Structures of 4-Iminopyrido[1,2-a]pyrimidines, Pyrido[1,2-a]pyrimidin-4-ones, Pyridopyrimidinium Olates, and Thiazolo[3,2-a]pyrimidine Analogues.

The Structure-Correlation Principle of Bürgi and Dunitz is invoked in an analysis of the structures of 2-chloro-8-methyl-4-(2-(4-picolinyl)imino-4H-pyrido[1,2-a]pyrimidine 8, 7-chloro-5-(2-thiazolyl)imino-5H-thiazolo[3,2-a]pyrimidine 9, 2-methylamino-4H-pyrido[1,2-a]pyrimidin-4-one 10, 7-methylthio-5H-thiazolo[3,2-a]pyrimidin-5-one 11, 2,3-dihydro-7-methylthio-5H-thiazolo[3,2-a]pyrimidin-5-one 12, and 1-methyl-2-[(o-tert-butylphenyl)imino]-1,2-dihydropyrido[1,2-a]pyrimidin-1-ium-4-olate 13, which have been determined by X-ray crystallography. The most notable structural peculiarities are the long 'amidine' and 'amide' C-N bonds (1.40-1.50Å) and the tilting of the 'amidine' C=N and 'amide' C=O groups towards a ring nitrogen atom (CH12040%5FIE1.gifNCX =114-118°). Also the 'amidine' C=N (1.28Å) and 'amide' C=O bonds (1.22-1.24Å) are long, i.e. in the normal range for resonance-stabilized amidines and amides in spite of the lack of such resonance in these compounds. These features mimic the transition states for ring opening to ketenes. The long amidine and amide C-N bonds and acute NCX angles are in accord with the observed thermal ring opening to ketenimines and ketenes, respectively. Extraordinarily long 'amidine' and 'amide' C-N bonds (up to 1.5Å) and acute NCX angles in the title compounds are interpreted in terms of the Structure-Correlation Principle of Buergi and Dunitz. The structural features are in accord with the observed thermal ring opening to ketenimines and ketenes, respectively. [ABSTRACT FROM AUTHOR]