Diastereoselective synthesis of ω-phosphonic acid analogues of 4-arylkainoids

Abstract

Radical cyclisation by the use of α,β-unsaturated phosphonate as a radical acceptor was applied to a synthesis of 7-phosphonomethylpyrrolo[1,2-c]oxazolidinones, synthetic intermediates on the route to phosphonic acid analogues of kainoids. The relative configuration at the 6;7;7a-positions was found to be highly controlled by steric effects due to the substituent at the 6-position. Thus, diethyl [{(6S*,7R*,7aS*)-6-(2-methoxyphenyl)-3-oxoperhydropyrrolo[1,2-c][1,3]oxazol-7-yl}methyl]phosphonate 29a and diethyl [{(6S*,7R*,7aS*)-3-oxo-6-phenylperhydropyrrolo[1,2-c][1,3]oxazol-7-yl}methyl]phosphonate 29b were prepared with high diastereoselectivity. When the substituent at the 6-position is a 1-naphthyl group, the diethyl [{(6S*,7R*,7aS*)-6-(1-naphthyl)pyrroloxazol-7-yl}methyl]phosphonate 29c and its (6S*,7R*,7aR*)-isomer 30c were formed in the ratio 29c∶30c ≈ 2∶1. The stereostructure of compound 29a was determined by X-ray crystallographic analysis. The 6-o-methoxyphenyl derivative 29a was converted into the corresponding phosphonic acid analogue 33.