Regiochemical switching in Diels-Alder cycloadditions by change in oxidation state of removable diene sulfur substituents. Synthesis of carbazoles by sequential heteroannulation and Diels-Alder cycloaddition

The palladium-catalyzed heteroannulation of N-carbobenzyloxy-o-iodoanilines with 1-phenylthio-1,3-butadiene afforded indolines 7, which were oxidized with DDQ to produce vinylogous 2-(phenylthio)indoles 8. The latter compounds underwent highly regioselective Diels-Alder cycloadditions with methyl propiolate in the presence of MeAlCl2 or AlCl3, with simultaneous elimination of benzenethiol, to afford methyl N-(carbobenzyloxy)carbazole-3-carboxylates 9 and, in some cases, the N-deprotected derivatives 11. This is the opposite regiochemistry of that observed previously with the corresponding sulfone analogues of 8. Thus, the regiochemistry of the cycloaddition can be effectively controlled by appropriate choice of oxidation state of the diene sulfur substituent.