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Synlett 2012; 23(16): 2341-2348
DOI: 10.1055/s-0032-1317135
DOI: 10.1055/s-0032-1317135
letter
Stereoselective Glycosylations by Nitrosyl Tetrafluoroborate-Catalyzed Activation of Glycosyl Trichloroacetimidate Derivatives
Further Information
Publication History
Received: 29 May 2012
Accepted after revision: 24 July 2012
Publication Date:
24 August 2012 (online)
![](https://www.thieme-connect.de/media/synlett/201216/lookinside/thumbnails/10.1055-s-0032-1317135-1.jpg)
Dedicated to Professor Richard R. Schmidt, Universität Konstanz, Germany on his 73rd birthday
Abstract
Efficient stereoselective glycosylation conditions have been developed by using nitrosyl tetrafluoroborate (NOBF4) promoted activation of glycosyl trichloroacetimidate derivatives. A series of mono- and disaccharide derivatives were synthesized in excellent yield, and high stereoselectivity at the glycosylation center was observed in every case.
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References
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- 46 General Procedure: A solution of glycosyl trichloroacetimidate derivative (1.0 mmol) and alcohol (2.0 mmol) or glycosyl acceptor (0.9 mmol) in anhydrous CH2Cl2 or CH2Cl2–Et2O (1:1) or MeCN (5 mL) was cooled to –15 °C under argon. To the cooled reaction mixture was added NOBF4 (35.0 mg, 0.3 mmol) and the reaction was stirred at the same temperature for the appropriate time (Table 1). The reaction mixture was diluted with CH2Cl2 and the organic layer was successively washed with sat. NaHCO3 and water, dried (Na2SO4), and concentrated. The crude product was purified over SiO2 (hexane–EtOAc) to give pure glycoside derivatives 30–48. All known compounds gave NMR spectra that matched the data reported in the cited references. Spectral data for those products that have not been previously reported are as follows: 2-(p-Methoxyphenoxy)ethyl 2,3,4,6-Tetra-O-acetyl-α-d-mannopyranoside (34): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 6.86–6.82 (m, 4 H, Ar-H), 5.35 (dd, J = 9.9, 3.4 Hz, 1 H, H-3), 5.30–5.26 (m, 2 H, H-2, H-4), 4.91 (d, J = 1.5 Hz, 1 H, H-1), 4.28–4.25 (m, 1 H, H-5), 4.14–4.06 (m, 4 H, H-6ab, -OCH 2ab-), 4.01–3.96 (m, 1 H, -OCH 2a-), 3.88–3.84 (m, 1 H, -OCH 2b-), 3.76 (s, 3 H, OCH 3), 2.16, 2.08, 2.03, 1.98 (4 s, 12 H, 4 COCH 3). 