The Diels−Alder reaction of 3-vinylfurans 5 and 27 with DMAD, N-phenylmaleimide, and dimethyl maleate afforded products derived both from addition to the furan ring diene system (intraannular addition) and to the furan 2,3-double bond 3-vinyl group diene system (extraannular addition). For example, compounds 6 and 7 were obtained from 5 and DMAD. In contrast, dienophiles containing a phenylsulfinyl group, such as 19−21, gave products derived exclusively from the extraannular reaction mode. These products are useful precursors of 4-substituted benzofurans, especially 4-hydroxybenzofurans.

No abstract