Studies on transition metal-catalyzed asymmetric reductions and reductive aminations

Abstract

The synthesis of enantioenriched NH lactams from ketoesters and ketoacids was achieved via direct asymmetric reductive amination and cyclization cascade reactions. The reaction showed wide substrate scope and good functional group tolerance. Various γ-, δ-, and ε-keto esters/acids were converted to the corresponding NH lactams of five-, six-, or seven-membered rings in generally high yields and good enantioselectivities. Structurally diverse chiral NH benzo-lactams were also synthesized smoothly. The amine salts, hydrogen gas and catalytic amount of metal complex applied in this method showcase the economy and high efficiency of this reaction. The scalable and concise preparation of key intermediates en route to the synthesis of larotrectinib, (+)-garenoxacin, and other bioactive molecules further illustrate the practicality of this strategy. An efficient and straightforward method to synthesize optically active N-unprotected α-amino acetals via ruthenium-catalyzed direct asymmetric reductive amination was developed. As versatile and valuable platform molecules, α-amino acetals could be converted to corresponding α-amino acids, amino alcohols, or other derivatives via convenient transformations. Two gram-scale reactions and the application towards the synthesis of a natural product, (+)-cytoxazone, highlight the practicality and potential of this methodology. An exclusive asymmetric 1,2-reduction of cycloalkenones was induced by using (R)-DTBM-C3*-TunePhos ligated copper hydride as the catalyst. The reduction of unsubstituted cyclohexenones resulted in allylic alcohols with moderate 71-77% ee, albeit with high chemoselectivity and yields. Reduction of α-brominated cycloalkenones resulted in excellent enantioselectivities of up to 98% ee and high yields. Five-, six-, and seven-membered substrates with diverse β-substitutions including aryl, alkyl, allyl, alkynyl were all well-tolerated under the optimized reductive conditions, thus giving the corresponding α-bromo-substituted allylic alcohols successfully. Acyclic as well as α-methylated substrates were also evaluated for reduction under these conditions. The products are versatile intermediates, and several elaborations of these α-bromo-substituted allylic alcohols were conducted.