Utility of the Catalytic, Asymmetric Acyl Halide-Aldehyde Cyclocondensation Reaction in Natural Product Synthesis

摘要

The ability of the catalytic, asymmetric acyl halide-aldehyde cyclocondensation (AAC) reaction to produce stereoenriched beta-lactone products has found extensive utility in natural product synthesis. The asymmetric Al(III)-catalyzed AAC-SN2' ring opening sequence was exploited in synthetic efforts towards the enantioselective total synthesis of the aspidospermane alkaloid (-)-rhazinilam. The synthetic sequence features an enantioselective cyclization of a tethered pyrrole moiety onto an optically-active allene to set the quaternary carbon stereocenter while concomitantly forming rhazinilam's tetrahydroindolizine core. In addition, attempts at forming the requisite biaryl bond via a Pd-catalyzed cross-coupling reaction are also discussed. Recently, it was found that the Cinchona alkaloids quinine and quinidine can catalyze the AAC reaction to produce disubstituted beta-lactones in high yield and in essentially enantiomerically and diastereomerically pure form. Reaction conditions were developed which allowed for the effective formation of masked polypropionate units by employing the Cinchona alkaloid-catalyzed AAC reaction. Based on the pseudoenantiomer of the Cinchona alkaloid used, different stereoarrays of polypropionate units are obtained. A variety of optically active aldehydes are viable in this transformation as reaction conditions can be optimized for a specific substrate. A matched/mismatched phenomenon was observed where the matched case produced the desired polypropionate unit in good yield and high diastereoselectivity and the mismatched case afforded an unexpected beta-lactone product in diminished yield and diastereoselectivity. (+)-Discodermolide is a marine, microtubule-stabilizing polyketide that can only be isolated in scarce amounts from nature. Due to our inability to harvest it in supple amounts, the total synthesis of (+)-discodermolide has been the focus of many research groups. Application of the cinchona alkaloid-catalyzed AAC reaction towards the catalytic, asymmetric total synthesis of an analogue of (+)-discodermolide is discussed.