From l-Dicarbonyl Compounds

Formally, one can also propose the formation of the 2,5-dihydro isomer, but even if this isomer does form during the reaction, it is easily transformed to the thermodynamically more stable 1,2-dihydro compound. This reaction is nearly identical to the classical Hantzsch s synthesis of dihydropyridines, with only the molar ratio of starting materials being changed. However, in contrast to the widely investigated Hantzsch reaction, there are few examples in the literature, and it needs detailed study. 

This original approach was first used in 1964 by Krohnke and coworkers159 for preparing dihydropyrimidines, but these spontaneously decomposed to the corresponding pyrimidines and could not be isolated. 

Hoffmann and co-workers49 160 first successfully isolated 1,2-dihydropyrim-idines 93 from a condensation of the dimethylacetal of acetoacetic aldehyde with different carbonyl-containing compounds in the presence of ammonia and ammonium nitrate [Eq. (32)]. 

Usually, these compounds are rather unstable at room temperature, although they could be stored in a freezer. Several authors have used this method for the preparation of 1,2-dihydropyrimidines.161-163 However, the majority of the compounds isolated were disubstituted in position 2, preventing further oxidation to corresponding pyrimidines. 

Salts of 1,2-dihydropyrimidines 95 were also prepared by a clever modification of this approach, which instead of a mixture of ammonia and carbonyl derivative, used a gem diamine (94) in condensation with acetylacetone or / -methoxyacrolein under acidic conditions164 [Eq. (33)]. 

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