Synthetic and ecological chemistry of natural products and analogues from sexually deceptive Chiloglottis orchids

Abstract

The work detailed in this thesis had its genesis in collaborative research efforts directed at understanding the ecological constraints on the evolution and speciation of sexually deceptive Chiloglottis orchids. Pollination by deception is a well documented phenomenon, particularly in the Orchidaceae, however only in recent decades has the chemical mimicry that so often drives these pollination syndromes become the subject of enquiry. This report commences with an outline of chemical mimicry and insect olfactory perception. Chapter 1 describes the conditions for pollination by food fraud and reproductive deception, with reference to insect pheromone systems, including their biological and behavioural functions, and chemical properties. Sexual deception, with a focus on the Australian genera, is given particular emphasis.

A number of synthetic routes for the construction of 2,5-dialk(en)yl-1,3-cyclohexanediones, which represent insect sex attractants are reported in Chapters 2 and 3. Chiloglottones are natural products that operate both as female sex pheromones in thynnine wasp species as well as deceptive allomones in Chiloglottis orchids. We developed and utilised two alternative approaches to successfully make a diverse array of chiloglottone analogues (Chapter 2). These routes have been disclosed in publications as indicated in the Author's Declaration (page ii). Chapter 3 reports a case study of mimetic chemistry of the sexually deceptive orchid C. turfosa, which presented diverse synthetic challenges that required the design of new approaches towards the target 2,5-dialkyl-1,3-cyclohexanediones.

Chapter 4 considers the interpretation of mass spectral data via high resolution analysis of synthetic 2,5-dialkyl-1,3-cyclohexanediones. This work, which was generated in cooperation with collaborators in Germany and has been reported in Proceedings of the National Academy of Sciences (see page ii), enables the rapid identification of likely structures from the mass spectra of new natural products that contain the 2,5-dialkyl-1,3-cyclohexanedione skeleton. This Chapter also describes our efforts to divulge the biosynthetic origin of chiloglottones in sexually deceptive Chiloglottis orchids. We discuss our journey towards revealing the circumstances required for in vivo chiloglottone biosynthesis, from the preparation of stable isotope labelled precursors, to manipulation of conditions for biosynthesis.

The ecological conclusions that were revealed through our appreciation of the chiloglottones from some 20 Chiloglottis orchid species are described in Chapter 5. This includes the discovery of further natural products and the analytical and field evaluation of synthetic material. The relationship between orchid chemistry and phylogeny suggests that pollinator driven selection is a probable means of speciation in this taxon. Show more