Annelated furans. VII. Synthetic routes to Di-O-methylstrepsilin

Abstract

Two unambiguous syntheses of di-O-methylstrepsilin are reported. Both schemes involved a Diels-Alder addition of dimethyl acetylenedicarboxylate to a 2-(β-methoxyvinyl)benzofuran in the key step. The synthetic di-O-methylstrepsilin so obtained was found to be identical with a sample obtained by methylation of the natural material, thereby confirming the structure of this compound and hence of strepsilin itself. The partial oxidation of several 1,2-bishydroxymethyldibenzofursns was studied. Vilsmeier-Haack formylation of 3-acetoxy-6-methoxy-4-methylbenzofuran (29) was found to proceed in an abnormal manner, giving rise to 3-chloro-6-methoxy-4-methylbenzofuran-2-carbaldehyde (31) as well as the unprecedented 3-(3'-chloro-6'- methoxy-4'-methylbenzofuran-2'-yl)-N,N-dimethylacrylamide (32). The mechanism for the formation of the latter compound has been discussed and interpreted as a Perkin-like condensation.