13C NMR (CDCl3, 125 MHz): δ = 171.1, 170.4, 170.2, 170.1 (4 COCH3), 154.5–115.1 (Ar-C), 98.1 (C-1), 69.9 (C-3), 69.4 (C-2), 68.9 (C-4), 67.8 (C-5), 67.3 (-CH2-), 66.5 (-CH2-), 62.8 (C-6), 56.1 (OCH3). MS (ESI): m/z = 521.1 [M + Na]+. Anal. Calcd for C23H30O12 (498.17): C, 55.42; H, 6.07. Found: C, 55.25; H, 6.30 2-(p-Methoxyphenoxy)ethyl 2,3,4-tri-O-acetyl-α-l-rhamnopyranoside (36): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 6.86–6.82 (m, 4 H, Ar-H), 5.31–5.27 (m, 2 H, H-2, H-3), 5.06 (t, J = 9.8 Hz each, 1 H, H-4), 4.83 (d, J = 1.5 Hz, 1 H, H-1), 4.11–4.09 (m, 2 H, -OCH 2-), 3.99–3.93 (m, 1 H, -OCH 2b-), 3.76 (s, 3 H, OCH 3), 2.15, 2.04, 1.98 (3 s, 9 H, 3 COCH 3), 1.21 (d, J = 6.3 Hz, 3 H, CCH 3). 13C NMR (CDCl3, 125 MHz): δ = 170.5, 170.4, 170.3 (3 COCH3), 154.5–115.1 (Ar-C), 98.0 (C-1), 71.5 (C-3), 70.2 (C-2), 69.5 (C-4), 67.9 (C-5), 66.9 (CH2), 66.8 (CH2), 56.1 (OCH3), 21.3, 21.2, 21.1 (3 COCH3), 17.7 (CCH3). MS (ESI): m/z = 463.1 [M + Na]+. Anal. Calcd for C21H28O10 (440.16): C, 57.27; H, 6.41. Found: C, 57.10; H, 6.60. Methyl 4-O-Acetyl-3-O-allyl-2,6-di-O-benzyl-α-d-glucopyranoside (37): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 7.32–7.23 (m, 10 H, Ar-H), 5.88–5.80 (m, 1 H, CH2=CH), 4.92 (t, J = 9.6 Hz each, 1 H, H-4), 4.77 (d, J = 12.0 Hz, 1 H, PhCH 2), 4.62 (d, J = 12.0 Hz, 1 H, PhCH 2), 4.56 (d, J = 3.5 Hz, 1 H, H-1), 4.48 (s, 3 H, OCH 3), 4.33–4.29 (m, 1 H, H-6a), 4.11–4.07 (m, 1 H, H-6b), 3.78–3.76 (m, 1 H, H-5), 3.75 (t, J = 9.5 Hz each, 1 H, H-3), 3.39–3.42 (m, 3 H, H-2, OCH 2-CH=CH2), 3.37 (s, 3 H, OCH 3), 1.95 (s, 3 H, COCH 3). 13C NMR (CDCl3, 125 MHz): δ = 169.7 (COCH3), 138.4–116.8 (Ar-C), 98.6 (C-1), 79.7 (C-5), 79.3 (C-3), 74.4 (PhCH2), 73.9 (PhCH2), 73.8 (C-5), 69.5 (OCH2-), 69.1 (C-6), 55.6 (OCH3), 21.2 (CHCH3). MS (ESI): m/z = 479.2 [M + Na]+. Anal. Calcd for C26H32O7 (456.21): C, 68.40; H, 7.07. Found: C, 68.22; H, 7.28. 6-Bromohexyl 3,4,6-Tri-O-acetyl-2-azido-2-deoxy-β-d-galactopyranoside (38): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 5.32 (d, J = 2.5 Hz, 1 H, H-4), 4.77 (dd, J = 10.0, 3.0 Hz, 1 H, H-3), 4.36 (d, J = 8.0 Hz, 1 H, H-1), 4.19–4.09 (m, 2 H, H-6ab), 3.98–3.94 (m, 1 H, -OCH 2-), 3.85–3.82 (m, 1 H, H-5), 3.37 (dd, J = 8.0 Hz each, 1 H, H-2), 3.60–3.55 (m, 1 H, -OCH 2-), 3.42–3.40 (m, 2 H, -CH 2-), 2.15, 2.05, 2.04 (3 s, 9 H, COCH 3), 1.89–1.85 (m, 2 H, CH 2-), 1.70–1.65 (m, 2 H, -CH 2-), 1.49–1.40 (m, 4 H, -CH 2). 