Ketene dithioacetals, reactions with amine

An interesting pyrrolo[l,2-tf]pyrimidine was described as the product of the reaction of the heterocyclic ketene-aminals 236 that was synthesized by cyclocondensation of ketene dithioacetals 237 and 1,3-diamino propane. These compounds reacted with diethyl oxomalonate that behaves as an hetero-enophile, yielding the corresponding products 238 in acceptable to good yield (Scheme 31). A mechanism that involves an aza-ene reaction, via adduct 239 which isomerizes to ketene aminal 240 to produce the lactam ring of 238, has been proposed <1999J(P1)321>. 

Cyclic ketene acetals can also react with amines to give 1,1-enediamines with elimination of ethylene glycol76. Treatment of 2-[(methoxycarbonyl)cyanomethylene]-1,3-dioxolane (38) with 1,3-diaminopropane or with 4,5-dimethyl-1,2-phenylenediamine gives the hexahydropyrimidine and the benzimidazoline derivatives 39 and 40, respectively (equation 11). Similarly to the reaction of ketene dithioacetals with amines, the reaction between ketene acetals and amines proceeds via monoamino-substituted intermediates and ketene 7V,Oacetals can be isolated when one molar amine is used72. 

EWG-Activated 2-methylene dithioles (= cyclic ketene dithioacetals with electron-withdrawing substituents) 38 undergo a ring-opening reaction mediated by primary aliphatic amines to give polyfunctionalized 2-(alkylamino) thiophenes (40) after hydrolysis in a one-pot procedure [82] ... 

Aminothiophenes can also be obtained from carbon disulfide-derived intermediates. For example, one of the methylthio groups of ketene dithioacetal 57, obtained from pentane-2,4-dione as shown, can be displaced with a secondary amine and the thiophene ring constructed by reaction with, for example, ethyl thioglycolate for final ring closure (Scheme 79) [127]. 

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