Studies in the synthesis of polycyclopentanoids: synthesis, oxa-di-π-methane rearrangement of annulated bicyclo[2.2.2]octenones and cyclopropane ring cleavage of tetracyclo[6.3.0.0.0]undecenones

Abstract

A short, new and general approach for the synthesis of linear polyquinanes having the cis:anti:cis tricyclopentanoidal framework is delineated. The key element of this approach is the photochemical 1,2-acyl shift or oxa-di-π-methane rearrangement of annulated bicyclo[2.2.2]octenones, having β,γ-unsaturated carbonyl chromophoric systems. An efficient method for the synthesis of a variety of annulated bicyclo[2.2.2]octenones via inverse demand π^4s+π^2s cycloaddition of cyclohexa-2,4-dienones (9-12) with olefins (18-26) is reported. The structure of the adducts has been established through the study of their high-field ¹H NMR and ¹³C NMR spectra and decoupling experiments. Synthesis of tricyclo[5.2.2.0^2,6]undecadienones (51-57) from readily available adduct 43 has been achieved. Oxa-di-π-methane rearrangement of various chromophoric systems to polyquinanes (60-68) is described. The adduct 32 did not undergo oxa-di-π-methane rearrangement upon sensitized irradiation, while adduct 40 rearranged inefficiently to compound 64. Studies on the cleavage of the cyclopropane ring of the tetracyclic ketones (60, 66, 67 and 73) with formic acid and acetyl methanesulfonate is also reported.