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

Sabine Amslinger; Corinne Aubert; Vincent Gandon; Max Malacria; Elisa Paredes; K. Peter C. Vollhardt

doi:10.1055/s-2008-1077950

LETTER

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

Sabine Amslinger^a, Corinne Aubert^b, Vincent Gandon^b, Max Malacria^b, Elisa Paredes^a, K. Peter C. Vollhardt*^a
^a Department of Chemistry, University of California at Berkeley, Berkeley, CA 94720-1460, USA
Fax: +1(510)6435208; e-Mail: kpcv@berkeley.edu;
^b Laboratoire de Chimie Organique (UMR CNRS 7611), Institut de Chimie Moléculaire (FR 2769), Université Pierre et Marie Curie-Paris 6, case 229, 4 place Jussieu, 75252 Paris cedex 05, France

Abstract

All articles of this category

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

Full Text

References

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

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.

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