The Biosynthesis of the Diterpenes

Biogenetic-Type Rearrangements of Terpenes

“…(221) . 00 (230) The first stage in the biogenesis of the most common types of diterpenes is a proton-initiated bicyclization of geranylgeranyl pyrophosphate (234) to the basic labdane or ent-labdane nuclei (e.g., copalol, 235-0H) (300) (301). The finding that angular methyl rearrangement occurs with facility in both isomers demonstrates once again that the migrating group need not necessarily be situated anti to the Ieaving group at a tertiary position.…”

“…If so, oxidation of the methyl group would obviously have to precede cyclization. Separate incorporation of mevalonate bearing tritium at positions 4 and 5 into rosenonolactone (249 with ketone at C1) without loss of the Iabel has excluded the intervention of olefinic intermediates having a double bond at either the 11 5 or 11 5 < 10 ) positions (21,300). An alternative to the combined cyclization-rearrangement-lactonization pathway is hydration of (248) (R = CH3) to a 10-hydroxyrosane, with oxidation and lactonization taking place at a later stage.…”

“…The biogenetic scheme (Scheme 26, updated to include new representatives and depicting classical cations for the sake of clarity*) begins with cyclization of copalyl pyrophosphate (235-0PP) to the pimarenyl cation (268) (enantiomer of 245) which further cyclizes upon the remairring double bond in the vinyl group to form the key intermediate (274), and the related atisine alkaloids, entails a 1 ,3-hydrogen shift ( C12--+-C16), Wagner-Meerwein rearrangement, and elimination. The elements of this biogenetic scheme are supported by the co-occurrence of these diterpenes (325) and biosynthetic investigations (300). Although a complete set of structures Stereoisomerie to those depicted below may be similarly generated from the c13 epimer of (268), the only known natural product of this type is phyllocladene (enantiomer of 272 with ethano bridgeanti to the Cg proton) and a few of its derivatives.…”

“…Biosynthetic investigations have established that 7ß-hydroxykauren-19-oic acid is the immediate predecessor of the gibberellins, undergoing an oxidative, pinacol-like rearrangement to gibberellin A12 aldehyde (288) with extrusion of C1 (300,(349)(350)(351). Chemically induced pinacol rearrangements of kauran-6ct,7ct-diol monosulfonates have, in fact, been commonly employed (351)(352)(353)(354)(355)(356) as a means of preparing gibberellin aldehydes (e.g., 287--+-288) (355).…”

“…5~., H2SO., HOAc -t--(94%) (243) Copalyl pyrophosphate (235-0PP) has been demonstrated to be a key branching point in the biosynthesis of tricyclic and tetracyclic diterpenes (24,300,308). It, therefore, seems likely that .…”