Identification of the First Alkenyl Chiloglottone Congener

Abstract

Orchids of the genus Chiloglottis rely on sexual deception to attract a male thynnine wasp, whereupon a courtship routine results in the deposition and transfer of pollinia and subsequent pollination of the orchid. The chemical cues responsible for this behavior were recognized as a new class of natural products and efforts to synthetically prepare them, to assist in structural identification and to aid in ecological studies, have been ongoing. Most recently the first alkenylated member of this class was proposed based on GC-MS analysis of a physiologically active compound present in an orchid extract. The current work reveals synthetic efforts that provide access to diastereomerically pure unsaturated chiloglottone analogues and in combination with laboratory-based electrophysiological assays and field-based studies have unambiguously allowed identification of 5-allyl-2-ethyl-1,3-cyclohexanedione (1d, chiloglottone 4) as the first alkenyl congener in the chiloglottone family. Convergent synthetic approaches, involving copper(I)-mediated conjugate addition to a glutaconic ester converging on cadmium-mediated desymmetrization of cyclic anhydrides or a Johnson-Claisen rearrangement of a mixed ketene acetal provided the appropriately substituted δ-keto acids suitable for carbocyclization to furnish the required chiloglottone analogues.