Photolability of potential calcium channel antagonists: Hexahydroquinoline derivatives

Photodegradation of three chlorinated derivatives—Compound I (2′-chloro-), Compound II (3′-chloro-) and Compound III (2′,3′-dichloro-) of methyl 2,6,6-trimethyl-4-phenyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate was studied in the conditions recommended by the International Conference on Harmonization (ICH) in the liquid phase. Photodegradation kinetics of the three compounds in methanolic solutions was studied by means of the spectrophotometric method. Reactions followed the first-order kinetic equation. The measured real quantum yields of photodegradation were 1.2 × 10−4, 4.0 × 10−5 and 3.0 × 10−5, respectively. Gas chromatography–mass spectrometry (GC–MS) was used to identify the photodegradation products, with the main products formed by oxidation of the dihydropyridine ring to pyridine derivatives. Mass fragmentation pathways of the photoproducts were also described.