Ningpoensines A–C: unusual zwitterionic alkaloids from Scrophularia ningpoensis

Abstract

Three novel zwitterionic alkaloids—ningpoensine A (1) and a pair of epimers, ningpoensines B/C (2a/2b)—with unprecedented molecular skeletons were isolated from the root of Scrophularia ningpoensis. Their structures were established by extensive spectroscopic analyses, and the absolute configuration of ningpoensine A was determined by quantum chemical calculations. A biosynthetic pathway leading to ningpoensines A–C from harpagide and natural amino acids is proposed. Ningpoensines B/C tended to promote wound closure in human embryonic keratinocytes.