Prochiral stereospecificity

The ability of the microorganism to discriminate between enantiomers is very important regarding resolution and asymmetric synthesis. The ability of microorganisms to exert prochirality stereospecific control in their catalyses overcomes this problem because it permits direct asymmetric synthesis of chiral products from symmetric starting materials. Norcadia corallina -276 is a... 

Enzymes are highly selective of the substrates with which they interact and in the reactions that they catalyze. This selective nature of enzymes collectively known as enzyme specificity can be best illustrated with oxidoreductases (dehydrogenases), which display substrate and bond specificities (e.g., acting on —CHOH—, versus —CHO versus —CH—CH— versus —CHNH2, and cis versus trans for unsaturated substrates), coenzyme specificity (e.g., NAD(H) versus NADP(H)), chiral stereospecificity (d- versus l- or R- versus S-stereoisomers), and prochiral stereospecificity (A versus B corresponding to proR- versus proS isomers and re face versus si face, respectively). The table lists some dehydrogenases and their coenzyme, substrate, product and stereospecificities (You, 1982) ... 

A prochiral monomer molecule is converted to a polymer with chiral groups in prochiral stereospecific or chirality-producing stereospecific polymerizations. An example of this is the polymerization converting benzo-furan to optically active polymers with the catalyst RAlCl2/optically active phenyl alanine. 

Further special cases can be distinguished among the chiral stereospecific and the prochiral stereospecific polymerizations. If one starts with... 

The remarkable stereospecificity of TBHP-transition metal epoxidations of allylic alcohols has been exploited by Sharpless group for the synthesis of chiral oxiranes from prochiral allylic alcohols (Scheme 76) (81JA464) and for diastereoselective oxirane synthesis from chiral allylic alcohols (Scheme 77) (81JA6237). It has been suggested that this latter reaction may enable the preparation of chiral compounds of complete enantiomeric purity cf. Scheme 78) ... 

Possible elements of chirality in stereospecific polymerizations will be briefly recalled in order to indicate the used terminology. First of all, upon coordination, a prochiral olefin such as propene gives rise to not superpos-able si and re coordinations.22 According to the mechanism described, the isotactic polymer is generated by a large series of insertions of all si- or all re-coordinated monomers, while the syndiotactic polymer would be generated by alternate insertions of si - and re-coordinated monomers. 

We can thus deduce that alcohol dehydrogenase stereospecifically removes the pro-R hydrogen from the prochiral methylene. 

During the reduction sequence, NADH transfers a hydride from a prochiral centre on the dihydropyridine ring, and is itself oxidized to NAD+ (nicotinamide adenine dinucleotide) that contains a planar pyridinium ring. In the oxidation sequence, NAD+ is reduced to NADH by acquiring hydride to an enantiotopic face of the planar ring. The reactions are completely stereospecific. 

Note also that transfer of hydride to the carbonyl compound is also stereospecific, as is removal of hydrogen from the prochiral centre of ethanol in the reverse reaction (see Section 3.4.7). 

Worthy of note in this reaction is that citrate displays prochirality (see Section 3.4.7). The methylene carbons may be considered prochiral, in that enzymic elimination of a proton is likely to be entirely stereospecific. In addition, the apparently equivalent side-chains on the central carbon are also prochiral and going to be positioned quite differently on the enzyme. This means that only one of these side-chains is involved in the dehydration-rehydration... 

They are able to polymerize a large variety of vinyl monomers. The polymer microstructure can be controlled by the symmetry of the catalyst precursor. Prochiral alkenes such as propylene can be polymerized to give stereospecific polymers,554 572-574 allowing production of polyolefin elastomers. They can give polyolefins with regularly distributed short- and long-chain branches which are new materials for new applications. 

Define chiral, enantomer, diastereomer, epimer, anomer (see Chapter 4), prochiral (see Chapter 9). What is meant by the statement that biochemical reactions are stereochemically specific Why is such stereospecificity to be expected in organisms (which are constructed from asymmetric units) See Chapter 9 for further discussion. 

R0P032 is pra-prochiral, since two oxygen atoms must be substituted to produce a chiral compound. Chiral phosphates have been synthesized de novo by using stereospecific chemical and enzymatic reactions with isotopic and/or atomic substitutions. For example, a chiral phosphorothioate may be synthesized from a prochiral phosphate by replacing an oxygen atom with a sulfur atom. Similarly, what would otherwise be a pro-prochiral phosphate has been synthesized as a chiral product by replacing one oxygen atom with sulfur and another... 

Ogston s contribution led to an interesting extension of concepts concerning stereochemistry of enzyme action. Compounds of the type Ca2bd are termed prochiral, and it is recognized that an enzyme that either synthesizes such a compound or uses it as a substrate nearly always does so stereospecifically. In the case of citrate synthase, for example, it is inherently likely that the planar carbonyl carbon of oxaloacetate lies flat on an enzyme surface and that only one side of the atom is available for attack by acetyl-coenzyme A. 

The molecule in Figure I is an example of a prochiral molecule. The asymmetrical enzyme binds only to hydrogen a and not to hydrogen b due to the spatial arrangement of its active site with respect to the other substituents on the prochiral carbon. All known dehydrogenases are stereospecific in this manner. 

Hill, R. K. Enzymatic Stereospecificity at Prochiral Centers of Amino Acids, in Bio-organic chemistry, Vol. 2, p. 111. (van Tamelen, E. E. ed.). New York Academic Press 1978... 

Alphand V, Mazzini C et al (1998) A new microorganism for highly stereospecific Baeyer-Villiger oxidation of prochiral cyclobutanones. J Mol Catal B Enzym 5 219-221... 

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