Stereocontrolled Synthesis of 6-<i>s</i>-<i>cis</i>- and 6-<i>s</i>-<i>trans</i>-Locked 9<i>Z</i>-Retinoids by Hydroxyl-Accelerated Stille Coupling of (<i>Z</i>)-Tri-<i>n</i>-Butylstannylbut-2-en-1-ol and Bicyclic Dienyl Triflates
作者:Beatriz Domínguez、Yolanda Pazos、Angel R. de Lera
DOI:10.1021/jo9917588
日期:2000.9.1
Analogues of 9-cis-retinoic acid with locked 6-s-cis and 6-s-trans conformations have been stereoselectively synthesized using a Stille coupling reaction between bicyclic dienyl triflates (5 and 6, respectively) and (Z)-tributylstannylbut-2-en-1-ol (7) to stablish the Z geometry of the polyenic side chain. The mild conditions (25 degrees C, 30 min) of this coupling stand in contrast to the reluctance of the isomeric (E)-tributylstannylbut-2-en-1-ol (18) to react with triflates 5/6. The significant rate differences experimentally observed in Stille reactions between isomeric (Z)- and (E)-tri-n-butylstannylalkenols in favor of the former isomer, even with highly hindered alkenyl triflates, is ascribed to internal coordination of palladium to the heteroatom in the presumably rate-limiting transmetalation step. Dienals and trienals with an E geometry, which are not efficiently available by direct coupling of the corresponding triflates and E-stannanes, can in turn be obtained by isomerization of their Z-isomers.