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Oxidation-reduction reactions stereospecificity

We should also expect stereoelectronic control when the hydroxyl group is replaced by another nucleophile in the reaction with cyclic oxonium ions. A recent report (110) shows that hydride transfer to cyclic oxonium ion is subject to stereoelectronic control. Tricyclic spiroketal 140 (Fig. 19) undergoes an acid-catalyzed oxidation-reduction reaction to give the equatorial bicyclic aldehyde 147 stereospecifically. Similarly, spiroketals 148 and M9 gave the corresponding equatorial bicyclic ketone 150. [Pg.28]

It is important to select stoichiometric co-reductants or co-oxidants for the reversible cycle of a catalyst. A metallic co-reductant is ultimately converted to the corresponding metal salt in a higher oxidation state, which may work as a Lewis acid. Taking these interactions into account, the requisite catalytic system can be attained through multi-component interactions. Stereoselectivity should also be controlled, from synthetic points of view. The stereoselective and/or stereospecific transformations depend on the intermediary structure. The potential interaction and structural control permit efficient and selective methods in synthetic radical reactions. This chapter describes the construction of the catalytic system for one-electron reduction reactions represented by the pinacol coupling reaction. [Pg.65]

Candida parapsilosis was found to be able to convert (k)-1,2-butanediol to (S)-l,2-butanediol through stereospecific oxidation and asymmetric reduction reactions [72]. The oxidation of (k)-1,2-butanediol to l-hydroxy-2-butanone and the reduction of l-hydroxy-2-butanone to (S)-l,2-butanediol were cataly-... [Pg.120]

They can also catalyze the opposite reaction, the coupling of quinol oxidation to quinone to the reduction of fumarate to succinate (Lemma et al., 1991). The m-configuration isomer of fumarate, maleinate, is neither produced in the oxidation reaction nor consumed as a substrate in the reduction reaction, i.e, the reaction is stereospecific in both directions. Depending on the direction of the reaction catalyzed in vivo, the members of the superfamily of succinate quinone oxidoreductases... [Pg.131]

The alkene reduction reactions most frequently observed are of a,3-unsaturated aldehydes, ketones, acids and esters. Examples of stereospecific reductions of acyclic substrates are given in Scheme 50.148.157-159 (j, (, e formation of (123), the double bond of (122) is reduced prior to the aldehyde function. The conversion of (124) to (125) involves oxidation of the intermediate alcohol to the carboxylic acid by bubbling air into the fermentation medium. Stereospecific reductions of a, 3-unsaturated ketones may be similarly effected (Scheme 61). The reduction of the chloro ketone (126) gives (127) initially. This epimerizes under the reaction conditions, and each enantiomer is then reduced further to (128) and (129), with the predominance of the (128) stereoisomer increasing with the size of the R-group. Reduction of ( )-(130) leads to (131) and (132). ... [Pg.205]

S)-(+)-carvone. The key steps were Baeyer-Villiger oxidation, Oppenhauer oxidation, Meerwein-Ponndorf-Verley reduction, a stereospecific Grignard addition, and an intramolecular Sn2 reaction. [Pg.29]

Dehydrogenases, reductases and a number of other enzymes, such as UDP-glucose epimerase, utilize NAD or NADP as an enzymatic cofactor and catalyze the oxidation/reduction of various substrates, facilitating the usually reversible stereospecific hydride transfer from the C4 position of the 1,4 dihydronicotinamide ring of NAD(P)H to substrate. The reaction catalyzed by lactate dehydrogenase and a schematic drawing of the putative hydride transfer reaction that takes place are shown in Fig. 15.1. [Pg.1393]

