a colorless oil; yield: 455 mg (83%).

¹H NMR (300 MHz, CDCl₃): δ = 3.69–3.63 (m, 4 H), 1.60–1.54 (m, 4 H), 1.40–1.36 (m, 4 H), 1.23 (br s, 1 H), 1.11–0.96 (m, 21 H).

¹³C NMR (75 MHz, CDCl₃): δ = 63.34, 62.92, 32.94, 32.77, 25.64, 25.56, 17.99, 12.02.

MS (ESI, MeOH): m/z = 297 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₅H₃₄NaO₂Si: 297.2226; found: 297.2225.

(4-{[(Triisopropylsilyl)oxy]methyl}phenyl)methanol (22′)[8e]

Prepared according to the general procedure from 22 [8e] (818 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:3)] as a colorless oil; yield: 507 mg (86%).

¹H NMR (300 MHz, CDCl₃): δ = 7.35–7.34 (m, 4 H), 4.84 (s, 2 H), 4.68 (s, 2 H), 1.60 (br s, 1 H), 1.18–1.15 (m, 3 H), 1.10–1.06 (m, 18 H).

¹³C NMR (75 MHz, CDCl₃): δ = 141.20, 139.37, 126.94, 125.96, 65.24, 64.84, 18.02, 12.05.

MS (ESI, MeOH): m/z = 317 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₇H₃₀NaO₂Si: 317.1913; found: 317.1912.

[4-({[tert-Butyl(dimethyl)silyl]oxy}methyl)phenyl]methanol (23′)[36]

NaAuCl₄·2 H₂O (0.8 mg, 0.002 mmol, 0.001 equiv) was added to a solution of disilyl ether 23 [36] (733 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 0.5 h. The mixture was then diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] to give a colorless oil; yield: 435 mg (86%).

¹H NMR (300 MHz, CDCl₃): δ = 7.33 (s, 4 H), 4.74 (s, 2 H), 4.68 (s, 2 H), 0.94 (s, 9 H), 0.10 (s, 6 H).

¹³C NMR (75 MHz, CDCl₃): δ = 140.88, 139.54, 126.94, 126.26, 65.13, 64.77, 25.93, 18.39, –5.27.

MS (ESI, MeOH): m/z = 275 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₄H₂₄NaO₂Si: 275.1443; found: 275.1447.

(±)-trans-[4-({[tert-Butyl(dimethyl)silyl]oxy}methyl)cyclohexyl]methanol (6)[26]

NaAuCl₄·2 H₂O (0.8 mg, 0.002 mmol, 0.001 equiv) was added to a solution of disilyl ether 24 (745 mg, 2 mmol) in MeOH (4 mL) at r.t., and the mixture was stirred at r.t. for 1 h. The mixture was then diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] to give a colorless oil; yield: 397 mg (77%).

¹H NMR (300 MHz, CDCl₃): δ = 3.46 (d, J = 4.8 Hz, 2 H), 3.41 (d, J = 4.5 Hz, 2 H), 1.82–1.80 (m, 4 H), 1.44–1.42 (m, 2 H), 0.96–0.92 (m, 4 H), 0.89 (s, 9 H), 0.04 (s, 6 H).

¹³C NMR (75 MHz, CDCl₃): δ = 68.70, 68.66, 40.73, 40.61, 29.00, 28.89, 25.95, 18.36, –5.37.

MS (ESI, MeOH): m/z = 281 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₄H₃₀NaO₂Si: 281.1913; found: 281.1919.

(4-{[tert-Butyl(dimethyl)silyl]oxy}phenyl)methanol (25′)[4j]

Prepared according to the general procedure from 25 [4j] (705 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:3)] as a colorless oil; yield: 439 mg (92%).

¹H NMR (300 MHz, CDCl₃): δ = 7.22 (d, J = 8.4 Hz, 2 H), 6.82 (d, J = 8.4 Hz, 2 H), 4.58 (d, J = 5.7 Hz, 2 H), 1.76 (t, J = 5.8 Hz, 1 H), 0.98 (s, 9 H), 0.19 (s, 6 H).

¹³C NMR (75 MHz, CDCl₃): δ = 155.31, 133.73, 128.52, 120.15, 65.10, 25.67, 18.20, –4.44.

