SITE SELECTIVITY OF THE DIELS-ALDER REACTIONS OF 3-[1-(TERT-BUTYLDIMETHYLSILYLOXY)VIN-1-YL]FURAN AND 3-(PROPEN-2-YL)FURAN - SYNTHESIS OF 4-SUBSTITUTED BENZOFURANS

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