From C-Glycosides to Pyranopyrans: An Approach to Thyrsiferol Using Titanium(III)-Promoted Redox Couplings
作者:Gisele A. Nishiguchi、R. Daniel Little
DOI:10.1021/jo050465d
日期:2005.6.1
were thwarted by the congestion imparted by the presence of the vicinal TBS-ether. Consequently, cyclization of the coupling adduct under conditions developed by Olah and Prakash and co-workers led to the cis-fused pyranopyran 27. X-ray analysis of this crystalline material confirmed each of the stereochemical assignments. After much effort, it was determined that the hydroxyl group at C12 could be removed
描述了一种在许多天然产物(包括甲状腺甾醇)中发现的吡喃并吡喃环系统的方法。该路线需要将香叶醇的α,β-不饱和酮(11)和乙酰乙醛二甲基乙缩醛(19)组装成二氢吡喃(23)和钛(III)促进的偶合,以得到60%的酮醇产率26。在此过程中形成的σ键对应于thyrsiferol(4)的pro-C 9 -C 10键。尝试在pro-C 11上颠倒立体化学由于邻近的TBS-醚的存在而导致的交通拥塞,使中枢受挫。因此,在Olah和Prakash及其同事开发的条件下,偶联加合物的环化反应导致了顺式吡喃吡喃[ 27]。对该晶体材料的X射线分析证实了每个立体化学分配。很大的努力后,它被确定在C羟基12可以通过用三-处理所述衍生的甲基黄原酸酯去除Ñ形成自由基的条件下丁基膦-硼烷络合物。尽管工作环境受阻,并且存在可能不稳定的C-Br键,但反应顺序仍然有效。