Cobalt-Mediated [2+2+2] Cycloaddition of Alkynyl Boronates to Indole and Pyrrole Double Bonds

Abstract

The complex [CpCo(C₂H₄)₂] facilitates the [2+2+2] cycloaddition of borylalkynes to N-(4-pentynoyl)indole and -pyrrole to furnish heterocyclofused borylcyclohexadienes regioselectively.

Keywords

alkynes - cycloadditions - heterocycles - polycycles - ­transition metals

References and Notes

• See, inter alia:
• 1a The Art of Drug Synthesis   Johnson DS. Li JJ. Wiley-Interscience; Hoboken: 2007. 
• 1b Palladium in Heterocyclic Chemistry: A Guide for the Synthetic Chemist   2nd ed.:  Li JJ. Gribble GW. Elsevier; Amsterdam: 2007. 
• 1c Microwave-Assisted Synthesis of Heterocycles   Van der Eycken E. Kappe CO. Springer; New York: 2006. 
• 2 See, for example: Sheldon RA. Arends I. Hanefeld U. Green Chemistry and Catalysis   Wiley-VCH; Weinheim: 2007. 
• 3 See: Young DD. Sripada L. Deiters A. J. Comb. Chem.  2007,  9:  735 ; and references therein
• For recent reviews, see:
• 4a Agenet N. Buisine O. Slowinski F. Gandon V. Aubert C. Malacria M. In Organic Reactions   Vol. 68:  RajanBabu TV. Wiley; Hoboken: 2007.  p.1 
• 4b Gandon V. Aubert C. Malacria M. Chem. Commun.  2006,  2209 
• 4c Chopade PR. Louie J. Adv. Synth. Catal.  2006,  348:  2307 
• 4d Miljanić O. Vollhardt KPC. In Carbon-Rich Compounds: From Molecules to Materials   Haley MM. Tykwinski RR. Wiley-VCH; Weinheim: 2006.  p.140 
• 4e Kotha S. Brahmachar E. Lahiri K. Eur. J. Org. Chem.  2005,  4741 
• 4f Yamamoto Y. Curr. Org. Chem.  2005,  9:  1699 
• 4g For an early review of CpCo-mediated [2+2+2] cycloadditions, see: Vollhardt KPC. Angew. Chem., Int. Ed. Engl.  1984,  23:  539 
• Other complexes based on Ru, Rh, and Ni, reported to effect the cocycloaddition of two alkyne units to double bonds, failed for these substrates. For recent leading references to such complexes, see:
• 5a Varela JA. Rubín SG. Castedo L. Saá C. J. Org. Chem.  2008,  73:  1320 
• 5b Shibata T. Tsuchikama K. Org. Biomol. Chem.  2008,  6:  1317 
• 5c Rayabarapu DK. Cheng C.-H. Acc. Chem. Res.  2007,  40:  971 
• 5d Tanaka K. Nishida G. Sagae H. Hirano M. Synlett  2007,  1426 
• Furans and thiophenes:
• 6a Boese R. Harvey DF. Malaska MJ. Vollhardt KPC. J. Am. Chem. Soc.  1994,  116:  11153 
• 6b Peréz D. Siesel BA. Malaska MJ. David E. Vollhardt KPC. Synlett  2000,  306 
• Indoles:
• 6c Eichberg MJ. Vollhardt KPC. In Strategies and Tactics in Organic Synthesis   Vol. 4:  Harmata M. Elsevier Science; Oxford: 2004.  Chap 14. p.365 
• 6d Eichberg MJ. Dorta RL. Grotjahn DB. Lamottke K. Schmidt M. Vollhardt KPC. J. Am. Chem. Soc.  2001,  123:  9324 
• 6e Eichberg MJ. Dorta RL. Lamottke K. Vollhardt KPC. Org. Lett.  2000,  2:  2479 
• 6f Boese R. Van Sickle AP. Vollhardt KPC. Synthesis  1994,  1374 
• 6g Grotjahn DB. Vollhardt KPC. J. Am. Chem. Soc.  1986,  108:  2091 
• 6h Pyrroles: Sheppard GS. Vollhardt KPC. J. Org. Chem.  1986,  51:  5496 
• 6i Imidazoles: Boese R. Knölker H.-J. Vollhardt KPC. Angew. Chem., Int. Ed. Engl.  1987,  26:  1035 
• Pyrimidines:
