Accessing monomers, surfactants, and the queen bee substance by acrylate cross-metathesis of long-chain alkenones

Abstract

Polyunsaturated long-chain alkenones are a unique class of lipids biosynthesized in significant quantities (up to 20% of cell carbon) by several algae including the industrially grown marine microalgae Isochrysis. Alkenone structures are characterized by a long linear carbon-chain (35-40 carbons) with one to four trans-double bonds and terminating in a methyl or ethyl ketone. Alkenones were extracted and isolated from commercially obtained Isochrysis biomass and then subjected to cross-metathesis (CM) with methyl acrylate or acrylic acid using the Hoveyda-Grubbs metathesis initiator. Within 1 h at room temperature alkenones were consumed, however complete fragmentation (i.e. conversion to the smallest subunits by double bond cleavage) required up to 16 h. Analysis of the reaction mixture by gas chromatography and comprehensive two-dimensional gas chromatography revealed a predictable product mixture consisting primarily of long-chain (mostly C17) acids (or methyl esters from CM with methyl acrylate) and diacids (or diesters), along with smaller amounts (~5%) of the honey bee “queen substance” (E)-9-oxo-decenoic acid. Together, these compounds comprise a diverse mixture of valuable chemicals that includes surfactants, monomers, and an agriculturally relevant bee pheromone.