The azomethine ylide strategy for β-lactam synthesis. A comprehensive mechanistic evaluation
作者:David Brown、Giles A. Brown、Sarah R. Martel、Denis Planchenault、Evelyn Turmes、Kenneth E. Walsh、Richard Wisedale、Neil J. Hales、Colin W. G. Fishwick、Timothy Gallagher
DOI:10.1039/b010046n
日期:——
The release of azomethine ylide reactivity from oxazolidinones such as 4a/4b and 7 is proposed to involve a stepwise fragmentation via12 and 13 followed by cycloaddition (to an alkene) leading to adduct 14, which then undergoes decarboxylation under the reaction conditions to give the observed product 15. In the case of CC-based dipolarophiles, the cycloaddition is concerted and stereospecific, and the cycloaddition step is rate determining. Extensive experimental, together with computational data, including racemisation and kinetic studies, as well as the changes in reactivity associated with varying key structural features associated with the β-lactam based oxazolidinones is presented in support of the favoured mechanistic postulate. The fragmentationâcycloadditionâdecarboxylation sequence is an alternative pathway for the release of an azomethine ylide from an oxazolidinone to that process already well established for N-alkyl oxazolidinones (concerted decarboxylation before cycloaddition). The N-acyl component associated with 4 may influence this change in mechanism, but specific structural features associated with the β-lactam system (ring strain and the presence of a malonyl moiety) are most likely responsible for the mechanistic divergence that is observed.
从恶唑烷酮(例如 4a/4b 和 7)中释放偶氮甲碱叶立德反应活性涉及通过 12 和 13 进行逐步裂解,然后进行环加成(与烯烃),得到加合物 14,然后在反应条件下进行脱羧,得到观察到的产物15。对于基于CC的亲偶极试剂,环加成是协同的和立体特异性的,并且环加成步骤是速率决定的。广泛的实验以及计算数据,包括外消旋和动力学研究,以及与β-内酰胺基恶唑烷酮相关的不同关键结构特征相关的反应性变化,都提出来支持所青睐的机械假设。断裂→环加成→脱羧顺序是从恶唑烷酮中释放偶氮甲碱叶立德的另一种途径,该途径已经为N-烷基恶唑烷酮建立了良好的过程(环加成之前的协同脱羧)。与 4 相关的 N-酰基成分可能会影响这种机制的变化,但与 β-内酰胺系统相关的特定结构特征(环应变和丙二酰基部分的存在)最有可能导致观察到的机械分歧。