2.4.6- Trisubstituted dihydropyrimidines

X-Ray analyses and solid-state IR spectra were recorded for a number of 1,4-and 1,6-dihydropyrimidines, demonstrating the dependency of the tautomeric composition in the crystal on the substitution in the pyrimidine ring and on the ability of these compounds to form intermolecular hydrogen bonds. Thus, 2,4,6-trisubstituted dihydropyrimidines exist in the solid state exclusively as 1,4-dihydro tautomers independently of the nature of the substituents (alkyl, aryl, heteroaryl, acyl, alkoxycarbonyl, etc.) [82H(19)493 86H(24)233 86T6429 90JMC1510]. Introduction of another alkyl substituent into the 4 position shifts the equilibrium toward the 1,6-dihydro isomer. For example, 2-phenyl-... 

Likewise, upon CIR of electron-deficient (hetero)aryl halides 11 and (hetero)aryl propargyl alcohols 12, and subsequent addition of amidinium salts 32, 2,4,6-trisubstituted pyrimidines 89 can be obtained in a consecutive three-component reaction in good yields (Scheme 48) [241]. Interestingly, in all cases the aromatic products 89 are found and not the expected dihydropyrimidines, regardless whether the reaction has been performed under an anaerobic or an aerobic atmosphere. Therefore, it can be assumed that the presence of the transition metal catalysts is beneficial for a terminal aromatizing dehydrogenation. 

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