Site Selectivity of the Diels−Alder Reactions of 3-[1-(<i>tert</i>-Butyldimethylsilyloxy)vin-1-yl]furan and 3-(Propen-2-yl)furan. Synthesis of 4-Substituted Benzofurans

Abstract

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.