Asymmetric Syntheses of (+)-Preussin B, the C(2)-Epimer of (-)-Preussin B, and 3-Deoxy-(+)-preussin B

Efficient de novo asymmetric syntheses of (+)-preussin B, the C(2)-epimer of (-)-preussin B, and 3-deoxy-(+)-preussin B have been developed, using the diastereoselective conjugate addition of lithium (S)-N-benzyl-N-(alpha-methylbenzyl)amide to tert-butyl 4-phenylbut-2-enoate and diastereoselective reductive cyclizations of gamma-amino ketones as the key steps to set the stereochemistry. Conjugate addition followed by enolate protonation generated the corresponding beta-amino ester. Homologation using the ester functionality as a synthetic handle gave the corresponding gamma-amino ketone. Hydtogenolytic N-debenzylation was accompanied by diastereoselective reductive cyclization in situ; reductive N-methylation then gave 3-deoxy-(+)-preussin B as the major diastereoisomeric product. Meanwhile, the same conjugate addition but followed by enolate oxidation with (+)-camphorsulfonyloxaziridine gave the corresponding anti-alpha-hydroxy-beta-amino ester. alpha-Epimerization by oxidation and diastereoselective reduction then gave access to the corresponding syn-alpha-hydroxy-beta-amino ester. Homologation of both of these diastereoisomeric alpha-hydroxy-beta-amino esters gave the corresponding beta-hydroxy-gamma-amino ketones. N-Debenzylation and concomitant diastereoselective reductive cyclization, followed by reductive N-methylation, provided the C(2)-epimer of (-)-preussin B and (+)-preussin B as the major diastereoisomeric products, respectively. The overall yields (from phenylacetaldehyde) were 19% for 3-deoxy-(+)-preussin B over seven steps, 8% for the C(2)-epimer of (-)-preussin B over nine steps, and 7% for (+)-preussin B over eleven steps.