Polymer-supported (-)-8-phenylmenthyl Auxiliary as an Effective Solidphase Chiral Inductor in the Addition of Nucleophiles to N-acyliminium Ions

Aim and objective: According to our interest in developing new methods for the construction of intricate molecules, a reliable polymer-supported (-)-8-phenylmenthyl chiral auxiliary for the addition of different nucleophiles to chiral-supported N-acyliminium precursors were developed.

Materials and methods: Merrifield resin was employed to anchor (-)-8-phenylmenthol, which was prepared by nitration of (-)-8-phenylmenthyl chloroacetate followed by reduction of nitro group and subsequent Merrifield resin coupling. Treatment of a suspension of polymer-supported chloroformate and piperidinone in the presence of Et3N resulted in attachment of the substrate onto the solid-support. Treatment of the resulting resin with LiEt3BH/MeOH afforded methoxypiperidine in 87% yield. Then, the addition of allyltrimethylsilane, TMSCN, 2-(trimethylsiloxy)propene and triisopropylsilyloxyfuran and others to the N-acyliminium ion derived from chiral 2- methoxypiperidine carbamate was studied.

Results: The stereochemical outcome of the addition of nucleophiles to the supported N-acyliminium ion derived from 2-methoxypiperidine carbamate was proposed through the Si-face, affording after resin cleavage 2-substituted piperidines in 70%-84% yields and selectivities ranging from 4:1-11.1. Moreover, the key intermediates of chiral piperidines have been employed for the synthesis of simple chiral alkaloids such as (R)-pipecolic acid, (R)-pelletierine, (S)-coniine and (R,R)-myrtine.

Conclusion: The proposed supported-chiral auxiliary for asymmetric approach may be expected to result not only in efficient solid-phase syntheses of a wide range of alkaloids but also in the development of useful new solid-phase methodologies, particularly for the asymmetric additions to iminium precursors. This work describes the first example of solid-phase synthesis by using supported (-)-8-phenylmenthyl as an effective chiral inductor and would be useful for the synthesis of chiral building block libraries.