J. Boyd, Wen-Kuang Lin
2001
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Abstract
The problem of aspartimide formation at Asp(OtBu) during Fmoc based solid phase synthesis is well documented [1]. In 1996 Karlstrom and Unden demonstrated that the more hindered 3-methylpent-3-yl (Mpe) s-ester of aspartate is remarkably resistant to piperidine induced aspartimide formation, and its use results in significantly cleaner crude products [2]. In our hands, the critical Mpe esterification step (via acid chloride 2) has been troublesome, with yields typically in the range of 15–25%. We describe significant yield enhancements and crystallization conditions for Fmoc-Asp(OMpe)-OH (5).