MS (ESI, MeOH): m/z = 261 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₃H₂₂NaO₂Si: 261.1287; found: 261.1285.

(2S)-2-((3aR,4S,7aR)-4-{[tert-butyl(dimethyl)silyl]oxy}-7a-methyloctahydro-1H-inden-1-yl)propan-1-ol (26′)[37]

Prepared according to the general procedure from 26 [37] (882 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] as a colorless oil; yield: 608 mg (93%).

¹H NMR (300 MHz, CDCl₃): δ = 4.01 (br s, 1 H), 3.63 (dd, J = 3.0, 10.2 Hz, 1 H), 3.37 (dd, J = 6.6, 10.2 Hz, 1 H), 1.95 (d, J = 12.4 Hz, 1 H), 1.87–1.73 (m, 2 H), 1.67 (dr s, 1 H), 1.63–1.49 (m, 2 H), 1.47–1.30 (m, 4 H), 1.27–1.23 (m, 1 H), 1.22–1.09 (m, 3 H), 1.02 (d, J = 6.6 Hz, 3 H), 0.93 (s, 3 H), 0.89 (s, 9 H), 0.01 (s, 3 H), 0.00 (s, 3 H).

¹³C NMR (75 MHz, CDCl₃): δ = 69.34, 67.76, 53.16, 52.83, 42.12, 40.58, 38.27, 34.41, 26.77, 25.78, 23.10, 17.97, 17.61, 16.66, 13.74, –4.84, –5.19.

MS (ESI, MeOH): m/z = 349 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₉H₃₈NaO₂Si: 349.2539; found: 349.2535.

(±)-trans-5-(1-{[tert-Butyl(dimethyl)silyl]oxy}-1-methylethyl)-2-methylcyclohex-2-en-1-ol (28′)

Prepared according to the general procedure from 28 (798 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] as a colorless oil; yield: 489 mg (86%).

IR (KBr): 3350, 2969, 2922, 2857, 1468, 1379, 1250 cm^–1.

¹H NMR (300 MHz, CDCl₃): δ = 5.57–5.56 (m, 1 H), 4.01 (br s, 1 H), 2.13–2.06 (m, 1 H), 2.02 (dq, J = 2.0, 13.6, Hz, 1 H), 1.86–1.81 (m, 1 H), 1.78 (s, 3 H), 1.61 (ddt, J = 2.4, 4.8, 12.0 Hz, 1 H), 1.43 (dt, J = 4.0, 13.6 Hz, 2 H), 1.21 (s, 3 H), 1.19 (s, 3 H), 0.85 (s, 9 H), 0.07 (s, 6 H).

¹³C NMR (75 MHz, CDCl₃): δ = 134.16, 125.86, 74.70, 69.00, 39.84, 32.88, 27.80, 27.75, 26.93, 25.93, 20.83, 18.28, –2.03.

MS (ESI, MeOH): m/z = 307 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₆H₃₂NaO₂Si: 307.2069; found: 307.2083.

(3,4,5-Trimethoxyphenyl)methanol (30′)[22]

Prepared according to the general procedure from 30 (625 mg), and purified by flash column chromatography [silica gel, EtOAc–PE (1:5)] as a white solid; yield: 361 mg (91%); mp 36–38 °C.

¹H NMR (300 MHz, CDCl₃): δ = 6.60 (s, 2 H), 4.63 (s, 2 H), 3.87 (s, 6 H), 3.84 (s, 3 H), 1.81 (br s, 1 H).

¹³C NMR (75 MHz, CDCl₃): δ = 153.24, 137.16, 136.68, 103.72, 65.32, 60.75, 55.98.

MS (ESI, MeOH): m/z = 221 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₀H₁₄NaO₄: 221.0790; found: 221.0792.

1-({[6-(Benzyloxy)hexyl]oxy}methyl)-4-methoxybenzene (29)[38]

NaAuCl₄·2 H₂O (39.8 mg, 0.1 mmol, 0.05 equiv) was added to a solution of silyl ether 1 (645 mg, 2 mmol) and 4-MeOC₆H₄CH₂OH (3) (1.24 mL, 10 mmol) in THF (4 mL) and the mixture was refluxed for 8 h. The mixture was then cooled to r.t., diluted with EtOAc­ (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:10)] to give a colorless oil; yield: 394 mg (60%).

