An Efficient Synthesis of Novel Dibenzo-Fused Nine-Membered Oxacycles Using a Sequential Baylis-Hillman Reaction and Radical Cyclization

Abstract

A short and high yield synthetic route to novel dibenz[b,g]oxonins, one of which has been characterized by X-ray crystallography, has been developed based on a sequential Baylis-Hillman reaction and radical cyclization. The regioselective radical cyclization followed a 9-endo-trig pathway.

dibenz[b,g]oxonin - radical cyclization - Baylis-Hillman reaction - medium-ring heterocycle - regioselective

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13

Crystal data of 4b: C₂₀H₁₉NO₄, MW = 337.4, monoclinic, space group = P2₁/n, a = 8.186 (5), b = 7.703 (5), c = 26.301 (5) Å, β = 98.59 (1)°, V = 1639.8 (1) ų, z = 4, F(000) = 712, T = 100 (2) K, µ (MoKα) = 0.096 mm^-1. A total 3353 unique reflections were measured (θ_max = 26.48°) on a CCD area detector using graphite monochromatized MoKα radiation (λ = 0.71073 Å). The structure was solved by direct methods with SHELXS-97 (Sheldrick, 1997) and refined by full-matrix least-squares method on F² using SHELXL-97 (Sheldrick, 1997). The non-hydrogen atoms were refined anisotropically and the hydrogen atoms located from the difference Fourier maps were allowed to ride on their parent atoms. Final R1 = 0.0878, wR2 = 0.0816 and GOF = 0.817 for all 3353 observed data. R1 = 0.0439 for 1918 data [F _o > 4σ(F _o)]. CCDC 649417.

As predicted by MO calculations and experimentally corroborated by Beckwith and others

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