A Metal-Free Cycloaddition Approach to Highly Substituted Aromatic Boronic Esters

Jane E. Moore; Mark York; Joseph P. A. Harrity

doi:10.1055/s-2005-863727

LETTER

A Metal-Free Cycloaddition Approach to Highly Substituted Aromatic Boronic Esters

Jane E. Moore^a, Mark York^b, Joseph P. A. Harrity*^a
^a Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, S3 7HF, UK
e-Mail: j.harrity@sheffield.ac.uk;
^b Department of Medicinal Chemistry, Organon Laboratories Ltd., Newhouse, Lanarkshire, ML1 5SH, UK

Abstract

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Abstract

Alkynylboronates participate in [4+2] benzannulation reactions to furnish highly functionalised aromatic boronic esters, further manipulation of these boronic esters has been achieved via oxidation and Suzuki coupling reactions.

Keywords

cycloaddition - alkynylboronates - regioselective - aromatic boronic esters

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References

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For previous [4+2] cycloaddition reactions of alkynylboronates see:

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Regiochemical assignments in compounds 8, 9, 10, 13 were carried out by NOE spectroscopy.

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5c Miyaura N. Suzuki A. Chem. Rev. 1995, 95: 2457

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Representative Experimental Procedure for Alkynylboronate Cycloaddition Reactions: Synthesis of 4′-Butyl-5′-{4,4,5,5-tetramethyl-[1,3,2,]dioxaborolan-2-yl}-[1,1′,2′,1′′]terphenyl-3′,6′-dicarboxylic Acid Dimethyl Ester (7b). 2-Oxo-4,5-diphenyl-cyclopenta-3,5-diene-1,3-dicarboxylic acid dimethyl ester (1, 460 mg, 1.32 mmol) was weighed into a round-bottomed flask together with 2-hex-1-ynyl-4,4,5,5-tetramethyl-[1,3,2]dioxaborolane (6b, 200 mg, 0.096 mmol). The flask was fitted with a reflux condenser and mesitylene (1 mL) was added. The resulting orange solution was stirred for 16 h at 160 °C under nitrogen. The dark brown reaction mixture was cooled to r.t., and purified by flash column chromatography (eluting solvent CH₂Cl₂-MeOH, 50:1 ratio) to give the title compound as a pale yellow solid (299 mg, 59% yield). Mp 123-125 °C. ¹H NMR (250 MHz, CDCl₃): δ = 0.85 (3 H, t, J = 7.0 Hz), 1.30 (12 H, s), 1.11-1.42 (2 H, m), 1.48-1.66 (2 H, m), 2.70 (2 H, t, J = 8.5 Hz), 3.30 (3 H, s), 3.33 (3 H, s), 6.52-6.79 (4 H, m), 6.93-7.05 (6 H, m). ¹³C NMR (62.9 MHz, CDCl₃): δ = 13.9, 23.3, 25.1, 33.8, 35.1, 51.6, 51.7, 84.3, 126.5, 126.8, 127.1, 127.2, 129.9, 130.2, 136.4, 136.7, 138.3, 138.6, 139.4, 140.4, 143.4, 169.6, 170.7. FT-IR (CHCl₃): 2954 (w), 1731 (s), 1562 (w), 1436 (m), 1361 (s), 1322 (s) cm^-1. HRMS: m/z calcd for C₃₂H₃₇BO₆: 528.2683; found: 528.2684. Anal. Calcd for C₃₂H₃₇BO₆: C, 72.73; H, 7.06. Found: C, 72.88; H, 7.01.