Thiocarbonyl and Selenocarbonyl Dienophiles

Cycloaddition of a thiocarbonyl compound and a 1,3-diene affords a 5,6-dihydro-2/f-thiapyran [Eq. (1)].  

Virtually all types of thiocarbonyl compounds have been found to react as heterodienophiles. In general, thiocarbonyl compounds are more reactive, versatile dienophiles than are the corresponding carbonyl compounds. One major difference between the two types of dienophile is that a dihydrothiapyran formed from a thiocarbonyl group often has a ten- 

Mechanistic studies in this area are essentially nonexistent. Recently, the regiochemistry of some [4 + 2] cycloadditions of thioaldehydes has been rationalized based on FMO theory. It is difficult to generalize the resulting stereochemistry of this type of Diels-Alder reaction since systematic studies are lacking and since many structurally diverse kinds of thiocarbonyl dienophiles have been used as 27t components. 

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