Inter- and intra-molecular reactions of allene-1,3-dicarboxylic acid esters with 2-vinylfurans and 2-vinylthiophenes. A potential route to a BC ring precursor of the nagilactones
Intermolecular reaction of 4-(2-furyl)but-3-en-2-yl acetate (10a) with dimethyl allene-1,3-dicarboxylate leads to products derived from cycloaddition across the endocyclic furan diene or from substitution in the 5-position. 4-(5-Methyl-2-furyl)but-3-en-2-yl acetate (10b) gives only cycloaddition products whereas polymerisation occurs in the reactions of the allene ester with the corresponding 2-vinylthiophenes. In contrast, intramolecular reactions of both furans and thiophenes result in cycloaddition across the exocyclic diene system followed by rearrangement and dehydrogenation. The resultant tricyclic benzofurans and benzothiophenes, isolated in low yields, contain two of the rings of the nagilactone skeleton.