Stereoselective syntheses of (+)-rhopaloic acid A and (−)-ent- and (±)-rac-rhopaloic acid A

Rhopaloic acid A (+)-1 and the related compounds (–)-ent-1 and (±)-rac-1 have been stereoselectively synthesized. The synthetic strategy consists of successive homologation of (2E,6E)-farnesol 7 and cyclization to form a tetrahydropyran ring, together with final introduction of an α-methylene group on the carboxylic moiety. The cyclization is carried out by intramolecular hetero-Michael addition leading to 2,5-disubstituted tetrahydropyrans. The stereochemistry can be rationalized by invoking a model of a chair-like transition state. The asymmetric synthesis is achieved by way of the Evans’ asymmetric alkylation procedure using (S)- or (R)-4-benzyloxazolidin-2-one as the chiral auxiliary. In the event, the configuration of natural rhopaloic acid A (+)-1 could be assigned as 2R,5S by comparison of the specific rotations of synthetic compounds with that of the natural product.