Synthesis of Sulfur-Containing Tricyclic Ring Systems by Means of Photoinduced Decarboxylative Cyclizations

Abstract

The intramolecular and intermolecular photoinduced electron transfer reactions of a series of mercaptoacetic acid and mercaptopropionic acid derivatives were investigated. In the intermolecular series, the phthalimidoalkylsulfanylalkylcarboxylates 1a-j and 2 were transformed into the tricyclic ring systems 3a-j and 4, respectively, with high regioselectivities. The mercaptoacetic acid and 2-mercaptopropionic acid derived substrates 1a-g and 2 readily cyclized in good to excellent yields (60-98%) but with low diastereoselectivities (except for 1d), whereas the corresponding 3-mercaptopropionic acid derived substrates 1h-j gave the corresponding tricyclic products 3h-j after prolonged irradiation, but with poor yields (11-20%). The intermolecular version - i.e., photodecarboxylative addition to N-methylphthalimide (5) as electron acceptor - was successful with mercaptoacetic acid, and 2-mercaptopropionic acid substrates 6a-c and the addition products 7a-c were obtained in high yields (57-90%). No ad-dition, however, was observed with 3-(methylsulfanyl)propionic acid (6d). The regioselectivity of decarboxylation proceeded in a controlled manner for the mercaptosuccinic acid derivatives in both the intramolecular (with 8a-c) and the intermolecular (with 9) versions. Comparison between sulfuractivated and nonactivated species (13, 15) or irradiation of 1a under nonactivating conditions showed that the carboxylate anion in the position alpha to the electron-donating sulfur atom acts as a superior leaving group. This efficiency is drastically reduced for carboxylate anions in the alpha position. With the former substrates, the photochemical cyclization proceeds with high product yields. Quantum yield measurements for decomposition (as a measure for cyclization) supported these observations. CV measurements indicated preorientation prior to electron transfer in the intramolecular pathway.