Regioselective synthesis of 7-acetyl-11c-methyl-4b,5,7,11c-tetrahydro[1]benzofuro[2′,3′:4,5]thiopyrano[2,3-b]indoles by sequential Claisen rearrangement of 2-(4′-aryloxybut-2′-ynylthio)-1-acetylindoles

Abstract

Hitherto unreported indole-annulated pentacyclic heterocycles containing oxygen, nitrogen and sulfur have been synthesised by thermal Claisen rearrangement followed by Lewis acid catalysed Claisen rearrangement. 9-Acetyl-4-aryloxymethyl-2,9-dihydrothiopyrano[2,3- b]indoles are regioselectively synthesised in 80–85 % yield by thermal rearrangement of 2-(4′-aryloxybut-2′-ynylthio)-1-acetylindoles. A second, Lewis acid catalysed, rearrangement gave 7-acetyl-11c-methyl-4b,5,7,11c-tetrahydro[1]benzofuro[2′,3′: 4,5]thiopyrano[2,3- b]indoles in yields of 85–90 %. The thermal Claisen rearrangement in refluxing N,N-diethylaniline for 3 h gave the same products in lower yield (50–54 %).