Unsaturated Thiocarbonyl Compounds 1 -Thiabutadienes

Thiocarbonyl compounds have been employed in Diels-Aldcr reactions to prepare thiopyranyl systems, and generally the carbon-sulfur double bond serves as the 2ir dienophile component of the [4 + 2] cycloaddition (Chapter 5). However, there are several reported examples of a,jS-unsaturated thioaldehydes, thioketones, and dithioesters participating as the diene partners of Diels-Alder reactions in which the thiocarbonyl group comprises a component of the 4ir diene system. 

The in situ generation of methyl vinyl thioketone in refluxing pyridine provided each of the four possible mixed Diels-Alder products derived from 477 and 27t methyl vinyl thioketone participation in [4 + 2] cycloadditions with methyl vinyl ketone [Eq. (I)].  

Interestingly, no a, 8-unsaturated thiocarbonyl dimer and no products derived from the 27t thiocarbonyl participation in Diels-Alder reactions were observed although the potential, reversible generation of such products could not be ruled out. In contrast, the low temperature dimerization of a,)8-unsaturated thioketones including methyl vinyl thioketone, generated by flash vacuum pyrolysis (FVP), provided 1 derived from the clean, regiospecific Diels-Alder dimerization with 47t and 27t thiocarbonyl participation [Eq. (2)].  

The in situ generation of the a,/3-unsaturated thioaldehyde, thiometha-crolein, has been reported to provide a 2 1 ratio of the Diels-Alder dimerization products 2 and 3 derived from the 4n and carbon-carbon double bond 2ir participation of thiomethacrolein in a [4 + 2] cycloaddition which proceeds with little regioselectivity [Eq. (3)].  

The C-2/C-4 addition of conjugating substituents (e.g., thiochalcone and related compounds) - or electron-donating substituents (e.g., j3-enaminothioketones, -thioaldehydes, -dithioesters, and a,/8-un-saturated dithioesters or thioamides ) to the a,/3-unsaturated thiocar- 

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Enaminothioketones and /3-enaminothioaldehydes (a,/3-unsaturated thiocarbonyl compounds bearing a C-4 electron-donating substituent have been the most thoroughly investigated thiabutadiene system capable of... 

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