• 6j Pellissier H. Rodriguez J. Vollhardt KPC. Chem. Eur. J.  1999,  5:  3549 
• 6k Boese R. Rodriguez J. Vollhardt KPC. Angew. Chem., Int. Ed. Engl.  1991,  30:  993 
• Pyridones:
• 6l Aubert C. Betschmann P. Eichberg MJ. Gandon V. Geny A. Heckrodt TJ. Lehmann J. Malacria M. Masjost B. Paredes E. Vollhardt KPC. Whitener GD. Chem. Eur. J.  2007,  13:  7443 
• 6m Aubert C. Gandon V. Heckrodt TJ. Malacria M. Paredes E. Vollhardt KPC. Chem. Eur. J.  2007,  13:  7466 
• 7a Geny A. Lebuf D. Rouquié G. Vollhardt KPC. Malacria M. Gandon V. Aubert C. Chem. Eur. J.  2007,  13:  5408 
• 7b Gandon V. Leboeu f D. Amslinger S. Vollhardt KPC. Malacria M. Aubert C. Angew. Chem. Int. Ed.  2005,  44:  7114 
• For a recent compilation of references to cycloadditions with alkynyl boronates, see:
• 8a Delaney PM. Browne DL. Adams H. Plant A. Harrity JPA. Tetrahedron  2008,  64:  866 
• For reviews on dienyl- and vinylboranes, see:
• 8b Albrecht K. Kaufmann DE. In Science of Synthesis   Vol. 6:  Kaufmann DE. Matteson DS. Thieme; Stuttgart: 2005.  p.697 
• 8c Vaultier M. Alcaraz G. In Science of Synthesis   Vol. 6:  Kaufmann DE. Matteson DS. Thieme; Stuttgart: 2005.  p.721 
• 8d For a monograph, see: Boronic Acids: Preparation and Applications in Organic Synthesis and Medicine   Hall DG. Wiley-VCH; Weinheim: 2005. 
• 8e See also: Gandon V. Aubert C. Malacria M. Curr. Org. Chem.  2005,  9:  1699 
• 9 See also: Gandon V. Aubert C. Malacria M. Vollhardt KPC. Chem. Commun.  2008,  1599 
• See, for example:
• 10a Murakami M. Usui I. Hasegawa M. Matsuda T. J. Am. Chem. Soc.  2005,  127:  1366 
• 10b Pereira S. Srebnik M. Organometallics  1995,  14:  3127 
• 11 Gandon V. Agenet N. Vollhardt KPC. Malacria M. Aubert C. J. Am. Chem. Soc.  2006,  128:  8509 
• 13a Grotjahn DB. Vollhardt KPC. J. Am. Chem. Soc.  1990,  112:  5653 
• 13b Grotjahn DB. Vollhardt KPC. Synthesis  1993,  579 
• 13c

  Aechtner, T.; Barry, D. A.; Grotjahn, D. B.; Vollhardt, K. P. C. unpublished results.

• 14 The acylation of indole was modified from that described in ref. 6b, according to: Teranishi K. Nakatsuka S. Goto T. Synthesis  1994,  1018 

12

Representative Cyclization Conditions; Synthesis of 5: To a degassed solution of N-(4-pentynoyl) indole (1;^6g,¹4 197 mg, 1 mmol) and 4,4,5,5-tetramethyl-2-(3-methylbut-1-ynyl)[1,3,2]dioxaborolane (2; 242 mg, 1.25 mmol) in anhyd THF (15 mL) at 0 ˚C under an atmosphere of argon was added η⁵-cyclopentadienylbis(ethene)cobalt (150 mg, 0.833 mmol) in anhyd, degassed THF (6 mL) over a period of 3 h via syringe pump. The ensuing dark-red solution was treated with bright yellow Fe(NO₃)₃˙9H₂O (337 mg, 0.834 mmol) in THF-H₂O (5 mL, 3:1). After stirring for 5 min at 0 ˚C, the brown mixture was poured into iced water (50 mL) and the solution was extracted with CH₂Cl₂ (3 × 100 mL), followed by sat. aq NaHCO₃ (3 × 150 mL). The extracts were dried (MgSO₄), filtered through a short silica gel column to give a red-orange foam, and then subjected to flash chromatography on silica gel (hexanes-acetone, 9:1) to yield 5 (189 mg, 58%) as a white solid; mp 91-92 ˚C.
