Dimroth 11,2.4 triazolo 1,5-« pyrimidine into

Brown, D. J., Nagamatsu, T. Isomerizations akin to the Dimroth rearrangement. III. The conversion of simple s-triazolo[4,3-a]pyrimidines into their [1,5-a] isomers. Aust. J. Chem. 1977, 30, 2515-2525. 

Bis-1,2.4-triazolo[4,3-fl 4, 3 -c]pyrimidine 43 and / /s-l,2.4-triazolo [4,3-n. r,5 -c]pyrimidine 45 are stable toward rearrangement in acid or alkali but system 43 did undergo a thermal Dimroth-like rearrangement into Mv-1.2,4-triazolo[l,5-<7 4, 3 -c ]pyrimidine 44 on fusion above 300°C (79AJC1585). 

It is important to note that besides these synthetic pathways a very important access to [ 1,2,4]triazolo[ 1,5 z] pyrimidine derivatives is the Dimroth rearrangement of [l,2,4]triazolo[4,3-c]pyrimidine compounds. This type of ring transformation is specifically discussed in Section 11.16.5.2 these possibilities are also reviewed in Section 11.16.7. As these isomerizations always take place into the direction of the [l,2,4]triazolo[l,5-c]pyrimidine ring, in several studies only these products are described without special (or any) note of the primarily formed [l,2,4]triazolo[4,3-c]pyrim-idine ring. Table 17 contains the stmctures of some [l,2,4]triazolopyrimidines and benzologues with a fusion site of the triazole ring that have been formed via transformation of the isomeric [ 1,2,4] triazolo[4,3-f]-pyrimidine compounds with or without isolation of these intermediates. 

The second important reaction path similar to AT synthesis starts with aminoguanidine derivatives (in this case part of a HP) and proceeds via condensation with a synthon Z (see principle in Scheme 8). In a first step l,2,4-triazolo-[4,3-a]pyrimidines (15) are formed these are often isolable and nearly always transformable into the more stable TPs by Dimroth rearrangement. 

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