13C NMR (CDCl3, 125 MHz): δ = 170.4, 170.1, 169.8 (3 COCH3), 102.4 (C-1), 70.9 (C-3), 70.6 (C-5), 70.5 (C-4), 66.3 (OCH2), 61.2 (C-6), 60.9 (C-2), 33.8 (CH2), 32.6 (CH2), 29.3 (CH2), 27.8 (CH2), 25.1 (CH2), 20.7, 20.6, 20.6 (3 COCH3). MS (ESI): m/z = 516.1 [M + Na]+. Anal. Calcd for C18H28BrN3O8 (493.10): C, 43.73; H, 5.71. Found: C, 43.57; H, 5.90. p-Methoxyphenyl (4-O-Acetyl-2,3,6-tri-O-benzyl-α-d-galactopyranosyl)-(1→3)-4,6-O-benzylidene-2-deoxy-2-N-phthalimido-β-d-galactopyranoside (41): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 7.68–7.10 (m, 24 H, Ar-H), 6.78 (d, J = 9.0 Hz, 2 H, Ar-H), 6.62 (d, J = 9.0 Hz, 2 H, Ar-H), 5.56 (d, J = 8.5 Hz, 1 H, H-1A), 5.32 (d, J = 2.4 Hz, H-4B), 5.15 (s, 1 H, PhCH), 4.92 (d, J = 3.4 Hz, 1 H, H-1B), 4.85 (dd, J = 8.5, 8.5 Hz, 1 H, H-2A), 4.63 (d, J = 11.1 Hz, 1 H, PhCH 2), 4.55 (dd, J = 10.5, 3.0 Hz, 1 H, H-3A), 4.50 (d, J = 11.1 Hz, 1 H, PhCH 2), 4.43 (d, J = 11.1 Hz, 1 H, PhCH 2), 4.28 (d, J = 11.1 Hz, 1 H, PhCH 2), 4.19–4.17 (m, 3 H, H-6abA, PhCH 2), 4.13 (d, J = 11.0 Hz, 1 H, PhCH 2), 3.78–3.74 (m, 2 H, H-5A, H-5B), 3.71–3.68 (dd, J = 10.0, 3.1 Hz, 1 H, H-2B), 3.62 (s, 3 H, OCH 3), 3.61–3.59 (dd, J = 10.0, 3.0 Hz, 1 H, H-3B), 3.33 (br s, 1 H, H-4A), 2.95 (t, J = 8.8 Hz each, 1 H, H-6aB), 2.72–2.69 (m, 1 H, H-6bB), 1.86 (s, 3 H, COCH 3). 13C NMR (CDCl3, 125 MHz): δ = 170.2 (COCH3), 169.4, 167.6 (PhthCO), 155.9–114.7 (Ar-C), 101.1 (PhCH), 99.1 (C-1A), 98.6 (C-1B), 76.4 (C-3B), 75.7 (C-2B), 75.4 (C-3A), 73.6 (PhCH2), 73.5 (PhCH2), 73.4 (2 C, C-4A, PhCH2), 72.2 (C-5B), 69.4 (C-4B), 68.8 (C-2A), 67.8 (C-6B), 67.3 (C-6A), 55.8 (OCH3), 52.7 (C-2A), 21.2 (COCH3). MS (ESI): m/z = 1000.3 [M +mNa]+. Anal. Calcd for C57H55NO14 (977.36): C, 70.00; H, 5.67. Found: C, 69.83; H, 5.90. p-Methoxyphenyl (2,3,4-Tri-O-benzoyl-6-O-tert-butyldiphenylsilyl-β-d-glucopyranosyl)-(1→4)-2,3,6-tri-O-benzyl-β-d-galactopyranoside (42): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 8.0–7.11 (m, 40 H, Ar-H), 6.94 (d, J = 9.0 Hz, 2 H, Ar-H), 6.71 (d, J = 9.0 Hz, 2 H, Ar-H), 5.80 (t, J = 9.6 Hz each, 1 H, H-3B), 5.63 (t, J = 9.7 Hz each, 1 H, H-4B), 5.52 (dd, J = 8.0, 8.0 Hz each, 1 H, H-2B), 5.23 (d, J = 8.0 Hz, 1 H, H-1B), 4.69 (d, J = 7.6 Hz, 1 H, H-1A), 4.67 (d, J = 12.0 Hz, 1 H, PhCH 2), 4.63 (d, J = 12.0 Hz, 1 H, PhCH 2), 4.46 (d, J = 10.8 Hz, 2 H, PhCH 2), 4.36 (d, J = 11.5 Hz, 1 H, PhCH 2), 4.05 (d, J = 2.3 Hz, 1 H, H-4A), 3.88–3.83 (m, 5 H, H-6abA,H-6abB, PhCH 2), 3.76–3.74 (m, 1 H, H-5B), 3.73 (s, 3 H, OCH 3), 3.64–3.61 (m, 1 H, H-5A), 3.55 (dd, J = 7.7, 7.7 Hz, 1 H, H-2A), 3.46 (dd, J = 10.0 Hz, 3.0 Hz, 1 H, H-3A), 1.03 (br s, 9 H, C(CH 3)3). 13C NMR (CDCl3, 125 MHz): δ = 166.2, 165.3, 165.2 (3 PhCO), 155.5–114.8 (Ar-C), 103.2 (C-1B), 101.6 (C-1A), 81.7 (C-3A), 79.8 (C-2A), 75.5 (PhCH2), 75.3 (PhCH2), 74.9 (PhCH2), 74.7 (C-3B), 71.1 (C-4B), 73.9 (C-2B), 73.8 (C-4A), 72.7 (C-5B), 71.0 (C-5A), 69.9 (C-6B), 63.3 (C-6A), 55.9 (OCH3), 27.1 (3 C, C(CH3)3), 19.5 (C(CH3)3). MS (ESI): m/z = 1291.5 [M + Na]+. Anal. Calcd for C77H76O15Si (1268.49): C, 72.85; H, 6.03. Found: C, 72.68; H, 6.25. p-Methoxyphenyl (2,3-Di-O-benzoyl-4,6-O-benzylidene-β-d-glucopyranosyl)-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (43): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 7.94–7.21 (m, 20 H, Ar-H), 6.84 (d, J = 9.0 Hz, 2 H, Ar-H), 6.71 (d, J = 9.0 Hz, 2 H, Ar-H), 5.68 (t, J = 9.5 Hz each, 1 H, H-3B), 5.52 (s, 1 H, PhCH), 5.49 (dd, J = 7.5, 7.5 Hz, 1 H, H-2B), 5.34 (s, 1 H, PhCH), 5.28 (d, J = 3.5 Hz, 1 H, H-1A), 5.03 (d, J = 7.5 Hz, 1 H, H-1B), 4.31 (t, J = 9.5 Hz each, 1 H, H-4B), 4.17–4.12 (m, 2 H, H-6abB), 3.98–3.95 (m, 1 H, H-5B), 3.87 (t, J = 9.5 Hz each, 1 H, H-3A), 3.71–3.69 (m, 1 H, H-6aA), 3.68 (s, 3 H, OCH 3), 3.66–3.62 (m, 2 H, H-4A, H-6bA), 3.59–3.55 (m, 1 H, H-5A), 3.24 (dd, J = 10.0, 10.0 Hz, 1 H, H-2A). 13C NMR (CDCl3, 125 MHz): δ = 165.5, 165.3 (2 PhCO), 155.6–114.7 (Ar-C), 102.2 (C-1B), 101.5 (PhCH), 101.4 (PhCH), 98.2 (C-1A), 80.1 (C-3B), 78.5 (C-2B), 76.7 (C-3A), 72.8 (C-4A), 72.2 (C-4B), 68.7 (C-5B), 68.6 (C-5A), 66.5 (C-6B), 63.5 (C-6A), 62.6 (C-2A), 55.6 (OCH3). MS (ESI): m/z = 880.2 [M + Na]+. Anal. Calcd for C47H43N3O13 (857.27): C, 65.80; H, 5.05. Found: C, 65.59; H, 5.25. p-Methoxyphenyl (4-O-Benzyl-2,3-O-isopropylidene-α-l-rhamnopyranosyl)-(1→3)-2-azido-4,6-O-benzylidene-2-deoxy-α-d-glucopyranoside (44): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 7.37–7.17 (m, 10 H, Ar-H), 6.94 (d, J = 9.0 Hz, 2 H, Ar-H), 6.77 (d, J = 9.0 Hz, 2 H, Ar-H), 5.42 (s, 1 H, PhCH), 5.40 (d, J = 3.5 Hz, 1 H, H-1A), 5.23 (br s, 1 H, H-1B), 