All new compounds gave satisfactory elemental analytical and/or HRMS data. For selected spectra, see: 3: ^¹H NMR (500 MHz, CDCl₃): δ = 1.21 (d, J = 7.0 Hz, 3 H, (CH ₃)₂CH], 1.25 (s, 6 H, Me), 1.32 (s, 6 H, Me), 1.48 [d, J = 6.5 Hz, 3 H, (CH ₃)₂CH], 1.80-1.85 (m, 1 H, CH₂), 1.98-2.03 (m, 1 H, CH₂), 2.52-2.66 [m, 3 H, CH₂, (CH₃)₂CH], 2.55 (d, J = 8.0 Hz, 1 H, CH), 3.24 (d, J = 7.5 Hz, 1 H, CH), 4.52 (s, 5 H, Cp), 5.01 (s, 1 H, CH), 6.98 (t, J = 7.5 Hz, 1 H, CH_arom), 7.23 (t, J = 7.5 Hz, 1 H, CH_arom), 7.77 (d, J = 7.5 Hz, 1 H, CH_arom), 8.20 (d, J = 7.5 Hz, 1 H, CH_arom). ^¹³C NMR (126 MHz, CDCl₃): δ = 20.5, 24.5, 26.0, 26.1, 30.4, 31.5, 32.5, 45.2, 63.4, 64.9, 72.1, 81.1, 83.4, 106.4, 116.1, 122.8, 126.4, 127.6, 135.2, 142.7, 169.8; C-B not detected. MS (FAB, NBA): m/z (%) = 515 (100) [M⁺].
4: ^¹H NMR (500 MHz, CDCl₃): δ = 0.62 [d, J = 6.5 Hz, 3 H, (CH ₃)₂CH], 1.29 [d, J = 7.0 Hz, 3 H, (CH ₃)₂CH], 1.33 (s, 6 H, Me), 1.35 (s, 6 H, Me), 1.75-1.81 (m, 1 H, CH₂), 2.28 (m, 1 H, CH₂), 2.43-2.48 (m, 1 H, CH₂), 2.69-2.76 (m, 1 H, CH₂), 3.08 [sep, J = 6.5 Hz, 1 H, (CH₃)₂CH], 3.83 (d, J = 11.0 Hz, 1 H, CH), 4.41 (d, J = 11.5 Hz, 1 H, CH), 4.67 (s, 1 H, CH), 4.68 (s, 5 H, Cp), 6.83 (t, J = 7.0 Hz, 1 H, CH_arom), 6.99-7.04 (m, 2 H, CH_arom), 7.68 (d, J = 7.5 Hz, 1 H, CH_arom). ^¹³C NMR (126 MHz, CDCl₃): δ = 19.5, 24.8, 25.5, 25.8, 29.5, 31.3, 33.3, 44.4, 62.1, 67.1, 76.3, 81.1, 82.8, 109.7, 115.5, 123.2, 124.4, 127.0, 133.6, 140.8, 169.8; C-B not detected. MS (FAB, NBA): m/z (%) = 515 (100) [M⁺].
5: ^¹H NMR (400 MHz, CDCl₃): δ = 0.93 [d, J = 6.8 Hz, 3 H, (CH ₃)₂CH], 0.96 [d, J = 6.8 Hz, 3 H, (CH ₃)₂CH], 1.32 (s, 6 H, Me), 1.35 (s, 6 H, Me), 2.47-2.57 (m, 2 H, CH₂), 2.65-2.81 (m, 2 H, CH₂), 3.46 [sep, J = 6.9 Hz, 1 H, (CH₃)₂CH], 4.57 (d, J = 12.8 Hz, 1 H, CH), 5.01 (dd, J = 1.2, 13.6 Hz, 1 H, CH), 5.88 (br s, 1 H, CH), 6.98 (t, J = 7.2 Hz, 1 H, CH_arom), 7.16 (t, J = 7.8 Hz, 1 H, CH_arom), 7.31 (d, J = 7.6 Hz, 1 H, CH_arom), 8.12 (d, J = 7.6 Hz, 1 H, CH_arom). ^¹³C NMR (101 MHz, CDCl₃): δ = 21.0, 21.5, 24.8, 25.1, 26.0, 30.9, 32.8, 41.2, 58.6, 83.3, 115.9, 118.4, 123.6, 124.4, 127.5, 133.8, 136.0, 140.1, 152.1, 170.2; C-B not detected. MS (70 eV, EI): m/z (%) = 391 (100) [M⁺], 286 (35), 260 (30). IR (neat): 2977, 1673, 1594, 1480, 1461, 1390, 1317, 1289, 1269, 1143, 1017, 857, 752, 732, 672 cm^-¹.
