Lewis Acid Catalysis in the Oxidative Cycloaddition of Thiophenes(1)

Yuanqiang Li; Thies Thiemann; Tsuyoshi Sawada; Shuntaro Mataka; Masashi Tashiro

doi:10.1021/jo961985z

Lewis Acid Catalysis in the Oxidative Cycloaddition of Thiophenes(1)

J Org Chem. 1997 Nov 14;62(23):7926-7936. doi: 10.1021/jo961985z.

Authors

Yuanqiang Li¹, Thies Thiemann, Tsuyoshi Sawada, Shuntaro Mataka, Masashi Tashiro

Affiliation

¹ Graduate School of Engineering Sciences, Kyushu University, 6-1, Kasuga-koh-en, Kasuga-shi, Fukuoka 816, Japan, and Institute of Advanced Material Study, Kyushu University, Fukuoka 816, Japan, and Lab. de Quimica Farmaceutica, Faculdade de Farmacia, Universidade de Coimbra, 3000 Coimbra, Portugal.

PMID: 11671894
DOI: 10.1021/jo961985z

Abstract

Thiophenes 1 were treated with m-chloroperbenzoic acid (m-CPBA) under BF(3).Et(2)O catalysis to afford thiophene S-monoxides. These could be reacted in situ as intermediary species with a number of dienophiles to provide arenes (with alkynes as dienophiles) or 7-thiabicyclo[2.2.1]hept-2-ene 7-oxides (with alkenes as dienophiles). It was also possible to isolate thiophene S-monoxides in solution and to cycloadd them in a second step. In either way it could be shown that the use of BF(3).Et(2)O enhances the yields of the oxidative cycloaddition of thiophenes considerably. Moreover a greater variety of dienophiles (29a, 29b, 29c) could be reacted with thiophenes than in the case of the noncatalyzed reaction. All cycloadditions catalyzed by BF(3).Et(2)O give only a single diastereoisomer as cycloadduct. The reactions show a high pi-facial selectivity, a fact that can be explained by the Cieplak-effect. Without added dienophiles, 2-methylthiophene (1e) gave a single dimer (36) of 2-methylthiophene S-monoxide, whereas 2,5-dimethylthiophene (1a) gave three dimers (32a-c). In the case of tetrasubstituted thiophenes, thiophene S-monoxides (e.g., 31b and 31c) could be isolated in substance.