Efficient Chiral Induction by Diene Iron-Tricabonyl Moiety. IV. : Asymmetric Total Synthesis of a Piperidine Alkaloid, SS20846A

Abstract

By LiClO₄-catalyzed cycloaddition with Danishefsky's diene 5, an optically active 1-azatriene Fe(CO)₃ complex was converted into the 2-substituted dehydropiperidinone 8, from which a piperidine alkaloid 1 (SS20846A) was synthesized in an enantiomerically pure form via successive reduction and removal of the protecting groups. Although the reduction of the ketone 3 proceeded with cis-selectivity even with a hindered reducing agent, the desired trans-alcohol 4 could be obtained by the reaction with sodium borohydride in the presence of cerium(III) chloride. The cis-selective reduction of 3 originates from the equatorial attack of a hydride on conformer A, in which the diene Fe(CO)₃ moeity is axially oriented due to the severe steric hindrance with the p-methoxyphenyl (PMP) group on the nitrogen atom. However, the cerium salt reverses the stereoselectivity of the hydride reduction of 3.