Facile entry into the thiazolo[3,2-a]indol-3(2H)-one system via an unusual reaction with thionyl chloride

This study included investigations into the canthin-6-one series. ..gamma..-oxo-3((methoxycarbonyl)methyl)-1H-indole-1-butanonic acid (1a) was treated with oxalyl chloride to give the acid chloride (lb) R/sub 1/ = CO/sub 2/CH/sub 3/; R/sub 2/ = Cl. Similar reaction of (1a) with refluxing thionyl chloride followed by methanol treatment did not give the expected methyl ester (1c) but instead gave orange fibrous needles of (2a) in 20% yield. The corresponding reaction of ..gamma..-oxo-3-methyl-1H-indole-1-butanoic acid (1d) with thionyl chloride followed by quenchings with methanol or neopentyl alcohol led respectively to (2b) R/sub 1/ = H; R/sub 3/ = CH/sub 3/ and 2c (R/sub 1/ = H; R/sub 3/ = CH/sub 2/C(CH/sub 3/)/sub 3/. The structural proof of (2) rests on its /sup 1/H and /sup 13/CNMR data along with comparison to (3) whose synthesis and structural elucidation by x-ray and /sup 13/C NMR spectroscopy have been reported. Proton-carbon coupling constants were determined by using the gated-decoupling technique. Analytical calculations were found for C/sub 15/H/sub 15/NO/sub 5/; C/sub 13/H/sub 13/NO/sub 3/; C/sub 16/H/sub 13/NO/sub 5/S; C/sub 14/H/sub 11/NO/sub 3/S; and C/sub 18/H/sub 19/NO/sub 3/S. 1 table. (DP)