¹H NMR (300 MHz, CDCl₃): δ = 7.33–7.35 (m, 5 H), 7.27 (d, J = 8.4 Hz, 2 H), 6.90 (d, J = 8.4 Hz, 2 H), 4.52 (s, 2 H), 4.45 (s, 2 H), 3.83 (s, 3 H), 3.50–3.44 (m, 4 H), 1.63–1.58 (m, 4 H), 1.41–1.39 (m, 4 H).

¹³C NMR (75 MHz, CDCl₃): δ = 159.06, 138.66, 130.76, 129.13, 128.27, 127.54, 127.39, 113.70, 72.79, 72.45, 70.35, 70.04, 55.19, 29.67, 26.01.

MS (ESI, MeOH): m/z = 351 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₂₁H₂₈NaO₃: 351.1936; found: 351.1939.

1,2,3-Trimethoxy-5-(methoxymethyl)benzene (31)[39]

NaAuCl₄·2 H₂O (39.8 mg, 0.1 mmol, 0.05 equiv) was added to a solution of silyl ether 30 (624 mg, 2 mmol) in MeOH (4 mL), and the mixture was refluxed for 24 h. The mixture was then cooled to r.t., diluted with EtOAc (10 mL), filtered through activated alumina, and concentrated in vacuo. The residue was purified by flash column chromatography [silica gel, EtOAc–PE (1:10)] to give a colorless oil; yield: 266 mg (63%).

¹H NMR (300 MHz, CDCl₃): δ = 6.57 (s, 2 H), 4.39 (s, 2 H), 3.87 (s, 6 H), 3.84 (s, 3 H), 3.41 (s, 3 H).

¹³C NMR (75 MHz, CDCl₃): δ = 153.26, 137.46, 133.85, 104.61, 74.85, 60.76, 58.11, 56.05.

MS (ESI, MeOH): m/z = 235 [M + Na]⁺.

HRMS-ESI: m/z [M + Na]⁺ calcd for C₁₁H₁₆NaO₄: 235.0946; found: 235.0948.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (21302119), the Research Fund for the Doctoral Program of Higher Education of China (20100202120002), and the Fundamental Research Funds for the Central Universities (GK201302017).

• References

• 1 Greene TW, Wuts PG. M. Protective Groups in Organic Synthesis . 3rd ed. Wiley; New York: 1999
• 2 Kocienski PJ. Protecting Groups . 3rd ed. Thieme; Stuttgart: 2005
• 3 For a recent review, see: Crouch RD. Tetrahedron 2013; 69: 2383 ; and references therein

