Bioorganic chemistry: sterols and acrylic terpene terminal expoxides

Abstract

Investigations into the bioorganic chemistry of sterol synthesis have led to various advances in both the organic and biological provinces. Along organic lines, there have been developed: (i) a general method for the highly selective terminal oxidation of acyclic terpenes, and (ii) stereoselective cyclizations of terpene terminal epoxides to 3-hydroxylated biand tricyclic terpenoids of the natural product type. In the biochemical realm, application of the above chemistry has led to: (i) the identification of squalene 2,3-oxide as a genuine intermediate in the conversion of squalene to lanosterol; (ii) a fuller theoretical understanding of the cyclization process, including the recognition of alternate biochemical cyclization pathways; and(iii) the discovery that structurally modified squalene 2,3-oxides are enzymically converted to triand tetracyclic products with revealing structural features.

Iodine(III)-mediated halogenations of acyclic monoterpenoids

“…For X = Br, this is the case when the bromide is the default reagent because of its slow addition (giving 3 ) or because the nucleophile is the co-solvent (water giving 4 and to a lesser extent EtOH yielding 4’ ). While the formation of 4 is related to the well-established NBS-mediated formation of bromohydrins in aqueous media [30], the formation of 3 is not so straightforward. For instance, submitting 1a to NBS in the presence of trifluoroacetic acid led to a complex mixture of products, the major one being allylic bromide 6a’ .…”

Flavours and Fragrances

“…[234][235][236] However, the first published work on the acid-catalysed cyclisation of homofarnesylic acid to a mixture of sclareolide diastereomers in the year 1960 came from Günther Lucius at VEB Chemische Fabrik Miltitz, Leipzig. [237] Over recent years, there have appeared further very elegant examples of the "domino synthesis", in particular to build the steroid scaffold (cf.…”

Synthesis of (±)-karahana ether and a (±)-labdadienoic acid by the electrophilic cyclization of epoxy allylsilanes

“…For the past fifty years, organic chemists have often sought biomimetic type routes in their syntheses of natural products (1). These routes, based on the presumed biosynthetic pathway, frequently were very efficient and led to the development of new synthetic reactions.…”

“…The conformation of the polyolefin and an anti-periplanar addition to the olefin determine the structure and stereochemistry of the final product. There have been a variety of successful natural products syntheses based on this concept (1,3). In most cases, the cyclization has been initiated by the attack of a Lewis acid or '~u t h o r to whom correspondence may be addressed.…”