[2+4] vs [4+2] Cycloaddition Reactions of ortho-Thioquinones with 1,3-Dienes

Mono-ortho-thioquinones, of general formula 1, can be generate under very mild conditions by reacting the corresponding ortho-hydroxythiophthalimides 2 with bases.1 Species 1, generate in situ, can be trapping through cycloaddition reactions with 1,3-dienes. This chemistry allowed us to study in detail the reactivity of thioquinones 1 and to demonstrate their double behaviour as hetero dienophiles, across the carbon-sulfur double bond, to give the spiro derivates 4 and as hetero dienes leading to the formation of the benzoxathiin derivatives 5.2 The reagents structure and the reaction conditions chosen make the difference in this reactions driving the interaction between o-thioquinones and 1,3-dienes to the formation of the [2+4] dihydrothiopyran spiro and/or the [4+2] benzoxathiin cycloadducts. A computational study was conduct in order to achieve a deep understanding of both [2+4] and [4+2] paths.