For recent references, see:
• 4a González-Calderó D, Benitez-Puebla LJ, Gonzalez-Gonzalez CA, Garcia-Eleno MA, Fuentes-Benitez A, Cuevas-Yañez E, Corona-Becerril D, González-Romero C. Synth. Commun. 2014; 44: 1258
• 4b Jadhav AH, Chinnappan A, Patil RH, Chung W.-J, Kim H. Chem. Eng. J. (Amsterdam, Neth.) 2014; 236: 300
• 4c Bothwell JM, Angeles VV, Carolan JP, Olson ME, Mohan RS. Tetrahedron Lett. 2010; 51: 1056
• 4d Martinez-Solorio D, Jennings MP. Tetrahedron Lett. 2008; 49: 5175
• 4e Bhure MH, Kumar I, Natu AD, Rode CV. Synth. Commun. 2008; 38: 346
• 4f Shah ST. A, Guiry PJ. Org. Biomol. Chem. 2008; 6: 2168
• 4g Yang Y.-Q, Cui J.-R, Zhu L.-G, Sun Y.-P, Wu Y. Synlett 2006; 1260
• 4h Das B, Reddy KR, Thirupathi P. Tetrahedron Lett. 2006; 47: 5855
• 4i Bhatt S, Nayak SK. Tetrahedron Lett. 2006; 47: 8395
• 4j Yeom C.-E, Kim YJ, Lee SY, Shin YJ, Kim BM. Tetrahedron 2005; 61: 12227
• 4k Kumar GD. K, Baskaran S. J. Org. Chem. 2005; 70: 4520
• 4l Karimi B, Zareyee D. Tetrahedron Lett. 2005; 46: 4661
• 4m Hua J, Jiang ZY, Wang YG. Chin. Chem. Lett. 2004; 15: 1430
• 4n Rani S, Babu JL, Vankar YD. Synth. Commun. 2003; 33: 4043
• 4o Yadav JS, Reddy BV. S, Madan C. New J. Chem. 2000; 24: 853
• 5a Yeom C.-E, Kim HW, Lee SY, Kim BM. Synlett 2007; 146
• 5b Jiang Z.-Y, Wang Y.-G. Chem. Lett. 2003; 32: 568
• 5c Jiang Z.-Y, Wang Y.-G. Tetrahedron Lett. 2003; 44: 3859
• 5d Crouch RD, Stieff M, Frie JL, Cadwallader AB, Bevis DC. Tetrahedron Lett. 1999; 40: 3133
• 5e Wilson NS, Keay BA. Tetrahedron Lett. 1997; 38: 187
• 6a Patel P, Chang C.-T, Kang N, Lee G.-J, Powell WS, Rokach J. Tetrahedron Lett. 2007; 48: 5289
• 6b Ikawa T, Hattori K, Sajiki H, Hirota K. Tetrahedron 2004; 60: 6901
• 6c Kim S, Jacobo SM, Chang C.-T, Bellone S, Powell W, Rokach J. Tetrahedron Lett. 2004; 45: 1973
• 6d Corey EJ, Jones GB. J. Org. Chem. 1992; 57: 1028
• 6e de Vries EF. J, Brussee J, van der Gen A. J. Org. Chem. 1994; 59: 7133
• 6f Cormier JF. Tetrahedron Lett. 1991; 32: 187
• 7a Li J, Menche D. Synthesis 2009; 1904
• 7b Reddy MS, Narender M, Nageswar YV. D, Rao KR. Synthesis 2005; 714
• 7c Karimi B, Zamani A, Zareyee D. Tetrahedron Lett. 2004; 45: 9139
• 7d Wang M, Li C, Yin D, Liang X.-T. Tetrahedron Lett. 2002; 43: 8727
• 7e Sabitha G, Syamala M, Yadav JS. Org. Lett. 1999; 1: 1701
• 7f DattaGupta A, Singh R, Singh VK. Synlett 1996; 69
• 7g Oku A, Kinugasa M, Kamada T. Chem. Lett. 1993; 165
• 8a Shah ST. A, Singh S, Guiry PJ. J. Org. Chem. 2009; 74: 2179
• 8b Ranu BC, Jana U, Majee A. Tetrahedron Lett. 1999; 40: 1985
• 8c Zhang W, Robins MJ. Tetrahedron Lett. 1992; 33: 1177
• 8d Corey EJ, Yi KY. Tetrahedron Lett. 1992; 33: 2289
• 8e Pilcher AS, Hill DK, Shimshock SJ, Waltermire RE, DeShong P. J. Org. Chem. 1992; 57: 2492
• 8f Metcalf BW, Burkhart JP, Jund K. Tetrahedron Lett. 1980; 21: 35
• 8g Corey EJ, Venkateswarlu A. J. Am. Chem. Soc. 1972; 94: 6190
• 9a Jadhav AH, Kim H. Tetrahedron Lett. 2012; 53: 5338
• 9b Yan L, Zhao F, Gan Y, Zhao J, Jiang Z. Synth. Commun. 2012; 42: 285
• 9c Wang B, Sun H.-X, Sun Z.-H. J. Org. Chem. 2009; 74: 1781
