Stereospecific transfer

The ready availability of the transketolase (TK E.C. 2.2.1.1) from E. coli within the research collaboration in G. A. Sprenger s group suggested the joint development of an improved synthesis of D-xylulose 5-phosphate 19, which was expensive but required routinely for activity measurements [27]. In vivo, transketolase catalyzes the stereospecific transfer of a hydroxyacetyl nucleophile between various sugar phosphates in the presence of a thiamine diphosphate cofactor and divalent cations, and the C2 donor component 19 offers superior kinetic constants. For synthetic purposes, the enzyme is generally attractive for its high asymmetric induction at the newly formed chiral center and high kinetic enantioselectivity for 2-hydroxyaldehydes, as well as its broad substrate tolerance for aldehyde acceptors [28]. 

But the results provided an exciting bonus. The reactions proved to be stereospecific. NAD+ that has been reduced with dideuterioethanol has both a hydrogen atom and a deuterium atom in the 4-position of the pyridine ring, but reductions of pyruvate or of acetaldehyde with NAD(D) prepared enzymically in this way quantitatively transfer deuterium, and leave the hydrogen atom in place. The 1954 diagram showing the stereospecific transfer of hydrogen from the 4-position of NADH is presented in (5). 

Nicotinamide-dependent enzymes operate in a highly stereospecific manner. This phenomenon was first demonstrated for alcohol dehydrogenase which catalyzes the direct and stereospecific transfer of the pro-(R) hydrogen at C-l of ethanol to the re face of NAD+, or, in the reverse direction, the pro-(R) hydrogen of NADH to the re face of acetaldehyde (equation 2) (B-71MI11001, B-79MI11000). Many other nicotinamide-dependent... 

Serine as a Cl donor. For many organisms, from E. coli to higher animals, serine is a major precursor of C-l units.395-397 The (3-carbon of serine is removed as formaldehyde through direct transfer to tetrahydro-folate with formation of methylene-THF and glycine (Eq. 14-30, Fig. 15-18, step c). This is a stereospecific transfer in which the pro-S hydrogen on C-3 of serine enters the pro-S position also in methylene-THF.398 The glycine formed in this reaction can, in turn, yield... 

A 3a-hydroxysteroid dehydrogenase active on 7a-hydroxy-5 -cholestan-3-one and 7a,12a-dihydroxy-5 -cholestan-3-one (cf. Fig. 3), was partially purified (about 300-fold) from rat Uver cytosol by Berseus [112,113]. NADPH was required as cofactor and hardly any activity was observed with NADH. The preparation was also active towards 3-oxo steroids of the Cjg, C21 and C24 series, and in these cases appreciable activity was obtained also with NADH. The mechanism of reduction involves a stereospecific transfer of a hydride ion from the 4A position of NADPH to the 3)S position of the steroid [114],... 

In the microsomal 5 a saturation of the double bond in 7a-hydroxy-4-cholesten-3-one there is a stereospecific transfer of a hydride ion from the 4B position of NADPH to the 5 a position of the steroid [169]. From the results of experiments on the stereochemistry of the addition of the proton to the 4 position, it was concluded that the reduction of the double bond is likely to involve a non-stereospecific addition of hydrogens or a cis addition rather than a trans addition. 

J -3-Keto isomerase catalyzes the isomerization of J -3-ketosteroids to zl -3-ketosteroids by stereospecific transfer of a hydrogen atom from C(4) to C(6). There is considerable evidence that it is the 40- and 6/5-hydrogens that are involved and that the reaction proceeds via an enolic intermediate. A low resolution (6 A) crystal structure determination has been published and the probable steroid-binding site identified via a bound inhibitor, 4-acetoxy-mercuric estradiol. The results of a higher resolution study (2.5 A) combined with the results of NMR studies and analysis of activity of mutant forms of the enzyme have helped to further define the probable active site of the enzyme [64]. 

The different conformations of the 4-ene-3-one A-rings of many hormonal steroids have been discussed in Section 3.2 (see Fig. 3.2.3). A 5-ene-3-one isomerase produces these steroids by a stereospecific transfer of a hydrogen atom from C4 to C6. The 4 - and 6P hydrogens are involved, and the reaction proceeds via an enolic intermediate. The isomerase is an elongated dimer in which the steroid is entrapped in a barrel of eight 3-strands (Fig. 9.6.4). 

The biosynthesis of sialic acid conjugates requires three dedicated en2ymatic steps along the general Leloir pathway, which consist of a carbohgation step to construct the special extended sialic acid backbone, followed by nucleotide activation of the sugar and its regio- and stereospecific transfer to a suitable acceptor moiety (Scheme 17.1). For the most common sialic acid, namely NeuSAc, synthesis... 

Palladium-catalyzed retro-allylation of homoallylic alcohols is extended to retro-propargylation of homopropargylic alcohols [20]. Despite the rigid and linear structure of the triple bond, the reactions of aryl halides with homopropargylic alcohols necessitate slightly modified conditions to afford allenylated arenes (Scheme 5.25). Not only gewi-disubstituted allenes but also tri- and tetrasubstituted ones are accessible. The reactions with diastereomerically pure homopropargylic alcohols threo- and erythro-17 result in stereospecific transfer to afford anti- and syn-18, respectively (Scheme 5.26). 

The oxidized and reduced forms of the coenzyme are drawn using a shorthand in which the R group attached to the heterocyclic ring represents a part of the molecule that does not directly participate in its reactions. The oxidation or reduction of the coenzyme occurs within the stx-membered pyridine ring. The pyridinium ion ring system of NAD is prochiral. The enzyme stereospecifically transfers a hydride ion from ethanol to the r -face and gives a product, NADH, that is also prochiral. 

In the reverse reaction, the reduction acetaldehyde by (/ )-NADH-4-zif. Only the deuterium is transferred yielding (J )-l-pH]-ethanol even though cleaving a C—D bond requires more energy than cleaving a C—H bond. We conclude that the arrangement of the reactants within the reactive site of the enzyme places the deuterium in a position to be transferred to the carbonyl carbon atom. The plane of NAD and the plane of acetaldehyde are arranged so that the two re faces are next to each other. As a consequence, the deuterium is stereospecifically transferred. 

Enolsilane derivatives provide siloxyallyl cations for [4+3] cycloaddition having a known and robust M. The stereospecific transfer of preformed enolsilane geometry to cycloadducts also enable the stereochemistry of cycloadducts to be controlled when the enolsilanes are prepared. These factors contribute to afford cleaner cycloadditions. 

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