7: ^¹H NMR (500 MHz, CDCl₃): δ = 0.85 (t, J = 6.8 Hz, 3 H, Me), 1.25 (m, 8 H, CH₂), 1.32 (s, 6 H, Me), 1.35 (s, 6 H, Me), 2.19-2.23 (m, 1 H, CH₂), 2.47-2.58 (m, 3 H, CH₂), 2.66-2.79 (m, 2 H, CH₂), 4.57 (d, J = 14.0 Hz, 1 H, CH), 5.01 (d, J = 13.5 Hz, 1 H, CH), 5.70 (s, 1 H, CH), 6.97 (t, J = 7.0 Hz, 1 H, CH_arom), 7.15 (t, J = 7.8 Hz, 1 H, CH_arom), 7.33 (d, J = 7.5 Hz, 1 H, CH_arom), 8.12 (d, J = 8.0 Hz, 1 H, CH_arom). ^¹³C NMR (126 MHz, CDCl₃): δ = 14.1, 22.6, 25.0, 25.7 (2 × C), 29.1, 29.8, 31.7, 32.7, 35.4, 41.2, 58.6, 83.3, 115.9, 123.6, 123.7, 124.5, 127.5, 133.5, 136.0, 140.1, 148.2, 170.3; C-B not detected. MS (70 eV, EI): m/z (%) = 433 (20) [M⁺], 432 (40), 431 (100), 361 (45), 285 (95). IR (neat): 2976, 2926, 2855, 1673, 1594, 1479, 1459, 1396, 1379, 1328, 1317, 1268, 1143, 1020, 857, 752 cm^-¹.
9: Diastereomer 1: ^¹H NMR (500 MHz, CDCl₃): δ = 1.32 (s, 6 H, Me), 1.36 (s, 6 H, Me), 1.50 (m, 2 H, CH₂), 1.57 (m, 2 H, CH₂), 1.68 (m, 1 H, CH₂), 1.80 (m, 1 H, CH₂), 2.52-2.58 (m, 2 H, CH₂), 2.67-2.80 (m, 2 H, CH₂), 3.44-3.50 (m, 1 H, CH₂), 3.83-3.89 (m, 1 H, CH₂), 4.20 (d, J = 11.5 Hz, 1 H, CH₂), 4.58-4.67 (m, 3 H, CH, CHO, CH₂), 5.04 (br d, J = 13.5 Hz, 1 H, CH), 6.00 (s, 1 H, CH), 6.97 (t, J = 7.5 Hz, 1 H, CH_arom), 7.17 (t, J = 7.8 Hz, 1 H, CH_arom), 7.34 (d, J = 7.5 Hz, 1 H, CH_arom), 8.14 (d, J = 8.0 Hz, 1 H, CH_arom). ^¹³C NMR (126 MHz, CDCl₃): δ = 19.5, 25.0 (2 × C), 25.4, 25.9, 30.7, 32.7, 41.3, 58.5, 62.2, 67.5, 83.8, 97.7, 110.1 (br, C-B), 116.1, 120.9, 123.6, 24.4, 127.7, 133.7, 135.1, 140.1, 143.0, 170.1. MS (70 eV, EI): m/z (%) = 463 (16) [M⁺], 462 (6), 461 (10), 363 (32), 361 (52). IR (neat): 3053, 2977, 2941, 2870, 1671, 1595, 1480, 1460, 1391, 1381, 1321, 1269, 1202, 1142, 1076, 1022, 869, 754, 736 cm^-¹. Diastereomer 2: ^¹H NMR (500 MHz, CDCl₃): δ = 1.32 (s, 6 H, Me), 1.36 (s, 6 H, Me), 1.48-1.59 (m, 4 H, CH₂), 1.66-1.83 (m, 2 H, CH₂), 2.52-2.58 (m, 2 H, CH₂), 2.67-2.80 (m, 2 H, CH₂), 3.44-3.50 (m, 1 H, CH₂), 3.83-3.89 (m, 1 H, CH₂), 4.17 (d, J = 12.0 Hz, 1 H, CH₂), 4.58-4.67 (m, 3 H, CH, CHO, CH₂), 5.03 (d, J = 13.5 Hz, 1 H, CH), 6.00 (s, 1 H, CH), 6.97 (t, J = 7.5 Hz, 1 H, CH_arom), 7.17 (t, J = 7.8 Hz, 1 H, CH_arom), 7.36 (d, J = 8.0 Hz, 1 H, CH_arom), 8.14 (d, J = 8.0 Hz, 1 H, CH_arom). ^¹³C NMR (126 MHz, CDCl₃): δ = 19.4, 24.9, 25.0, 25.4, 25.9, 30.5, 32.7, 41.3, 58.5, 62.1, 68.0, 83.8, 98.6, 110.2 (br, C-B), 116.1, 120.9, 123.6, 124.5, 127.7, 133.7, 135.2, 140.1, 143.3, 170.1.
