Stereoselective syntheses of (+)-rhopaloic acid A and (−)-ent- and (±)-rac-rhopaloic acid A
作者:Ryukichi Takagi、Asami Sasaoka、Hiroko Nishitani、Satoshi Kojima、Yoshikazu Hiraga、Katsuo Ohkata
DOI:10.1039/a707204j
日期:——
Rhopaloic acid A (+)-1 and the related compounds (â)-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 Evansâ 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.
跳罁酸A (+)-1及其相关化合物(-)-ent-1和(±)-rac-1已被立体选择性地合成。合成策略包括连续的高同系化(2E,6E)-金合欢醇7和环化形成四氢吡喃环,同时最终引入羧基上的α-亚甲基。环化是通过分子内的杂迈克尔加成进行的,导致形成2,5-二取代的四氢吡喃。立体化学可以通过椅状过渡态模型来合理化。不对称合成是通过埃文斯不对称烷基化程序实现的,使用(S)-或(R)-4-苄氧基唑啉-2-酮作为手性辅助剂。通过比较合成化合物的特定旋转与天然产物的特定旋转,可以确定天然跳罁酸A (+)-1的构型为2R,5S。