Enantiomerically Pure Insect Pheromones: The Carbohydrate Synthetic Approach

Abstract

1. Introduction During the past two decades, there has been a rapidly growing interest in the field of intra- and interspecific chemical communication. Insect pheromones in particular have been intensely studied with respect to their potential use as a component of integrated pest management.1 Structural identification and chemical synthesis of these so-called “semiochemicals” have developed in harmony with the biological research evaluating structure-activity relationships. Most of the pheromones identified, studied, and synthesized come from the order Lepidoptera: very often these pheromones consist of long-chain olefinic compounds in which regio- and cis/trans-isomerism determine their biological activity. Several well documented reviews have appeared, emphasizing both coupling of alkyl halides or similar electrophilic substrates with metallo-organic reagents (“Wurtz-chemistry”) and carbonyl olefination, such as the Wittig reaction and its huge variety of modifications for regio- and stereoselective C-C bond formation. 2-8 The importance of chirality in pheromone perception by insects has been recognized and the role of optical isomerism and diastereoisomerism in behavioral discrimination has been firmly established through the combination of synthesis and bioassay. Some reviews bring good insight into the diverse strategies employed in synthesizing chiral pheromones, 8,9 originating principally from the orders Coleoptera and Hymenoptera. In spite of the increasing efficiency of asymmetric synthesis in the semiochemical field 10 , only few methods afford highly optically pure compounds with known absolute configuration. On the other hand, optical resolution is not always satisfactory and the absolute configuration of the resolved material has to be determined by some means. 11 Enantiospecific synthesis starting from naturally occuring chiral molecules such as amino or hydroxy acids, terpenes and sugars appears to be the safest path leading to highly optically pure pheromones. The prominent role of readily available carbohydrate starting materials within this “chiral synthon approach” has been clearly demonstrated. 12 The present review offers a 1976 to 1990 literature survey of the use of hexoses (mainly D-glucose and derivatives) and pentoses for the enantiospecific synthesis of chiral pheromones, issuing principally from the order Coleoptera. 13