11/12 mixture, major regioisomer 11: ^¹H NMR (400 MHz, CDCl₃): δ = 0.18 (s, 9 H, SiMe₃), 1.33 (s, 6 H, Me), 1.37 (s, 6 H, Me), 2.45-3.03 (m, 4 H, CH₂), 4.60 (d, J = 14.0 Hz, 1 H, CH), 5.01 (br d, J = 14.0 Hz, 1 H, CH), 6.01 (br s, 1 H, CH), 6.98 (t, J = 7.4 Hz, 1 H, CH_arom), 7.17 (t, J = 7.8 Hz, 1 H, CH_arom), 7.38 (d, J = 7.6 Hz, 1 H, CH_arom), 8.14 (d, J = 7.6 Hz, 1 H, CH_arom). ^¹³C NMR (101 MHz, CDCl₃): δ = 0.75, 24.6, 25.6, 26.1, 32.8, 42.1, 58.4, 83.9, 116.0, 122.7, 123.6, 124.6, 127.5, 130.8, 135.7, 140.2, 147.6, 170.5; C-B not detected. Minor regioisomer 12: ^¹H NMR (400 MHz, CDCl₃): δ = 0.36 (s, 9 H, SiMe₃), 1.25 (s, 6 H, Me), 1.26 (s, 6 H, Me), 2.45-3.03 (m, 4 H, CH₂), 4.52 (d, J = 12.8 Hz, 1 H, CH), 5.01 (dd, J = 1.6, 14.0 Hz, 1 H, CH), 6.11 (s, 1 H, CH), 6.98 (dt, J = 1.2, 7.4 Hz, 1 H, CH_arom), 7.17 (t, J = 7.8 Hz, 1 H, CH_arom), 7.38 (d, J = 7.6 Hz, 1 H, CH_arom), 8.14 (d, J = 7.6 Hz, 1 H, CH_arom). ^¹³C NMR (101 MHz, CDCl₃): δ = 1.38, 24.8, 25.0, 25.1, 32.7, 43.3, 59.5, 83.8, 115.7, 122.5, 123.5, 124.3, 127.8, 129.1, 134.9, 140.2, 147.6, 170.5; C-B not detected. MS (70 eV, EI): m/z (%) = 421 (52) [M⁺], 420 (66), 404 (28), 348 (22), 347 (20), 322 (30), 278 (65), 83 (100). IR (neat): 2979, 1667, 1594, 1537, 1481, 1460, 1398, 1317, 1247, 1216, 1142, 1091, 1058, 1020, 983, 960, 844, 755, 701, 667 cm^-¹.