4.76 (d, J = 11.5 Hz, 1 H, PhCH 2), 4.48 (d, J = 11.5 Hz, 1 H, PhCH 2), 4.32 (t, J = 9.5 Hz each, 1 H, H-3A), 4.18–4.14 (m, 3 H, H-2B, H-6abA), 4.03–3.98 (m, 1 H, H-5A), 3.85–3.80 (m, 1 H, H-5B), 3.70 (s, 3 H, OCH 3), 3.64 (t, J = 10.0 Hz each, 1 H, H-4B), 3.51 (t, J = 9.5 Hz each, 1 H, H-4A), 3.37 (dd, J = 10.0, 3.5 Hz, 1 H, H-2A), 3.04 (dd, J = 10.0, 6.0 Hz, 1 H, H-3B), 1.42, 1.33 (2 s, 6 H, 2 CH 3), 0.68 (d, J = 6.0 Hz, 3 H, CCH 3). 13C NMR (CDCl3, 125 MHz): δ = 155.6–114.8 (Ar-C), 109.2 (C(CH3)2), 102.3 (PhCH), 98.0 (C-1A), 97.6 (C-1B), 81.1 (C-3B), 80.2 (C-4A), 78.4 (C-2B), 75.9 (C-3A), 73.1 (C-4B), 72.9 (PhCH 2), 68.7 (C-6A), 65.1 (C-5A), 64.2 (C-5B), 63.6 (C-2A), 55.7 (OCH3), 28.0 (CH3), 26.4 (CH3), 17.1 (CH3). MS (ESI): m/z = 698.2 [M + Na]+. Anal. Calcd for C36H41N3O10 (675.27): C, 63.99; H, 6.12. Found: C, 63.82; H, 6.35. Ethyl (2,3,4-Tri-O-benzyl-α-l-rhamnopyranosyl)-(1→3)-2-O-acetyl-4,6-O-benzylidene-1-thio-α-d-mannopyranoside (45): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 7.46–7.16 (m, 20 H, Ar-H), 5.56 (s, 1 H, PhCH), 5.42 (d, J = 2.5 Hz, 1 H, H-2A), 5.24 (br s, 1 H, H-1A), 4.93 (br s, 1 H, H-1B), 4.88 (d, J = 9.5 Hz, 1 H, PhCH 2), 4.73 (d, J = 10.0 Hz, 1 H, PhCH 2), 4.67 (d, J = 9.5 Hz, 1 H, PhCH 2), 4.58 (d, J = 9.5 Hz, 1 H, PhCH 2), 4.52 (d, J = 10.5 Hz, 1 H, PhCH 2), 4.47 (d, J = 10.5 Hz, 1 H, PhCH 2), 4.29–4.22 (m, 2 H, H-5A, H-6aA), 4.19 (dd, J = 8.5, 3.0 Hz, 1 H, H-3A), 4.04–4.0 (m, 1 H, H-5B), 3.87 (t, J = 8.0 Hz each, 1 H, H-4A), 3.85–3.81 (m, 1 H, H-6bA), 3.78 (dd, J = 8.5, 3.0 Hz, 1 H, H-3B), 3.61 (br s, 1 H, H-2B), 3.55 (t, J = 8.0 Hz each, 1 H, H-4B), 2.66–2.56 (m, 2 H, SCH 2CH3), 2.07 (s, 3 H, COCH 3), 1.28 (t, J = 7.4 Hz each, 3 H, SCH2CH 3), 1.14 (d, J = 6.0 Hz, 1 H, CCH 3). 13C NMR (CDCl3, 125 MHz): δ = 169.7 (COCH3), 139.0–125.9 (Ar-C), 101.3 (PhCH), 94.8 (C-1B), 83.2 (C-1A), 80.4 (C-4B), 79.3 (C-3B), 76.8 (C-4A), 74.7 (PhCH2), 74.6 (C-2B), 72.3 (PhCH2), 71.6 (PhCH2), 70.3 (C-2A), 70.2 (C-3A), 68.4 (C-6A), 67.8 (C-5B), 64.4 (C-5A), 25.3 (SCH2CH3), 20.8 (COCH3), 17.6 (CCH3), 14.7 (SCH2 CH3). MS (ESI): m/z = 793.3 [M + Na]+. Anal. Calcd for C44H50O10S (770.31): C, 68.55; H, 6.54. Found: C, 68.40; H, 6.38. Ethyl (2,3,4,6-Tetra-O-benzyl-α-d-glucopyranosyl)-(1→3)-2-O-acetyl-4,6-O-benzylidene-1-thio-α-d-mannopyranoside (46): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 