Thioacylketenes

Six-membered sulfur heterocycles such as 352 and 353 have been synthesized by [4 + 2] cycloaddition of enaminothiones (351) to electrophilic olefins and acetylenes209-211, like acrylonitrile and dimethyl acetylenedicarboxylate (equation 76)209. Thioacylketene thioacetals212 or l,2-dithiole-3-thiones213 have been used in the preparation of thiopyranthiones. 

The reaction of thiosalicyclic acid 614 with two equivalents of carbodiimides may involve a thioacylketene intermediate 615, which undergoes a [2+4] cycloaddition reaction with the second equivalent of the carbodiimide to give the cycloadduct 616.5 ... 

An interesting and reactive class of 4ir thiabutadienes capable of useful participation in normal (HOMOdiene controlled) Diels-Alder reactions are the stable thioacylketene thioacetals,22 generated by the photoaddition of olefins to l,2-dithiole-3-thiones23 [Eq. (10)]. 

Olefinic, acetylenic, and typical electron-deficient dienophiles participate in apparent concerted [4 + 2] cycloadditions with the thioacylketene thioacetals (Scheme 8-V), while enamines provide modest yields of [4 + 2] cycloadducts via a polar, stepwise addition-cyclization.22c... 

Analogous thioacylketene thioacetals have been reported to give a [4 + 2] adduct with sulfene (equation 103) under similar reaction conditions209. 

Olefinic, acetylenic, and typical electron-deficient dienophiles participate in apparent concerted [4 2] cycloaddidons with the thioacylketene... 

An analogous thioacylketene 516 was created from 2-mercaptobenzoic acid 515 and captured by cycloaddition to a carbodiimide to afford the adduct 517 (Scheme 170) (1962JOC3365). 

Remarkably, two multifunctional thioacylketenes react smoothly and cleanly with diisopropylcarbodiimide (Scheme 171). In both reactions the only isolated products are the result of [4 + 2] addition to the thioacylketene moiety. Possible modes of reaction involving cyanoketene and carboethox-yketene groups were not observed (1976TL2961). 

Thioacylketenes are only stable at very low temperatures and even at -196 °C only thietons, their rearrangement products, can be detected by infrared spectroscopy (band at 1790 cm ). a-Imidoylketenes are obtained by flash vacuum pyrolysis of Meldrum s acid derivatives, but only an oxoketenimine (band at 2076 cm ) rearrangement product is detected at -77 K . Also, the unstable methyleneketenes (R2C=C=C=0) are obtained from suitable precursors. In the absence of trapping agents their cyclodimers are isolated (see Section 4.1.2). 

Thioacylketen dithioacetals participate with alkenes and sulphenes in 1,4-cycloaddition reactions. Thio-Claisen rearrangement of 1-allylthio-l-aminoalkenes to thioamides or isothiocyanates, and of 5 -allyl-5 -methyl-keten mercaptals to dithioesters, has been described. 

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