tert-Butylcyanoketene (TBCK) combines with 2-(dimethylamino)thiazole (la) and the 5-methyl derivative 1b in benzene to give in each case 2:1 cycloadducts (from 2 mol of TBCK and 1 mol of 1) which proved by spectroscopic and X-ray analyses to be pairs of diastereomeric -lactones condensed with the thiazole ring across its former C4-C5 bond, namely, thiazolo[4,5-d]-2-tetrahydropyranones 3 and 4. From the reaction of 1a, an open-chain substitution product at C5 was also obtained in low yield. At room temperature the main products were the annulated lactones 3; in refluxing benzene the ratios reversed in favor of 4. Thermolysis of 3 in benzene and methanol showed that they rearrange to the diastereomers 4 through open-chain intermediates, one of which was trapped as an adduct with methanol. A reaction scheme with competing pathways involving dipolar intermediates accounts for both the formation of the adducts and their conversion. The distribution of the diastereomers 3 and 4 at room temperature is accounted for in terms of conformational effects controlling the stereochemistry of the cyclization of their open-chain precursors, whereas their relative stabilities appear to be related to the strains of the lactone ring. The failure to obtain a [2 + 2] cycloadduct is briefly examined on the basis of frontier molecular orbital theory. © 1980, American Chemical Society. All rights reserved.
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