Studies on Monoterpene Glucosides and Related Natural Products. LII. Mechanism for Iridane Skeleton Formation from Acyclic Monoterpenes in the Biosynthesis of Secoiridoid Glucosides and Indole Alkaloids

Biosynthesis significance of iridoids in chemosystematics

“…Damtoft's group 15,16 demonstrated that 6 is a precursor for some iridoids, however, these findings conflict with others results published in the literature [17][18][19] . It was found also that deoxyloganic acid (8) was incorporated into asperuloside (9) in Theligonum cynocrambe (Rubiaceae) while 6 was not 16 .…”

Monoterpenoids

“…255 The epimeric alcohol (285) can be made by adding phenylsulphenyl chloride to car-3-ene (263) to give (286), which can be solvolysed to the acetate (287). Oxidation of (287) to the sulphoxide (288) and thermal elimination of phenylsulphenic acid leads ultimately to (285).z55 The keto-epoxide (289) gives a mixture of ( 290) and the Favorskii-type product (29 1) when it is treated with NaOEt. 256 The eucarvone epoxide (292), which can be derived from car-3-en-2-one (293), yields the ethers (294) when it reacts with HClO, and R'OH (R' = alkyl) and the chlorohydrin (295) with HCl.…”

The biosynthesis of C5—C20terpenoid compounds

“…The enzyme (system ?) was dependent on divalent metal cations, and represents the first example of the preparation of a soluble C , ,-cyclase from a higher plant.s4 y-Bisabolene (44), which is a notional intermediate in the formation of several classes of sesquiterpene, may be formed either by cyclization of (2E, 6E)-farnesyl diphosphate (41) or of its ( 2 2 , 62)-isomer (45). Very detailed studies with intact callus cultures and derived enzymic extracts from Andrographis paniculata have revealed that (a) the y-bisabolene that was biosynthesized indeed had the 2 configuration (as had been previously shown); (b) the immediate precursor is the (6E)-rather than the (62)-farnesyl diphosphate; and (c), consistent with (b), in the paniculide B that was derived (and probably in the ybisabolene), the carbon atom in the ring that was derived from C-2 of mevalonate was anti to the side-chain (H, as shown).…”

“…Very detailed studies with intact callus cultures and derived enzymic extracts from Andrographis paniculata have revealed that (a) the y-bisabolene that was biosynthesized indeed had the 2 configuration (as had been previously shown); (b) the immediate precursor is the (6E)-rather than the (62)-farnesyl diphosphate; and (c), consistent with (b), in the paniculide B that was derived (and probably in the ybisabolene), the carbon atom in the ring that was derived from C-2 of mevalonate was anti to the side-chain (H, as shown). In addition, both mevalonate and (41) were incorporated into (44) without loss of hydrogen, respectively, from C-5 or from C-1 : cyclization to the hypothetical bisabolenyl cation therefore did not involve a prior E -+ 2 isomerization of the terminal doublebond by a redox mechanism. Thus there must exist a (2E,6E)farnesyl-diphosphate isomerase-cyclase linked system that is independent of a previously identified (2E, 6E) -+ (22, 6E) isomerase system that is responsible for the formation of (2E, 6E)-farnesol in these culture^.^^ These studies illustrate some of the particular advantages of plant tissue cultures in biosynthetic studiesthe ready preparation of an active cell-free system and high incorporations of tracers.…”