Nucleophilic attack, 1,2-dithiolium ring

The initial attack at C(3) on a 1,2-dithiolium ring by a carbon nucleophile, arising from reaction of an exocyclic amidic anion on DMAD explains the formation of the fused isothiazoloimidazole (71) (Scheme 5) <82ZOR2626>. 

Dithioles with hydroxy and thiol substituents are protonated forms of l,2-dithiole-3-ones (2a) and -3-thiones (2b) respectively, which are regenerated on deprotonation. The reactions with nucleophiles of 3-alkoxy and 3-alkylthio-1,2-dithiolium salts give dealkylation products only in minor amounts. Instead, nucleophilic attack occurs on the ring (Section 3.11.4.2(iv)). A l,2-dithiolium-4-thiolate (98) readily acylates on an amine function to form a l,2-dithiolo-l,2-dithiole (99). This unusual reactivity is explained by the relative preferences for cyclic and acyclic forms of mesoionic forms <91EGP293120>. 

The susceptibility of 1,3-dithiolium cations to nucleophilic attack at C-2 has been exploited in the development of a useful route to 1,4,2-dithiazines where amino compounds are used as nucleophiles. The adducts (176), formed by the action of the dithiocarbamate derivative (174) on dithiolium rings (175), readily experience ring expansion (a sulfur C to N bond migration with concomitant displacement of the dithiocarbamate) at ambient temperature to afford the 1,4,2-dithiazines (177) (Scheme 19) <88BCJ2232>. An almost identical process, using NH3/I2 as the nucleophilic species, has been used to make the 1,4,2-dithiazines (28a-d) the same mechanism is thought to be involved <92CC478>. 

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