Alkoxycarbonylcarbene Transfer to Semicyclic Enaminones – A Route to Cyclopenta[b]pyrrole and Indole Ring Systems

Abstract

Copper-catalyzed decomposition of alkyl diazoacetates in the presence of semicyclic enaminones 1a–o leads primarily to enamino esters 2 which constitute formal products of C–C insertion of the carbene unit. In the case of N-methyl (1a–e) and N-benzyl (1f–i) enaminones, 2a–i are accompanied by 2,3,5,6-tetrahydroindoles 3a–i, in which two carbene moieties are incorporated. At 250 °C, compounds 2b–e, which could not be isolated in pure form, undergo cyclocondensation to form 1,2,3,5-tetrahydrocyclopenta-[b]pyrroles 4b–e. In contrast, 2f–i can be isolated as Z,E mixtures and are transformed thermally into 4f–i only in the presence of silica gel. Carbene transfer to N-phenyl enaminones 1j–m and N-methyl enamino esters 1n,o leads only to the 1:1 adducts, enamino esters 2j–o, which do not undergo the cyclocondensation reaction under the previous conditions. Dehydrogenation of tetrahydroindoles 3c–e with tetrachloro-p-benzoquinone can be controlled to give either 1,2-dihydroindole-6,7-dicarboxylates 5 or indole-6,7-dicarboxylates 6.