Reduction reactions Sodium cyanoborohydride-Zinc

Esters of racemic pyrazolidine-3-carboxylic acid (5-azaproline, 16) are obtained by a two-step reaction via addition of diazomethane to acrylic acid esters to form the corresponding dihydropyrazoline derivatives, which are converted into the desired pyrazolidine-3-carb-oxylic acid derivative by hydrogenation over palladium on charcoal,11621 or by reduction with sodium cyanoborohydride,[164l or with zinc in acetic acid. Details are given in Table 6J1651... 

Selective reduction of either the pyridine or the benzene rings in quinolines and isoquinoline can be achieved the heterocyclic ring is reduced to the tetrahydro level by sodium cyanoborohydride in acid solution,by sodium borohydride in the presence of nickel(II) chloride, by zinc borohydride," or, traditionally, by room temperature and room pressure catalytic hydrogenation in methanol. However, in strong acid solution it is the benzene ring which is selectively saturated " longer reaction times can then lead to decahydro-derivatives. 

---