14/15 mixture, major regioisomer 14: ^¹H NMR (500 MHz, CDCl₃): δ = 1.07 (s, 6 H, Me), 1.13 (s, 6 H, Me), 2.60 (m, 2 H, CH₂), 2.79 (m, 2 H, CH₂), 4.70 (d, J = 13.5 Hz, 1 H, CH), 5.11 (d, J = 13.5 Hz, 1 H, CH), 6.07 (br s, 1 H, CH), 7.05 (t, J = 7.5 Hz, 1 H, CH_arom), 7.20-7.40 (m, 6 H, CH_arom), 7.52 (d, J = 7.5 Hz, 1 H, CH_arom), 8.21 (d, J = 8.0 Hz, 1 H, CH_arom). Minor regioisomer 15: ^¹H NMR (500 MHz, CDCl₃): δ = 1.16 (s, 6 H, Me), 1.20 (s, 6 H, Me), 2.60 (m, 2 H, CH₂), 2.79 (m, 2 H, CH₂), 4.81 (d, J = 14.0 Hz, 1 H, CH), 5.17 (d, J = 14.0 Hz, 1 H, CH), 5.95 (br s, 1 H, CH), 6.67 (t, J = 7.5 Hz, 1 H, CH_arom), 7.12 (t, J = 7.5 Hz, 1 H, CH_arom), 7.20-7.40 (m, 6 H, CH_arom), 8.17 (d, J = 8.0 Hz, 1 H, CH_arom). ^¹³C NMR (126 MHz, CDCl₃; 14/15 mixture): δ = 24.2, 24.6 (2 × C), 24.7, 26.0, 26.1, 30.9, 32.8, 33.1, 36.6, 41.9, 44.0, 58.1, 58.9, 83.5, 83.6, 116.0, 116.2, 120.8, 123.0, 123.1, 123.8, 124.7, 125.0, 127.1, 127.2, 127.7, 127.8, 127.9 (2 × C), 129.0, 132.2, 132.4, 134.9, 140.2, 140.3, 141.4, 142.8, 144.7, 144.9, 169.4, 169.9; C-B (2 × C) not detected. MS (70 eV, EI):
m/z (%) = 425 (18) [M⁺], 424 (33), 423 (86), 307 (52), 297 (60), 169 (41), 84 (100). IR (neat): 3015, 2979, 2930, 1705, 1655, 1594, 1482, 1408, 1367, 1317, 1141, 1066, 1023, 853, 755, 702, 667 cm^-¹.
17: ^¹H NMR (500 MHz, CDCl₃): δ = 0.88 [d, J = 7.0 Hz, 3 H, (CH ₃)₂CH], 0.91 [d, J = 7.0 Hz, 3 H, (CH ₃)₂CH], 1.15 (s, 6 H, Me), 1.20 (s, 6 H, Me), 2.47 (m, 2 H, CH₂), 2.57 (dd,
J = 6.4, 14.3 Hz, 1 H, CH₂), 2.66 (dd, J = 6.4, 13.0 Hz, 1 H, CH₂), 3.34 [sep, J = 7.0 Hz, 1 H, (CH₃)₂CH], 4.18 (br d, J = 14.6 Hz, 1 H, CH), 4.70 (d, J = 14.4 Hz, 1 H, CH), 5.36 (m, 1 H, CH), 5.83 (br s, 1 H, CH), 6.80 (m, 1 H, CH). ^¹³C NMR (126 MHz, CDCl₃): δ = 21.2, 24.4, 24.7, 24.9, 26.6, 30.9, 31.6, 45.4, 55.5, 82.9, 117.4, 117.9, 125.0, 132.5, 150.8, 167.1; C-B not detected. MS (70 eV, EI): m/z (%) = 341 (12) [M⁺], 340 (11), 339 (30), 326 (11), 298 (10), 224 (6), 198 (8), 185 (9), 129 (16), 83 (100). IR (neat): 2976, 2872, 1716, 1643, 1518, 1468, 1412, 1371, 1220, 1144, 1110, 1078, 1016, 983, 951, 924, 885, 858, 831, 787, 744, 674 cm^-¹.
19: ^¹H NMR (500 MHz, CDCl₃): δ = 1.04 [d, J = 6.9 Hz, 6 H, (CH ₃)₂CH], 1.21 (s, 12 H, Me), 1.95 (quint, J = 6.0 Hz, 2 H, CH₂), 2.50 (sep, J = 7.0 Hz, 1 H, CH), 2.54 (td, J = 2.0, 6.0 Hz, 2 H, CH₂), 3.95 (t, J = 6.0 Hz, 2 H, CH₂), 5.38 (s, 1 H, CH), 6.16 (m, 1 H, CH), 6.42 (m, 1 H, CH), 6.59 (m, 1 H, H-2), 6.61 (br s, 1 H, CH). ^¹³C NMR (126 MHz, CDCl₃): δ = 21.7, 23.9, 24.9, 25.6, 37.2, 45.5, 82.7, 103.1, 108.3, 119.9, 120.7, 129.6, 131.8, 165.0; C-B not detected. MS (70 eV, EI): m/z (%) = 327 (10) [M⁺], 312 (10), 284 (26), 184 (40), 59 (100). IR (neat): 3100, 2972, 2871, 1695, 1620, 1458, 1380, 1331, 1259, 1107, 1077, 970, 851, 756, 722, 667 cm^-¹.