Rearrangement 1,2-imino shift

Isomerization of 5-imino-l,2,4-dithiazolidine-3-thiones (34) into l,2,4-thiadiazolidine-3,5-di-thiones (51) by a Dimroth rearrangement (see Section 4.13.6.1.1) is a reversible process <91JPR107>. The equilibrium position strongly depends on the nature of substituents. When R = R = Me or Pr the equilibrium position is (34) (51) =1 2, with R = R = Et the ratio is 1 9. When R = Ar, R = Aik, the equilibrium is almost wholly shifted to the right. The forward process is usually accelerated by bases, the reverse by acids. When both substituents are Ar no isomerization occurs <91JPR107>. Isomerization of the 3,5-diimino analogue (52) to a monothione isomer (53), accelerated by pyridine, reaches an equilibrium ratio of (52) (53) =1 9 <89BSB879>. 

Dimethyl 2-(arylamino)thiazole-4,5-dicarboxylates 546 are produced from 3-aryl-5-benzoyl-2-imino-l,3,4-thiadiazolines 542 and dimethyl acetylenedicarboxylate. It is suggested that the initial adducts 543 rearrange to the betaines 544 by a proton shift and that the betaines cyclize to the hypervalent sulphur compounds 545. Subsequent loss of benzoyl cyanide yields the products . 

The calculated values of Table II confirm our previous statement [6] that H2 of adenine and the Imino protons of uracil (or thymine) and guanine are those for which the dimerization shift Is, to a rather large extend, not Influenced by the "rearrangement effect" but unfortunately Table II shows also that the shifts of these protons are not the most sensitive to the exact conformation of the polynucleotide chain. 

The rearrangement is analogous to the di-n-methane rearrangement and is considered as 1,2-shift of imino group from C(3) to C(4), followed by a o bond formation between C(3) and C(5). For example, the photorearrangement of P,y-unsaturated imine 55 gives cyclopropyl imine 56, which on hydrolysis gives cyclopropane aldehyde 57 [62]. 

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