Biogenetic route

Scheme 1 Proposed biogenetic route for the formation of 8 and 9 as the putative precursors of xestospongin and araguspongine alkaloids...

The synthesis of corynantheine-type alkaloids along a biogenetic route has been widely investigated by Brown et al. (175, 176). 

A new biogenetic route to the aporphines has been proposed which does not involve phenolic oxidative coupling, and which proceeds through the intermediacy of protoberberinium salts.6 The alkaloid polycarpine (44) must be derived biogenetically from palmatine (45), and indeed oxidation of (45) in vitro with m-chloroperoxybenzoic acid, followed by hydrolysis, leads to polycarpine (44). Since it is known that photocyclization of simple benzylisoquinoline... 

An approach to the cephalotaxine skeleton, based upon the presumed biogenetic route, has been reported (164) and involves the oxidation of the 1-phenethylisoquinoline derivative (260) with VOF3 (Scheme 54). Alkaline cleavage of the dienone (261) gave 262 which, as the hydrochloride salt, was reduced 263. A-trifluoroacetylation followed by O-methylation yielded 264 which, after hydrogenolysis to 265, was oxidized with potassium ferricyanide to give the dienone (266). 

The dibenzazonines are intermediates in the biosynthesis of Erythrina alkaloids (72). Several biogenetic routes have been proposed to explain the formation of dibenzazonines in plants. 

The defence secretion of the milliped Polyzonium rosalbum contains the spiro-monoterpenoid polyzonimine (35) 97 the isolation of the spirocyclic nitro-compound (36)98 might suggest a non-geranyl pyrophosphate biogenetic route from a pyr-rolizidine alkaloid. The production of verbenone in the bark beetle Dendroctonus sp. may be associated with chemical communication.99... 

The proof of structure of dehydroesermetholemethine gave considerable further support to the 3a-methylpyrrolo[2,3-6]indole structure for physostigmine proposed by Robinson. The Robinson structure for the alkaloid also was derived easily from tryptophan, thus offering an obvious biogenetic route of synthesis by the plant (this will be discussed in more detail later). 

The synthetic pathway (Chart IV) leading to dihydroeinchon-amine is probably a reversal of the biogenetic route. The same authors have also synthesized 10-methoxydihydrocinchonamine (30, 31). In none of these syntheses were any of the 3-epi compounds isolated. 

Rauwolfia species have been the source of other bases closely related to ajmaline, and the structures of some of them can be regarded as trapped intermediates of the plausible, but as yet still hypothetical, biogenetic route to the major alkaloid, ajmaline itself. 

Mixtures of enantiomers rather than optically pure compounds can be rationalized by different assumptions (a) one enzyme with low enan-tioselectivity catalyzes the biogenetical process, (b) at least two enzymes with different enantioselectivities compete in the reaction, and (c) different pathways eventually leading to the same final products, however with opposite configurations, are involved. By addition of chemically synthesized precursors we aimed to trace some of the biogenetical routes involved in the biogenesis of chiral pineapple volatiles. 

Although biosynthesis of the phenethylisoquinoline alkaloids has not yet been studied in full, that of androcymbines, homoaporphines, and homoerythrinans has been examined by work with radioactive tracers. In this section tracer experiments as well as hypothetical biogenetic routes in the synthesis of the phenethylisoquinoline alkaloids are discussed. 

Follow Nature s Biogenetic Routes to Avoid Protecting Croups 221... 

Comparison of structures (17) and (1) clearly shows that the former could serve as a biogenetic model for the first step of the lycodoline to serratinine rearrangement (1)— (2). In point of fact, treatment of (17) with acid gave compound (18), thus lending some credence to the proposed biogenetic route.5... 

In a recent paper, Lounasmaa and Hanhinen (204) presented evidence to suggest that the bond formation between C-5 and C-16 takes place not after, as had been generally accepted (222-224), but before the D-ring formation (151 - 155— 156, Scheme 20). Once intermediate 156 is formed, transformation to sarpagine (and eventually to ajmaline) structures can take place by normal biogenetic routes (e.g., 156—> 157 —> 158, Scheme 21). 

The ability of plants to utilize both the major biogenetic routes (to CCLXIX and CCLXX) is amply demonstrated by the various Stryehnos species, some of which elaborate the strychnine family of alkaloids, some of which (the South American Stryehnos) elaborate yohimbine types (curare alkaloids). Even more striking is the biosynthesis in the one plant Gdsemium sempervirens (not a Stryehnos) of both gelsemine... 

Enantioselectivity of Biogenetic Routes. The enantiomeric composition of natural aroma compounds is known to reflect the enantioselectivity of their biogenesis (22). Thus, by elaborating the chiral composition of Cp-norisoprenoids in fruits, we expected to obtain some information about step II m the formation of Ci3-notiso-prenoids, i.e. the enzymatic transformation(s) of the primary carotenoid degradation products into labile aroma precursors (cf. Fig. 3). 

A possible biogenetic route to triterpene quinones with an extra double bond (e.g., netzahualcoyone and derivatives) may involve dehydrogenation and subsequent pristimerin regrouping. Indeed, (Table V) two epimeric di-triterpene quinone ethers umbelatin a and p (118,119) have been isolated from the roots of Rzedowskia tolantonguensis [68]. The structures of pristimerin and netzahualcoyone were ascertained by the standard spectral methods. The H... 

What is known about the biogenetic routes leading to the biosynthesis of the dimeric akaloids vinblastine and vincristine in C. roseus is represented in Fig. (4) to (6). Enzymes and genes that have been characterized are indicated, and the subcellular compartmentalization of the pathway is presented in Fig. (3). 

Withajardins are closely related to acnistins, in this case C-21 is directly bonded to C-25 giving rise to a bicyclic lactone side chain with a six-membered homocycle. Withajardins A-E (113-117) were isolated from plants of Deprea orinocensis (Kunth) Raf. collected in El Jardin, Colombia. A common precursor has been proposed in the biogenetic routes to acnistins, withajardins and withametelins [55-57]. 

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