Hydrolysis separating diastereomers

Three general methods exist for the resolution of enantiomers by Hquid chromatography (qv) (47,48). Conversion of the enantiomers to diastereomers and subsequent column chromatography on an achiral stationary phase with an achiral eluant represents a classical method of resolution (49). Diastereomeric derivatization is problematic in that conversion back to the desired enantiomers can result in partial racemization. For example, (lR,23, 5R)-menthol (R)-mandelate (31) is readily separated from its diastereomer but ester hydrolysis under numerous reaction conditions produces (R)-(-)-mandehc acid (32) which is contaminated with (3)-(+)-mandehc acid (33). 

Acetylsultam 15 is also used for stereoselective syntheses of a-unsubstituted /1-hydroxy-carboxylic acids. Thus, conversion of 15 into the silyl-A/O-ketene acetal 16 and subsequent titanium(IV) chloride mediated addition to aldehydes lead to the predominant formation of the diastereomers 17. After separation of the minor diastereomer by flash chromatography, alkaline hydrolysis delivers /f-hydroxycarboxylic acids 18, with liberation of the chiral auxiliary reagent 1919. 

Although it is claimed that the Strecker reaction of 2 results in the exclusive formation of one isomer of 3 and that selective elimination of the minor isomer during isolation of the intermediate compound 4 is not possible, it is apparent that during the workup of the hydrolysis product 4, fractional precipitation or crystallization or other separation of the diastereomers may... 

In contrast to hydrolysis and photolysis, an enantioselective and/or regioselective degradation of pyrethroids is expected since various kinds of enzymes in microbes participate in their metabolism in soil and sediment. The diastereomers should be separated by using a chiral GC or HPLC to examine the fate of each isomer [85]. Sakata et al. [86] have conducted the first extensive aerobic soil metabolism study... 

Eaton and Leipzig chose to resolve racemic trishomocubanone by reaction with /-ephedrine and separation of the diastereomers by fractional crystallization. Subsequent acid hydrolysis delivered the enantiomeric ketones Successive removal of a diagonal CHj bridge from —)-451 furnishes (—)-ditwist-brendane (Cj symmetry) 454) and (—)-twist-brendane C symmetry) 455). Also,... 

In Fig. 3 the sequences of 12 modified as well as two natural peptides which serve as control sequences are given [54]. The diastereomers of all Tfm-substituted peptides were separately studied regarding their protease stability. The results of this hydrolysis study are summarized in Fig. 4. 

The imidazolidine-4-one 29, obtained by heating the methanol solution of an a-aminoamide and a 4-substituted benzaldehyde, is a mixture of two diastereomers which can be separated by chromatography. They are oxidized by MCPBA, separately or as a mixture, into the two diastereomeric l-hydroxyimidazolidine-4-ones 30. Hydrolysis of 30 by ethanolic HC1 and hydroxylamine hydrochloride gives the optically pure TV-hydroxyamino acid amide 31 (Scheme 9).[12]... 

The difference between the solvolysis rate of diastereomers 40 and 40a made it possible to purify and isolate the former isomer by partial hydrolysis of their mixture5813. A similar approach allowed earlier24 the separation and purification (up to 99.9%) of both isomers of some l-organyl-3,7,10-trimethylsilatranes. This was performed by partial... 

It is important to note that 5,10-dideaza-5,6,7,8-tetrahydrofolic acid (369) is, as prepared above by catalytic hydrogenation of (388) (followed by saponification), a mixture of two diastereomers (dl and ll). Their separation was achieved by reaction of (369) with ( + )-10-camphorsulphonic acid to give a mixture (ddl and dll) of salts separated by fractional crystallization. Subsequent hydrolysis then gave (369) (isomer A,[a]589 = +31.09) and (isomer B, [a]S89 = -21.06). The initially isolated mixture of diastereomers had [a]s89 = + 10-22. 

Formation of (R)- and (S)-2-butyl tartrate. The reaction of a pure enantiomer of one compound with a racemic mixture of another compound produces a mixture of diastereomers. Separation of the diastereomers, followed by hydrolysis, gives the resolved enantiomers. 

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