Biosynthesis, natural distribution, and biological activities of acyclic monoterpenes and their derivatives

Monoterpenoids are C₁₀ isoprenoid natural products with diverse functions as volatile and non-volatile secondary metabolites. Their biosynthesis from acyclic diphosphate precursors is often accompanied by one or more ring closures, which stabilize the linear, carbocationic intermediates generated during typical terpene synthase reaction sequences. The subset that remains uncyclized following the elimination of the diphosphate group correspondingly retains higher reactivity than their cyclized counterparts, a property that may explain the additional transformations that geraniol and other acyclics undergo to yield monoterpene aldehydes, acids, esters, glycosides, and iridoids, many of which possess allelochemical activity as pollinator attractants, feeding deterrents, or insect repellents. Essential oils of lemon grass (or citronella) (Cymbopogon), rose-scented geraniums (Pelargonium), and lemon-scented gums (Corymbia) are rich sources of the acyclic monoterpene alcohols we collectively refer to in this review as ‘citronelloids’. Their formation from geranyl diphosphate may be brought about by the tps-g or tps-b subclade of terpene synthases or, alternatively, by a subclade of Nudix hydrolases which act preferentially on prenyl diphosphate substrates, yielding monoterpene alcohols in a two step process that involves a monophosphate intermediate. In some members of the Lamiid and Campanulid clades, further oxidation of geraniol and a non-classical, reductive cyclization leads to the iridoids, a group of mostly non-volatile, bicyclic monoterpenes with potent anti-herbivory and anti-inflammatory properties. Here we review the recent literature on acyclic monoterpene biosynthesis, their natural distribution across the plant kingdom, and their emerging roles in medicine as biologically active natural products.