Nitrogen bridgehead compounds. Part 30. Vilsmeier–Haack formylation of 6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones

Abstract

Depending on the substituents at C(3) and on the reaction conditions, Vilsmeier–Haack formylation (POCl₃–DMF) of 6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidin-4-ones (I) led to 9-(dimethylamino-methylene)- or 9-(ethoxymethylene)-6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidines (II) and (IV), respectively, or 9-formyl-1,6,7,8-tetrahydro-4H-pyrido[1,2-a]pyrimidines (III). Compounds (II) may be converted into (III) or (IV). Owing to the enhanced activity of their 9-CH₂ group, the 3-carboxylic acid derivatives of (I) form compounds (II) even on the action of dimethylformamide diethyl acetal, and compounds (IV) on the action of triethyl orthoformate in Ac₂O. Compounds (III) may be converted back into (I) by hydrolysing the 9-formyl group. Their spectral data show that compounds (II) and (IV) exist exclusively in the E-configuration, and compounds (III) predominantly in the 1,6,7,8-tetrahydrotautomeric form.