Stereochemistry, tautomerism, and reactions of acridinyl thiosemicarbazides in the synthesis of 1,3‐thiazolidines

Abstract

Abstract magnified image Acridin‐9‐yl hydrazine upon treatment with various isothiocyanates (RNCS, R = methyl, allyl, phenyl, p ‐methoxy phenyl, and p ‐nitro phenyl) yielded the corresponding thiosemicarbazides with acridine substituted on the carbazide‐type side. The alkyl‐substituted compounds were present in solution as equilibria consisting of the major H‐10, H‐12 tautomer (either E or Z or both about the C 13 ‐N 14 bond) and the minor H‐10, SH tautomer (either E or Z or both). The major species for the aromatic‐substituted compounds was the H‐10, H‐12 E tautomer, with the evident minor species being the H‐10, H‐12 Z tautomer. The thiosemicarbazides were each quantitatively converted into the analogous semicarbazides upon treatment with mesitylnitrile oxide wherein all structures were present in solution as the H‐10 tautomers with Z conformation about the C 13 ‐N 14 bond. Methylation of the compounds with methyl iodide yielded S‐methylated compounds wherein the Z configuration dominated in each case over the E configuration along the N 12 ‐C 13 double bond. Treatment of the thiosemicarbazides with methyl bromoacetate resulted in the formation of 1′,3′‐thiazolidin‐4′‐ones wherein the Z configuration predominated in each case over the E configuration along the N 12 C 13 double bond. With bromoacetonitrile as the bifunctional electrophile, the initial 1′,3′‐thiazolidin‐4′‐imines that formed spontaneously underwent Dimroth‐type rearrangement to the regiosiomeric 1′,3′‐thiazolidin‐4′‐imines.