Partial hydrogenation of substituted pyridines and quinolines: a crucial role of the reaction conditions

Hydrogenation of pyridyl and quinolyl compounds 2-substituted with a carbonyl group (1a–c and 2b,c) using PtO2 and 1 equiv. of HCl (conditions A) provides clean and total formation of the desired amino alcohol (hydrogenation of the heterocyclic ring and of the carbonyl) while under conditions B1 and/or B2 (concentrated HCl or pure CF3CO2H) the heterocyclic ring remains untouched and other aromatic parts are hydrogenated providing complex mixtures. When the heterocyclic ring is substituted by an alkyl group (quinaldine 3) conditions A provide mixtures while under conditions B2 (pure CF3CO2H) the benzene ring is cleanly hydrogenated leading to a pure product.