Studies of heterocyclic compounds. Part XIII. Pyrrolo[2,1-b]thiazole thioaldehydes: synthesis and spectral studies

Abstract

Pyrrolo[2,1 -b]thiazole-5- and 7-thiocarbaldehydes, a class of stable heterocyclic thioaldehydes, have been synthesised by a modification of the Vilsmeier reaction. 6-Methyl- and 2,3,6-trimethylpyrrolo[2,1-b]thiazole-5-[²H]-thiocarbaldehyde have been obtained by using [²H₇]dimethylformamide in the Vilsmeier reaction. ¹H N.m.r. spectral data for the thioaldehydes in CDCl₃ are recorded. Restricted rotation about the ring–CHS bond was detected in 6-methyl-, 2,6-dimethyl-, and 6,7-dimethyl-pyrrolo[2,1-b]thiazole-5-thiocarbaldehyde, by using the change in shape of the 3-H and CHS signals with temperature as a probe. The coalescence temperatures for solutions in (CD₃)₂SO lie in the range 78–86°. ¹H N.m.r. spectral data indicate that pyrrolo[2,1 -b]thiazole-5-thiocarbaldehydes exist either in a syn-configuration (6-methyl-, 2,6-dimethyl-, 6,7-dimethyl-, and 3-methyl-6-t-butyl-) or in an anti-configuration (3,6-dimethyl- and 2,3,6-trimethyl-), and that the 7-thiocarbaldehydes exist in a syn-configuration. Normal ranges for the thioformyl proton chemical shifts are: 5-syn CHS, δ 10·2–10·4; 5-anti CHS, 11·0–11·1; 7-syn CHS, 10·5–10·8 p.p.m. The preparation of some new pyrrolo[2,1-b]thiazoles is described.