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The cycloadditions of 2-methoxy-3-phenylthiobuta-1,3-diene, the 4'-chlorophenylthio and 2'-pyrimidylthio derivatives, and 2-acetoxy-3-phenylthiobuta-1,3-diene and its 4'-methoxyphenylthio derivative with unsymmetrical dienophiles are examined. The effect of silica gel, boron trifluoride etherate, and magnesium bromide on the regiochemistry is probed. In situ generation of the diene from the precursor cyclobutene and cycloaddition is a promising technique for sluggish dienophiles such as cyclohex-2-en-1-one. The use of a cisoid diene, 2,3-dimethylene-l,4-oxathiin, has also been examined. By variation of the substituents and use of thermal vs. Lewis acid catalysis, either sulfur or oxygen dominance of regiochemistry is possible. The former, combined with desulfurization, offers an approach to 1,3-substituted cyclohexanes via cycloaddition chemistry-a type of substitution not available in the normal Diels-Alder reaction. Thus, sulfur serves as a regiochemical control element. Rationalization of these results in terms of current concepts is presented. The frontier-orbital approach modified by consideration of charge-transfer (polar) interactions best accounts for the results. The site of complexation of the Lewis acids in these reactions also appears open to question.

The internal competition between substituents in the 2,3 positions of a diene-one being a sulfur and the other an oxygen substituent-is discussed. To examine this question, a general approach to 2-alkoxy(acyloxy)-3-alkyl(aryl)thiobuta-I ,3-dienes has been developed. Bromocyclobutanone undergoes displacement with sulfur nucleophiles without competing rearrangements of the benzylic acid type. These compounds have been 0-alkylated and 0-acylated to give 1 -alkoxy(acyloxy)-2alkyl(ary1)thiocyclobutenes. Thermal opening of the cyclobutenes, preferably by the technique of flash vacuum pyrolysis, gave the desired dienes normally in overall yields of 35-6770.

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