Asymmetric Synthesis of 3,4-Annulated Cyclopentenones from Cycloalkenyl Ketones and Vinyl Carbamates by Diastereoselective Carbonyl Addition/Conrotatory 4π Ring Closure

Abstract

1-Lithiated O-vinyl carbamates add with high induced diastereoselectivity onto enantiomerically pure cycloalk-1-enyl (1-dibenzylamino)alkyl ketones. Subsequent O-carbamoyl migration, followed by a torquoselective, antarafacial 4π ring closure, leads to homochiral 2-substituted bicyclo[n.3.0]alk-1-ene-2-ones.

Key words

carbanion - cyclopentenones - asymmetric synthesis - electrocyclic reaction - torquoselectivity

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11

A partial base-induced epimerization of 6ca may be the origin.

12

X-ray crystal structure analysis for 6af: formula C₂₉H₃₈O₂Si, M = 446.68, colorless crystal 0.30 × 0.25 × 0.10 mm, a = 10.154 (1), b = 16.328 (1), c = 15.949 (1) Å, β = 94.90 (1)˚, V = 2634.6 (3) Å^³, ρ_calc = 1.126 g cm^-³, µ = 0.943 mm^-¹, empirical absorption correction (0.765 ≤ T ≤ 0.912), Z = 4, monoclinic, space group P2₁ (No. 4), λ = 1.54178 Å, T = 223 K, ω and φ scans, 21759 reflections collected (±h, ±k, ±l), [(sin θ)/λ] = 0.60 Å^-¹, 7576 independent (R _int = 0.034) and 7417 observed reflections [I ≥ 2 σ(I)], 587 refined parameters, R = 0.035, wR ^² = 0.094, Flack parameter 0.04 (2), max. residual electron density 0.16 (-0.13) eÅ^-³, two almost identical molecules in the asymmetric unit, hydrogen atoms calculated and refined as riding atoms. CCDC 670192 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: [phone], E-mail: [e-mail].