Cine Substitution in 2-Oxabicyclo[4.2.0]octanones and Subsequent Unusual Rearrangements(1)

Reaction of 2-chlorooxabicyclo[4.2.0]octanone 5 with several nucleophiles was examined and found to differ significantly from those of carbon analog 1. MeO(-) and PhS(-) led either to products of cine substitution 9 or of ring opening to cyclobutenones 8. With most enolates cine substitution occured via C-alkylation of the intermediate oxidoallyl cation in spite of formation of a new C-C bond between two quaternary carbons; with nitroalkanes O-alkylation was preferred. With azide as a nucleophile, further transformations occurred, among them an oxy-promoted electrocyclic cyclobutane opening, with incorporation of a phenyl triazole unit and final formation of the unusual product 19a. Evidence for a mechanism explaining formation of 19a was obtained by isolation of intermediates. Thermolysis or photolysis of 8e or9b led via electrocyclic ring opening to a vinyl ketene which was trapped by MeOH, alkenes, dienes, or oxygen to produce polyfunctional unsaturated esters 29 and 30 or 8-membered ring lactone 31, fused cyclobutanones 33 and 34, pyranone 38, or gamma-lactone 39, respectively.