Stereocontrolled Synthesis of 6-s-cis- and 6-s-trans-Locked 9Z-Retinoids by Hydroxyl-Accelerated Stille Coupling of (Z)-Tri-n-Butylstannylbut-2-en-1-ol and Bicyclic Dienyl Triflates
摘要:
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.
The Total Synthesis of (±)-11-<i>O</i>-Debenzoyltashironin
作者:Silas P. Cook、Alessandra Polara、Samuel J. Danishefsky
DOI:10.1021/ja0670254
日期:2006.12.1
The total synthesis of the putative non-peptidyl neurotrophic factor 11-O-debenzoyltashironin has been accomplished. The key transformation involves a biomimetic oxidative dearomatization/transannular Diels-Alder sequence that closes the tetracyclic carbon skeleton present in the natural product.