7.40–6.92 (m, 25 H, Ar-H), 5.48 (s, 1 H, PhCH), 5.31 (d, J = 3.5 Hz, 1 H, H-1B), 5.30 (br s, 1 H, H-2A), 5.24 (br s, 1 H, H-1A), 4.91–4.42 (7 d, J = 11.0 each, 7 H, PhCH 2), 4.32–4.26 (m, 3 H, H-2B, PhCH 2), 4.22–4.16 (m, 2 H, H-5A, H-6aB), 3.83 (t, J = 10.0 Hz each, 1 H, H-4A), 3.80 (t, J = 9.5 Hz each, 1 H, H-3B), 3.78–3.74 (m, 1 H, H-6bB), 3.70–3.62 (m, 2 H, H-6abA), 3.55 (t, J = 9.5 each, 1 H, H-4B), 3.44 (dd, J = 10.0, 3.5 Hz, 1 H, H-3A), 2.70–2.58 (m, 2 H, SCH 2CH3), 2.18 (s, 3 H, COCH 3), 1.29 (t, J = 7.4 Hz each, 3 H, SCH2CH 3). 13C NMR (CDCl3, 125 MHz): δ = 170.0 (COCH3), 138.9–126.4 (Ar-C), 102.4 (PhCH), 97.5 (C-1B), 83.5 (C-1A), 81.1 (C-3B), 79.3 (C-5A), 78.7 (C-3A), 77.4 (C-4B), 75.4 (PhCH2), 74.7 (PhCH2), 73.7 (PhCH2), 73.4 (C-2B), 71.6 (PhCH2), 71.6 (2 C, C-2A, C-4A), 68.6 (C-6A), 68.4 (C-6B), 64.5 (C-5B), 25.5 (SCH2CH3), 21.1 (COCH3), 14.9 (SCH2 CH3). MS (ESI): m/z = 899.3 [M + Na]+. Anal. Calcd for C51H56O11S (876.35): C, 69.84; H, 6.44. Found: C, 69.68; H, 6.68. 2-(p-Methoxyphenoxy)ethyl [2,3,4-Tri-O-acetyl-6-O-(p-methoxybenzyl)-α-d-mannopyranosyl]-(1→2)-3-O-benzyl-4,6-O-benzylidene-α-d-mannopyranoside (48): Colorless oil. 1H NMR (CDCl3, 500 MHz): δ = 7.50–6.81 (m, 18 H, Ar-H), 5.67 (s, 1 H, PhCH), 5.44–5.43 (m, 1 H, H-2B), 5.37 (dd, J = 9.9, 3.5 Hz, 1 H, H-3B), 5.24 (t, J = 10.0 Hz each, 1 H, H-4B), 5.07 (br s, 1 H, H-1B), 4.95 (br s, 1 H, H-1A), 4.82 (d, J = 12.1 Hz, 1 H, PhCH 2), 4.60 (d, J = 12.1 Hz, 1 H, PhCH 2), 4.49, 4.39 (2 d, J = 12.0 Hz each, 2 H, PhCH 2), 4.24–4.20 (m, 1 H, H-5A), 4.12 (t, J = 9.9 Hz each, 1 H, H-4A), 4.07 (br s, 1 H, H-2A), 4.06–4.02 (m, 1 H, OCH 2), 4.0–3.93 (m, 3 H, H-5B, H-6abA), 3.89–3.86 (m, 2 H, H-6abB), 3.85–3.77 (m, 1 H, OCH 2), 3.75, 3.74 (2 s, 6 H, 2 OCH 3), 3.56–3.52 (m, 2 H, OCH 2), 3.48 (dd, J = 10.0, 3.0 Hz, 1 H, H-3A), 2.09, 1.99, 1.94 (3 s, 9 H, 3 COCH 3). 13C NMR (CDCl3, 125 MHz): δ = 170.4, 170.2, 170.1 (3 COCH3), 151.6–114.1 (Ar-C), 101.9 (PhCH), 100.1 (2 C, C-1A, C-1B), 79.4 (C-4A), 77.4 (C-4A), 76.1 (C-4B), 73.6 (PhCH2), 73.5 (MPhCH2), 70.6 (C-3B), 69.8 (OCH2), 69.7 (OCH2), 69.1 (2 C, C-5A, C-2B), 68.0 (C-5B), 67.3 (C-2A), 66.5 (C-6B), 64.4 (C-6A), 56.1 (OCH3), 55.6 (OCH3), 21.3 (2 C), 21.1 (3 COCH3). MS (ESI): m/z = 939.3 [M + Na]+. Anal. Calcd for C49H56O17 (916.35): C, 64.18; H, 6.16. Found: C, 64.0; H, 6.38