Pyrimidines and their Fused Ring Systems

Reactions of 2-formyl-galactal, presented as an unsaturated sugar derivative with push-pull functionalization, with guanidinium and amidinium salts, respectively were carried out imder basic conditions to furnish the substituted 5-( 1,2,4-tri-O-benzyl-D-lyxo- l,2,3,4-tetrahydroxy-butyl)pyrimidines 92 (Fig. 16). Treatment of the 2-formyl pentose glycals with 2-aminobenzimid-azole and 3-amino-1,2,4-triazole, respectively afforded 3-(l,2,4-tri-0-benz-yl-D-lyxo-l,2,3,4-tetrahydroxy-butyl)benzo[4,5]imidazo[l,2-a]pyrimidine 94 

The protected 2-formyl-L-arabinal reacted with thiourea and cyanamide in the presence of sodium hydride to afford, via ring transformations, the 5- [ lR,2S-l,2-bis(benzyloxy)-3-hydroxypropyl]- 1,2-dihydropyrimidines. Similarly, treatment with 3-amino-2H-1,2,4-triazole yielded 6-[lR,2S-l,2-bis(benzyloxy)-3-hydroxypropyl][l,2,4]-triazolo[l,5-a]pyrimidine [151]. Reaction of 27 with 2-aminoimidazole and 2-aminobenzimidazole gave 96 and its bezo-analogues [64]. 

The synthesis of the mesoionic thiazolopyrimidine acyclonucleosides 97 incorporating the 2,3-dihydroxypropyl moiety was carried out starting by reaction of 2-bromothiazole with excess l-amino-2,3-propanediol acetonide via an aromatic nucleophilic substitution reaction to yield l-(2-thiazolylamino)- 

3- propanediol acetonide, which was condensed with substituted bis(2,4,6-trichlorophenyl) malonic esters to form a series of protected acyclonucleo-sides 98. The deprotection of the isopropyUdene group of 98 was done by using p-toluenesulfonic acid catalyst in methanol to give 97 [152], 

Reaction of levoglucosenone with urea, thiourea, or J -cyano- or W-ni-troguanidine resulted in the formation of the pyrimidine systems 99 in a stereospecific manner [153,154], Its reaction with a-aminoazoles yielded azolo[l,5-a]pyrimidine systems fused with a carbohydrate fragment. The reaction occurs much more smoothly than in the case of other a,jS-unsaturated ketones. The reactions of levoglucosenone with /3-dicarbonyl compounds (dimedone, barbituric acid) in the presence of a base resulted in pyran ring closure [153,154]. 

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Most of the examples that were reported to experience a translocation process of one of the heteroatom substituents with one of the heteroatoms of the ring belong to pyrimidines and their fused ring systems. 

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