Pyrimidine racemic mixtures

Small fluctuations in the ratio of the two enantiomers are considered to be present in racemic mixtures of chiral molecules [14,101]. We thought that, when a reaction system involves asymmetric autocatalysis with amplification of ee, the initial small fluctuation of ee in racemic mixtures that arises from the reaction of achiral reactants can produce an enantiomerically enriched product. We anticipated that when z-P Zn was treated with pyrimidine-5-carbaldehydes without adding any chiral substance, extremely slight enan-tioenrichment would be induced statistically in the initially formed zinc alkoxide of the alkanol, and that the subsequent amplification of chirality by asymmetric autocatalysis would produce the pyrimidyl alkanol with detectable enantioenrichment (Scheme 19). 

Figure 9 shows the examples of separations of racemic mixtures of methylphenobarbital and mephenytoin performed under optimal conditions available (37). It has been found that ( -CD complexation resuTts in a distinct enantioselectivity in the case of mephenytoin and barbiturates which have a chiral center in the pyrimidine ring. The resolution of barbiturate enantiomers is due to the different stabilities of their diastereo-isomeric -CD complexes, while the separation of mephenytoin enantiomers results from the difference in their adsorption on the RP phase. The latter case should be considered further. It has been already suggested (18) that the adsorption of CD complexes in which guest molecules are entirely immersed in the CD cavity is low on RP phases. The distinct adsorption arises from the part of the molecule which is outside the cavity. Taking into account this fact and the remarkable difference in the adsorption of -CD mephenytoin diastereoisomers one may conclude that a significant difference must exist between immersion of mephenytoin enantiomers in the -CD cavity. 

Dihydropyrimidinases (EC 3.5.2.2) are involved in the reductive pathway of pyrimidine degradation, catalyzing the hydrolysis of 5,6-DHU and 5,6-dihydrothymine to the corresponding Namino adds. However, dihydropyrimidinases have been more commonly known as hydantoinases [32, 33], as this enzyme can be used in the production of optically pure amino acids starting from racemic mixtures of 5-monosubstituted hydantoins using the so-called hydantoinase process, ... 

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