A monocarbocyclic sesterterpenoid biosynthetic precursor of leucosceptroids from Leucosceptrum canum and its metabolic isomerization by a specialist insect

Abstract

Pre-leucosceptroid (1), a rare monocarbocyclic sesterterpenoid featuring a cyclopentane ring with a terminal furan moiety, was isolated from the leaves of Leucosceptrum canum. Discovery of 1 suggested a two-step cyclization in the formation of the skeleton of leucosceptroids in plants, indicating its important role as a biosynthetic precursor of leucosceptroids. Its isomer, isopre-leucosceptroid (2), was isolated from the excrement of Nacna malachitis larvae, a specialist insect feeding on L. canum, suggesting that metabolic isomerization of α-hydroxyl keto from 1 to 2 might be a new insect detoxification mechanism. Their structures including the absolute configurations were determined by extensive spectroscopic analysis and quantum chemical calculations.