Purine derivatives, Dimroth rearrangement

The reaction of adenosine, and some other derivatives, with DMAD has been shown to yield a mixture of different pyrimido[2, l-/]purines depending on the reaction conditions. The kinetically favoured products (80) usually crystallise from the reaction medium, but if they are soluble then the thermodynamically favoured products (81) result via a Dimroth rearrangement 95H(41)1197],... 

However, using isothiocyanates, the corresponding acyclic thioureas cannot be isolated and immediate ring closure to the purine derivative takes place. The reaction is further complicated by a possible Dimroth rearrangement (see Houben-Weyl, Vol. E4, p 1200), e.g. the formation of 10 and rearrangement to 11. 

The first step is H2O addition at C-2 with fission of the pyrimidine ring forming the imidazole derivative 11. Its tautomer 13 recyclizes to the purine system 12 in a Dimroth rearrangement [148] (see also p 202). 

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