The catalytic mechanism of the asymmetric reduction of alkenes catalyzed by ene-reductases has been studied in great detail [977] and it has been shown that a hydride (derived from a reduced flavin cofactor) is stereoselectively transferred onto Cp, while a Tyr-residue adds a proton (which is ultimately derived from the solvent) onto Cot from the opposite side (Scheme 2.134). As a consequence of the stereochemistry of this mechanism, the overall addition of [H2] proceeds in a trans-fashion with absolute stereospecificity [978]. This reaction is generally denoted as the oxidative half reaction . The catalytic cycle is completed by the so-called reductive half reaction via reduction of the oxidized flavin cofactor at the expense of NAD(P)H, which is ultimately derived from an external H-source via another... [Pg.166]

OYEs are oxidoreductases that catalyze the addition of the elements of hydrogen to an electron-poor olefinic double bond in a stereospecific trans-manner [6]. The hydride is delivered from reduced flavin mononucleotide (FMNH2) in a Michael-type process, the proton being donated by a tyrosine residue [8] or by water [9,10] in the so-called oxidative half-reaction (Figure 5.1). NAD(P)H in the reductive half-reaction ensures reduction of the flavin [6]. Mechanistically... [Pg.113]

Iron-complexed diene systems have a reduced electron density due to 7t-donation to the iron center. This makes them less reactive towards electrophilic attack that stands for the majority of reactions at olefmic systems. But also oxidation, reduction, and cycloaddition reactions proceed more slowly or can be completely suppressed when the diene is ligated to an iron center. Moreover, the iron complex blocks one face of the diene system. Incoming reagents, whether at the diene unit or at the periphery, are directed anti to the iron complex fragment. This allows stereospecific reactions that are otherwise difficult to achieve. The stereodirecting effect can be exploited for reactions... [Pg.636]

A facile method for the stereospecific labeling of carbon atoms adjacent to an oxygenated position is the reductive opening of oxides. The stereospecificity of this reaction is due to virtually exclusive diaxial opening of steroidal oxides when treated with lithium aluminum hydride or deuteride. The resulting /ra/w-diaxial labeled alcohols are of high stereochemical and isotopic purity, with the latter property depending almost solely on the quality of the metal deuteride used. (For the preparation of m-labeled alcohols, see section V-D.)... [Pg.204]

Figure 11-4. Mechanism of oxidation and reduction of nicotinamide coenzymes. There is stereospecificity about position 4 of nicotinamide when it is reduced by a substrate AHj. One of the hydrogen atoms is removed from the substrate as a hydrogen nucleus with two electrons (hydride ion, H ) and is transferred to the 4 position, where it may be attached in either the A or the B position according to the specificity determined by the particular dehydrogenase catalyzing the reaction. The remaining hydrogen of the hydrogen pair removed from the substrate remains free as a hydrogen ion. Figure 11-4. Mechanism of oxidation and reduction of nicotinamide coenzymes. There is stereospecificity about position 4 of nicotinamide when it is reduced by a substrate AHj. One of the hydrogen atoms is removed from the substrate as a hydrogen nucleus with two electrons (hydride ion, H ) and is transferred to the 4 position, where it may be attached in either the A or the B position according to the specificity determined by the particular dehydrogenase catalyzing the reaction. The remaining hydrogen of the hydrogen pair removed from the substrate remains free as a hydrogen ion.
We have developed the efficient synthesis of the SERM drug candidate 1 and successfully demonstrated the process on a multiple kilogram scale to support the drug development program. A novel sulfoxide-directed borane reduction of vinyl sulfoxides was discovered. The mechanistic details of this novel reaction were explored and a plausible mechanism proposed. The sequence of asymmetric oxidation of vinyl sulfoxides followed by stereospecific borane reduction to make chiral dihydro-1,4-benzoxathiins was applied to the asymmetric synthesis of a number of other dihydro-1,4-benzoxathiins including the sweetening agent 67. [Pg.162]


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Oxidative stereospecific oxidation

Reaction oxidation-reduction

Reaction stereospecificities

Reduction stereospecific

Stereospecific oxidation/reduction

Stereospecific reactions

Stereospecific reactions reductive

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