研究了不同取代的5-dethia-5-oxacephams的手性与它们各自的分子结构之间的关系。发现这些化合物中的β-内酰胺单元的酰胺发色团是非平面的,在氮原子上具有浅金字塔形状。由于非平面性,β-内酰胺系统固有地变得不对称,这由高强度的n-> pi * CD带支持。还发现在研究的草甘膦中内酰胺部分的螺旋度受C(6)碳原子处的绝对构型控制。在此基础上,
Strategies for the Formation of 1-Dethia-1-oxa-cephams
摘要:
The paper describes three possible routes for the formation of 1-dethia-1-oxa-cephams. The first two routes: (a) [2+2]cycloaddition to chiral vinyl ethers and (b) condensation of 4-acetoxyazetidin-2-one to chiral alcohols, are followed by the ring closure step involving N-alkylation. The third route (c) consists of N-alkylation prior to the cyclization step. In order to compare routes (a), (b) and (c), diastereomeric 1-dethia-3-(4-methoxybenzyloxy)-1-oxacephams were synthesized using three possible strategies. While the comparison of stereoselectivities of the [2+2]cycloaddition method (a) and the condensation (b) shows unequivocally the advantage of the former, the route (c) leads to the reverse direction of asymmetric induction relative to the first two steps and offers the highest asymmetric induction. (C) 2000 Elsevier Science Ltd. All rights reserved.