Reductive condensation

The alcohol-unsaturate couplings developed in our laboratory provide products of carbonyl addition. In contrast, related hydrogen auto-transfer processes provide products of alcohol substitution via pathways involving oxidation-condensation-reduction and the use of preactivated nucleophiles. For recent reviews, see [22-25]. 

The reaction of an orf/io-ester, e.g., HC(OEt)3, with a secondary bisamine in the presence of an anunonium salt yields imidazolidinium salts (Scheme The necessary secondary diamines can be generated by a classical condensation-reduction sequence or by applying the palladium-catalyzed Buchwald-Hartwig amination." The latter reaction offers convenient access to imidazolidinium salts with chiral backbones starting from chiral diamines, a number of which are commercially available. ... 

The types of compound obtainable by the condensation of tellurium tetrachloride and mixed ethers may be exemplified by the case of phenetole. The primary product obtained in the reaction is p-phenetyl telluritriehloride (I), which on heating with more phenetole gives Ws-p-phenetyl telluridichloride (II). The latter also occurs if an excess of phenetole be used in the initial condensation. Reduction of the trichloride by aqueous potassium metabisulphite at 0° C. produces bis-p-phenetyl ditelluride (III). 

Figure 8.3 Steps in fatty acyl-CoA elongation condensation, reduction, dehydration and reduction. The condensation step appears to regulate chain length specificity.

Figure 22.22. Fatty Acid Synthesis. Fatty acids are synthesized by the repetition of the following reaction sequence condensation, reduction, dehydration, and reduction. The intermediates shown here are produced in the first round of synthesis.

Representative examples of the reactions of various carbonyl compounds with Grignard reagents under these conditions are listed in Table 5. It is emphasized that enolization, aldol reaction, ester condensation, reduction and 1,4-addition are remarkably suppressed by the use of cerium chloride. Various tertiary alcohols, which are difficult to prepare by the conventional Grignard reaction, can be synthesized by this method. 

Despite their enormous structural diversity, polyketide metabolites are related by their common derivation from highly functionalised carbon chains whose assemblies are controlled by multifunctional enzyme complexes, the polyketide synthases (PKSs) which, like the closely related fatty acid synthases, catalyse repetitious sequences of decarboxylative condensation reactions between simple acyl thioesters and malonate, as shown in Fig. 3 [7]. Each condensation is followed by a cycle of modifying reactions ketoreduction, dehydration and enoyl reduction. In contrast to fatty acid biosynthesis where the full cycle of essentially reductive modifications normally follow each condensation reduction, the PKSs can use this sequence in a highly selective and controlled manner to assemble polyketide intermediates with an enormous number of permutations of functionality along the chain. As shown in Fig. 3, the reduction sequence can be largely or entirely omitted to produce the classical polyketide intermediate which bears a carbonyl on every alternate carbon and which normally cyclises to aromatic polyketide metabolites. On the other hand, the reductive sequence can be used fully or partially after each condensation to produce highly functionalised intermediates such as the Reduced polyketide in Fig. 3. Basic questions to be answered are (i) what is the actual polyketide intermediate... 

As indicated in Scheme 12, the labelling studies are consistent with formation of triene (36) from thioesters (34) and (35) which themselves may be built up by the assembly sequence shown. Diol (33), in which the stereochemistry at C-3 is unknown, is proposed as a common intermediate, elimination to give either an E or Z alkene leading respectively to (34) or, after further condensation, reduction and elimination, to (35). Experiments to test this sequence with the putative intermediates fed in the form of their N-acetylcysteamine (NAC) thioesters have yet to be completed, but, significantly, feeding studies in which the putative triketide (34) has been fed as the NAC thioester, as the ethyl ester or... 

Fatty Acid Synthesis Consists of a Series of Condensation, Reduction, Dehydration, and Reduction Reactions... 

The commonest naturally-occurring inositol is myu-inositol, and its structure is readily memorised because in the preferred conformation only one hydroxyl group is axial. Its 1-L phosphate ester is biosynthesised from its isomer glucose-1-phosphate by an oxidation-intramolecular aldol condensation-reduction sequence, see Section 6.8.4. 

