Consideration of molecular arrangements in regio- and enantioselective reduction of an NAD model compound controlled by carbonyl oxygen orientation

Abstract

The regio- and enantioselectivity of the reduction of an NAD model compound having axial chirality with respect to the C₃(quinolinium)–C(carbonyl) bond, 3-piperidinylcarbonyl-1,2,4-trimethylquinolinium ion (1), by using several reducing agents is described. Reaction of 1 with sodium hydrosulfite affords the 1,4-reduced product, 3-piperidinylcarbonyl-1,2,4-trimethyl-1,4-dihydroquinoline (2), with low enantioselectivity, whereas sodium borohydride promotes 1,2-reduction, affording 3-piperidinylcarbonyl-1,2,4-trimethyl-1,2-dihydroquinoline (3) as the sole product in a moderate enantioselectivity. When 1 was reduced by the chiral NADH model compound, 2,4-dimethyl-3-(N-α-methylbenzylcarbamoyl)-1-propyl-1,4-dihydropyridine (Me₂PNPH (4)), the regioselectivity and enantioselectivity of the reaction were significantly altered by the stereochemistry of 1 and 4. An achiral NADH model compound, 1-propyl-1,4-dihydronicotinamide (PNAH (5)) exhibited both high regio- and enantioselectivities. The product selectivity reflects the change in molecular arrangement in the transition state of the reaction and reveals the relative importance of the parameters governing the molecular arrangement in the reaction.