Synthesis and X-ray structure of methyl 2-oxopyrimido[2,1-b]benzothiazole-4-carboxylate from condensation of 2-aminobenzothiazole and dimethyl but-2-ynedioate

Abstract

The product from 2-aminobenzothiazole and dimethyl but-2-ynedioate has been shown by X-ray analysis to be the title compound (IIIa). Crystals of (IIIa) are monoclinic, space group P2₁/c, with a= 4.801(5), b= 13.68(1), c= 20.37(2)Å, β= 90.44(5)°, and Z= 4. The structure was solved by direct methods and refined by full-matrix least-squares to R 0.099 for 1 931 observed photographic intensities.

The molecule consists of an exocyclic carbonyl group and a methyl ester group linked to a slightly warped C₆–C₃NS–C₄N₂ ring skeleton. The carboxy-group makes a dihedral angle of 34° with the mean plane of the heterocyclic ring system. The sulphur atom participates in the molecular π-bonding, and bond-length variations in the tricyclic ring system are well accounted for in terms of simple Hückel MO theory.

A reaction mechanism is suggested which involves an initial Michael addition on an acetylenic carbon by the ring nitrogen, forming an enamine with the fumarate structure. Subsequent ring-closure involving the β-methoxycarbonyl- and imino-groups leads to the formation of the product.