• 9d Itoh A, Kodama T, Masaki Y. Chem. Pharm. Bull. 2007; 55: 861
• 9e Iida A, Okazaki H, Misaki T, Sunagawa M, Sasaki A, Tanabe Y. J. Org. Chem. 2006; 71: 5380
• 9f Khan AT, Ghosh S, Choudhury LH. Eur. J. Org. Chem. 2004; 2198
• 9g Zubaidha PK, Bhosale SV, Hashmi AM. Tetrahedron Lett. 2002; 43: 7277
• 9h Bartoli G, Cupone G, Dalpozzo R, De Nino A, Maiuolo L, Procopio A, Sambri L, Tagarelli A. Tetrahedron Lett. 2002; 43: 5945
• 9i Tandon M, Begley TP. Synth. Commun. 1997; 27: 2953
• 9j Maiti G, Roy SC. Tetrahedron Lett. 1997; 38: 495
• 9k Lee AS.-Y, Yeh H.-C, Tsai M.-H. Tetrahedron Lett. 1995; 36: 6891
• 10 Arcadi A, Pietropaolo E, Alvino A, Michelet V. Org. Lett. 2013; 15: 2766
• 11 Xu M, Hou Q, Wang S, Wang H, Yao Z.-J. Synthesis 2011; 626
• 12 Yan B, Liu Y. Org. Lett. 2007; 9: 4323
• 13 Alfonsi M, Arcadi A, Aschi M, Bianchi G, Marinelli F. J. Org. Chem. 2005; 70: 2265
• 14 Georgy M, Boucard V, Debleds O, Zotto CD, Campagne J.-M. Tetrahedron 2009; 65: 1758
• 15 Georgy M, Boucard V, Campagne J.-M. J. Am. Chem. Soc. 2005; 127: 14180
• 16 Cuenca A, Mancha G, Asensio G, Medio-Simón M. Chem. Eur. J. 2008; 14: 1518
• 17 Jiang C, Wang S. Synlett 2009; 1099
• 18 Terrasson V, Marque S, Georgy M, Campagne J.-M, Prim D. Adv. Synth. Catal. 2006; 348: 2063
• 19 Collington EW, Finch H, Smith IJ. Tetrahedron Lett. 1985; 26: 681
• 20 Lee AS.-Y, Yeh H.-C, Yeh M.-K, Tsai M.-H. J. Chin. Chem. Soc. (Taipei) 1995; 42: 919
• 21 da Silva Rocha KA, Hoehne JL, Gusevskaya EV. Chem. Eur. J. 2008; 14: 6166
• 22 Baumert M, Albrecht M, Winkler HD. F, Schalley CA. Synthesis 2010; 953
• 23 Zhang L, Wang S, Zhou S, Yang G, Sheng E. J. Org. Chem. 2006; 71: 3149
• 24 Davies HM. L, Hedley SJ, Bohall BR. J. Org. Chem. 2005; 70: 10737
• 25 Hardcastle IR, Liu J, Valeur E, Watson A, Ahmed SU, Blackburn TJ, Bennaceur K, Clegg W, Drummond C, Endicott JA, Golding BT, Griffin RJ, Gruber J, Haggerty K, Harrington RW, Hutton C, Kemp S, Lu X, McDonnell JM, Newell DR, Noble ME. M, Payne SL, Revill CH, Riedinger C, Xu Q, Lunec J. J. Med. Chem. 2011; 54: 1233
• 26 Li P, Yamamoto H. Chem. Commun. 2009; 5412
• 27 Marković D, Steunenberg P, Ekstrand M, Vogel P. Chem. Commun. 2004; 2444
• 28 Johnson DA, Taubner LM. Tetrahedron Lett. 1996; 37: 605
• 29 Elgendy EM, Khayyat SA. Russ. J. Org. Chem. 2008; 44: 823
• 30 Ohmiya H, Tanabe M, Sawamura M. Org. Lett. 2011; 13: 1086
• 31 Lewis EA, Adamek TL, Vining LC, White RL. J. Nat. Prod. 2003; 66: 62
• 32 Mai K, Patil G. J. Org. Chem. 1986; 51: 3545
• 33 Inagaki M, Hiratake J, Yamamoto Y, Oda J. Bull. Chem. Soc. Jpn. 1987; 60: 4121
• 34 Reddy CS, Smitha G, Chandrasekhar S. Tetrahedron Lett. 2003; 44: 4693
• 35 Zhang S, Xu L, Trudell ML. Synthesis 2005; 1757
• 36 Itoh A, Kodama T, Masaki Y. Synlett 1999; 357
• 37 Fraga R, Zacconi F, Sussman F, Ordóñez-Morán P, Muñoz A, Huet T, Molnár F, Moras D, Rochel N, Maestro M, Mouriño A. Chem. Eur. J. 2012; 18: 603
• 38 Shen Z, Sheng L, Zhang X, Mo W, Hu B, Sun N, Hu X. Tetrahedron Lett. 2013; 54: 1579
• 39 Ohira S, Shirane F, Nozaki H, Yahiro S, Nakayama M. Bull. Chem. Soc. Jpn. 1989; 62: 2427