Phospho group chirality

Considerable ingenuity was required in both the synthesis of these chiral compounds695 697 and the stereochemical analysis of the products formed from them by enzymes.698 700 In one experiment the phospho group was transferred from chiral phenyl phosphate to a diol acceptor using E. coli alkaline phosphatase as a catalyst (Eq. 12-36). In this reaction transfer of the phospho group occurred without inversion. The chirality of the product was determined as follows. It was cyclized by a nonenzymatic in-line displacement to give equimolar ratios of three isomeric cyclic diesters. These were methylated with diazomethane to a mixture of three pairs of diastereoisomers triesters. These dia-stereoisomers were separated and the chirality was determined by a sophisticated mass spectrometric analysis.692 A simpler analysis employs 31P NMR spectroscopy and is illustrated in Fig. 12-22. Since alkaline phosphatase is relatively nonspecific, most phosphate esters produced by the action of phosphotransferases can have their phospho groups transferred without inversion to 1,2-propanediol and the chirality can be determined by this method. 

Evidence for an in-line SN2-like mechanism of most enzymatic phospho group transfer reactions comes largely from study of chiral phospho groups.663/692-695 A chiral phosphate can be introduced at either the a or (3 phosphorus of ATP by substitution of one of the oxygen atoms by sulfur. A chiral phospho group in the P position can be formed by substituting one oxygen by S and a second by lsO. 

Notice that the negative charge is largely localized on sulfur in these phosphorothioate compounds.696 More general is the use of 170 and lsO to form a chiral phospho group ... 

Figure 12-22 Method for determining chirality of phospho groups containing 160, lvO, and lsO. From Buchwald and Knowles.701...

Most kinases transfer chiral phospho groups with inversion and fail to catalyze partial exchange reactions that would indicate phosphoenzyme intermediates. However, nucleoside diphosphate kinase contains an active site histidine which is phosphorylated to form a phosphoenzyme.869 The enzyme catalyzes phosphorylation of nucleoside diphosphates other than ADP by a nucleotide triphosphate, usually ATP. 

When [180]bicarbonate is a substrate, two labeled oxygen atoms enter the oxaloacetate, while the third appears in P . A concerted, cyclic mechanism could explain these results. However, study of kinetic isotope effects,291 use of a substrate with a chiral thio-phospho group,292 and additional lsO exchange studies293 have ruled out this possibility. A transient carboxyl phosphate (Eq. 13-53) is evidently an intermediate.294 295 The incorporation of the lsO from bicarbonate into phosphate is indicated by the asterisks. 

The first example of a catalytic asymmetric Horner-Wadsworth-Emmons reaction was recently reported by Arai et al. [78]. It is based on the use of a chiral quaternary ammonium salt as a phase-transfer catalyst, 78, derived from cinchonine. Catalytic amounts (20 mol%) of organocatalyst 78 were initially used in the Homer-Wadsworth-Emmons reaction of ketone 75a with a variety of phospho-nates as a model reaction. The condensation products of type 77 were obtained in widely varying yields (from 15 to 89%) and the enantioselectivity of the product was low to moderate (< 43%). Although yields were usually low for methyl and ethyl phosphonates the best enantioselectivity was observed for these substrates (43 and 38% ee, respectively). In contrast higher yields were obtained with phosphonates with sterically more demanding ester groups, e.g. tert-butyl, but ee values were much lower. An overview of this reaction and the effect of the ester functionality is given in Scheme 13.40. 

Takagi, R., Hashizume, M., Nakamura, M., Begum, S., Hiraga, Y., Kojima, S., and Ohkata, K., Stereochemical considerations on the stereoselective cyclopropanation reactions of 3-aryl-2-phospho-noacrylates induced hy the (—)-8-phenyhnenthyl group as a chiral auxiliary, J. Chem. Soc., Perkin Trans. 1, 179, 2002. 

Asymmetric versions of the phospho-Mukaiyama aldol reaction have also been exploited under conditions of stoichiometric stereo control. Thus, the research teams of Shibuya [18] and Spunta [19] independently used stereocontrol within the carbonyl substrate (Scheme 6) whereas the groups of Evans [20], Spilling [21] and Kee [22] preferred to use chiral phosphorus(III) esters (Scheme 7). 

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