Thiocarbonyl compounds hetero-Diels—Alder reactions

The last years have seen a great development of the hetero Diels-Alder reactions of thiocarbonyl compounds, due to their rather high reactivity, as reported by many groups, including those of Kirby, Bonini, Capozzi, DeglTnnocenti, Vallde, Okazaki, Koizumi, Saito and Huisgen. A simple orbital frontier treatment allows comparison of the reactivity of C=S vs C=0 [119]. 

Table 2. Hetero Diels-Alder reactions of thiocarbonyl compounds as dienophiles...

In search for control of absolute stereochemistry, the reaction of thio-chalcones was investigated with unsaturated amides bearing an Evans chiral oxazolidinone [223] and dimenthyl fumarate [224, 225]. For the first time with thiocarbonyl compounds, the efficiency of Lewis acid addition was demonstrated, and reactions could be conducted at room temperature. With EtAlCl2 (Table 4, entry 2) or A1C13 (entry 3), levels of induction up to 92% were attained for the endo isomer. Yb(OTf)3 in DMSO also caused the acceleration of the reaction with chiral acrylamides with p-facial selectivity [226]. This group has also reported [227] an intramolecular hetero Diels-Alder reaction with divinyl thioketones and the double bond of an allyloxy group (Table 4, entry 4). 

Due to the high reactivity and high sensitivity of thioaldehydes, Lewis acid catalysis has not been applied to thia Diels-Alder reactions. However, Heim-garten et al. clearly demonstrated the suitability of Lewis acids to make less reactive thiocarbonyl compounds, e. g. thiazole thiones, react as heterodieno-philes in hetero Diels-Alder reactions [412]. 

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