ER fatty acid chain elongation, which uses two-carbon units provided by mal-onyl-CoA, is a cycle of condensation, reduction, dehydration, and reduction reactions similar to those observed in cytoplasmic fatty acid synthesis. In contrast to the cytoplasmic process, the intermediates in the ER elongation process are CoA esters. These reactions can lengthen both saturated and unsaturated fatty acids. Reducing equivalents are provided by NADPH. 

This hydrocarboration method is a valuable tool in industrial and laboratory synthesis, since it allows introduction of the one-carbon unit of carbon monoxide into unsaturated substrates and construction of new carbon skeletons with aldehyde functions or derivatives thereof formed by reduction, oxidation, condensation and other conversions. Hydroformylation, mainly catalyzed by cobalt, rhodium, or platinum complexes is an unsymmetrical 1,2-addition leading to linear and branched products if terminal olefins are used as the substrate. Since linear products are normally the industrial products wanted54, considerable efforts have concentrated on the control of regiochemistry. Other problems of the hydroformylation method arise from side reactions such as hydrogenation, double bond migration, and subsequent reactions of the products (e.g., condensation, reduction, dccarbonylation)54. 

Condensation (reduction) of the stiffness matrix for the resolution of a static problem is a mathematically exact procedure. Let us assume that the degrees of freedom are ordered in such a way the first are to be maintained and the last to be deleted. If subindex c denotes the degrees of fi eedom to be maintained, and e those to be deleted, the static problem can be stated as ... 

Basically, this reaction consists of elongating the acetyl group by C2 imits derived from malonyl-CoA, so that each step takes place by condensation, reduction, dehydration and further reduction [99] (Figura 6) ... 

Other methods are known, sometimes technically very simple, such as "dry" stirring in the absence of any solvent or reagent. More sophisticated are the transformation of a metal into finely divided particles by vaporization and low temperature condensation, reduction of a salt, etc. Reviews and a recently published book provide detailed analyses in these domains with their respective advantages and limitations. 

In strong contrast to 1,4-diene [86], the corresponding divinylsilanes (X = Si) produce aminomethyl-substituted silacyclohexanes via a hydroformylation-aldol condensation-reductive amination pathway (Scheme 5.110) [94]. In case of R H, in the final product both substituents adopt trans geometry to each other. Such silacyclohexanes display spasmolytic and antiarthritic activity. 

Operation of the FAS system involves an enzyme-catalyzed cycle of condensation, reduction, dehydration and a second reduction (Figure 3.2), and is dependent upon the supply of a carbon source and energy provision in the forms of ATP, NADH and NADPH. The extreme importance of acetyl-CoA carboxylase activity to the functioning of FAS may be deduced from the observed close proximity of each system to each other in plastidic membranes of maturing rape seed (Slabas and Smith, 1988). 

Many of the metabolic pathways are significantly conserved between yeast and mammalian cells (Figure 2). The first three steps of de novo ceramide synthesis, the condensation, reduction and acylation, are almost identical in these organisms except that yeast fatty acyl-CoAs are elongated into very long-chain fatty acyl-CoAs prior to subsequent incorporation into dihydroceramide. 

The condensation, reduction, cyclisation and methylation steps shown above result in an a-methoxy-y-pyrone unit appended to a linear polyketide-derived side-chain of varying degrees of reduction. However, the majority of y-pyrone-containing natural products isolated from sacoglossan molluscs do not possess linear side-chains, instead exhibiting complex carbon frameworks commonly characterised by cyclic motifs. For example, 9,10-deoxytridachione (2) [13] possesses a 1,3-cyclohexadiene unit, whilst ocellapyrone A (5) is endowed with a bicyclo[4.2.0]octadiene appendage (Fig. 1.1). 

In 2001, Hinterding and coworkers reported on a Claisen condensation/reduction sequence. They used an intramolecular Claisen-type reaction to cleave off the Evans... 

SCHEME 2.143 Claisen-like condensation/reduction sequence. 

SCHEME 2.102 Lewis acid/chiral Br0nsted acid-catalyzed Friedlander condensation-reduction cascade reaction. 

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