The above derivatives and some further ones have been found to be photoreactive also in the solid state.63'67-70 In this case the reaction may take a different course, however. Thus, with halomethasone (57) and prednicarbate (58) the observed processes involve the C(17) side chain (see Scheme 21).70

2.2 Drugs Acting on the Central Nervous System

2.2.1 Barbituric acid derivatives. 5,5-Dialkyl derivatives of barbituric acid (59) are used as hypnotics and tranquillisers. These compounds undergo two types of photochemical processes (Scheme 22). The first one involves initial cleavage of the C(4)-C(5) bond and leads to an isocyanate, positive evidence for which has been obtained by irradiation in the solid state (path a).71 This intermediate in turn adds nucleophilic solvents to give an amide (60) in water and a urethane in ethanol (61). This reaction is observed for barbital (59a, R, R-Et, R"=H) and its Ar(3)-methyl derivative (59b, R, R-Et, R"=Me).72 In a variation of this process, a second C-C bond is cleaved and CO is eliminated (path b). This leads to a hydantoin (62), as it occurs with mephobarbital (59c, R=Et, R-Me, R"=H).73'74

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