Transformation biomimetic

The chelated complex of the benzylamine derivative 505 underwent a remarkable oxidative transformation by treatment with thallium trifluoroace-tate to give narwedine (506) in one step by biomimetic oxidation[459]. 

The biomimetic approach to total synthesis draws inspiration from the enzyme-catalyzed conversion of squalene oxide (2) to lanosterol (3) (through polyolefinic cyclization and subsequent rearrangement), a biosynthetic precursor of cholesterol, and the related conversion of squalene oxide (2) to the plant triterpenoid dammaradienol (4) (see Scheme la).3 The dramatic productivity of these enzyme-mediated transformations is obvious in one impressive step, squalene oxide (2), a molecule harboring only a single asymmetric carbon atom, is converted into a stereochemically complex polycyclic framework in a manner that is stereospecific. In both cases, four carbocyclic rings are created at the expense of a single oxirane ring. 

Tandem reaction strategies can accomplish several synthetic objectives in a single step.6 The rapidity with which they can build up molecular complexity is a most useful and impressive virtue. For example, cation-induced, biomimetic polyolefinic cyclizations7 are among the most productive and atom-economical8 single-step transformations known in organic chemistry. In one of the most spectac-... 

It is probable that the negative charge induced by these three electrons on FeMoco is compensated by protonation to form metal hydrides. In model hydride complexes two hydride ions can readily form an 17-bonded H2 molecule that becomes labilized on addition of the third proton and can then dissociate, leaving a site at which N2 can bind (104). This biomimetic chemistry satisfyingly rationalizes the observed obligatory evolution of one H2 molecule for every N2 molecule reduced by the enzyme, and also the observation that H2 is a competitive inhibitor of N2 reduction by the enzyme. The bound N2 molecule could then be further reduced by a further series of electron and proton additions as shown in Fig. 9. The chemistry of such transformations has been extensively studied with model complexes (15, 105). 

Worobey BL (1989) Nonenzymatic biomimetic oxidation systems theory and application to transformation stndies of environmental chemicals. Handbook Environ Chem 2E 58-110. 

Biosynthesis of the spirobenzylisoquinoline alkaloid ochotensimine (282) via the quinomethide intermediate (Scheme 49) was proposed by Shamma and Jones (7J0). On the basis of this hypothesis, several biomimetic transformations of phenolic protoberberines to spirobenzylisoquinolines have been realized by the base-induced rearrangement via the quinomethide. 

Another biomimetic transformation was reported by Nalliah et al. (155). 13-Oxotetrahydroberberine (ophiocarpinone) metho salt (291), derived from berberinephenolbetaine (121), was irradiated in methanol in the presence of sodium hydride to afford the spirobenzylisoquinoline 294 in 45% yield... 

The reaction of photosynthesis is as follows Biological applications are based primarily on the use of C02 as food for plant growth. In a similar way as the plants take advantage of sunlight and C02 for biomass, or other products, imitating nature, improving its results. Therefore, this technology is also known as biomimetic transformation. 

Knowledge regarding biosyntheses has induced several biomimetic approaches towards steroids, the first examples being described by van Tamelen [10] and Corey [11]. A more efficient process was developed by Johnson [12] who, to synthesize progesterone 0-10 used an acid-catalyzed polycyclization of the tertiary allylic alcohol 0-7 in the presence of ethylene carbonate, which led to 0-9 via 0-8 (Scheme 0.3). The cyclopentene moiety in 0-9 is then transformed into the cyclohexanone moiety in progesterone (0-10). 

Witkop and Goodwyn reported (288) that ozonolysis of yohimbine (74) and its derivatives led to the corresponding quinolone derivatives. This reaction has been thoroughly investigated by Winterfeldt (289). For example, autooxidation of lactam 347 resulted in quinolone 599, which upon treatment with phosphoryl chloride, followed by catalytic reduction, gave pyrrolo[3,4-h]quioline derivative 600 (290). This transformation was also used as a key step in the biomimetic synthesis of camptothecin (601), performed by Winterfeldt et al. (291, 292). 

A fitting way to end this chapter is with Sakai s biomimetic syntheses of 11-methoxykoumine and koumine [492,473]. Thus, the presumed biogenetic intermediate 18-hydroxy-taberpsychine (390), which was synthesized from 18-hydroxygardnutine, was acetylated and transformed into koumine (391) [473]. 

In this approach, the cis-disubstituted tetrahydrofuranone 75 [accessible with high diastereoselectivity via radical cyclization of the acylselenide 74 with tris(tri-methylsilyl) silane and triethylborane] was transformed into the corresponding enyne 76 by reduction and Wittig olefination. The subsequent electrophilic cyclization was carried out in analogy with the biomimetic synthesis of panacene (Scheme... 

Biorefineries New catalytic pretreatment of plant materials Valorization, pretreatment or disposal of co-products and wastes from biorefinery by catalytic treatments New and/or improved catalytic processes for chemicals production through the integration of the biorefinery concept and products into the existing chemical production chain New advanced catalytic solutions to reduce waste emissions (solid, air and, especially, water) New catalysts to selectively de-oxygenate products from biomass transformation Catalysts to selectively convert chemicals in complex multicomponent feedstocks New biomimetic catalysts able to operate under mild conditions Small catalytic pyrolysis process to produce stabilized oil for further processing in larger plants... 

A quite detailed review of transannular cyclizations was published201 wherein their important role in biomimetic syntheses of sesquiterpenes as well as explanation of the biogenetic formation of the polycyclic natural compounds from their monocyclic precursors is discussed. The great significance of these transformations for the synthesis of natural products is also emphasized in a series of reviews which describe the cyclizations to form terpene derivatives, e.g., of the germacrane and humulene systems202-206. 

Recently, Moody et al. reported a biomimetic synthesis of calothrixin B (378) by oxidation of Hibino s 6-formylindole[2,3-fl]carbazole 1555 (870). The key intermediate 6-formyl-indole[2,3-fl]carbazole was readily obtained in six steps from indigo (1458). Using Somei s procedure, indigo (1458) was transformed to the cis-chlorohydrin 1461 in three steps and 50% overall yield (see Scheme 5.247). The reduction of the chlorohydiin 1461 gave 5-hydroxy-indolo[2,3-fl]carbazole 1564, and subsequent Vilsmeier formylation delivered the desired 6-formyl-indole[2,3-fl]carba-zole 1565 in 45% yield. Reaction of hydroxy-indolocarbazole 1565 with an excess of chloromethyl methyl ether (MOMCI) afforded the tiis-MOM-protected compound 1555. Following Hibino s approach, the tris-MOM-protected indolocarbazole 1555... 

Biocatalysis and Biomimetic Systems, Artificial Photosynthetic Transformations Through (Willner and Willner). 

Several cases warrant special mention. As an example, ring opening, hydrogenation, and subsequent transformations of the 5(47/)-oxazolone 459 derived from 2,3-dihydroxy-4-methoxybenzaldehyde affords a biomimetic synthesis of racemic stizolobinic acid 463 as shown in Scheme 7.151. ... 

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