Optically pure secondary alcohols, formation

Formation of Optically Pure Secondary Alcohols bv Yeast Alcohol Dehydrogenase... 

Capillary gas chromatographic determination of optical purities, investigation of the conversion of potential precursors, and characterization of enzymes catalyzing these reactions were applied to study the biogenesis of chiral volatiles in plants and microorganisms. Major pineapple constituents are present as mixtures of enantiomers. Reductions, chain elongation, and hydration were shown to be involved in the biosynthesis of hydroxy acid esters and lactones. Reduction of methyl ketones and subsequent enantioselective metabolization by Penicillium citrinum were studied as model reactions to rationalize ratios of enantiomers of secondary alcohols in natural systems. The formation of optically pure enantiomers of aliphatic secondary alcohols and hydroxy acid esters using oxidoreductases from baker s yeast was demonstrated. 

This resolution method was found to be appUcable to some cyanohydrins 15 and secondary alcohols 16. Very interestingly, the racemic cyanohydrins were converted into a pure optically active isomer in almost quantitative yield in the presence of 72 For example, when a solution of racemic l-cyano-2,2-dimethy 1-1-phenyl-propanol 15a) (1.0 g, 5.3 mmol) and 72 (2.1 g, 5.3 mmol) in methanol (2 ml) was kept in an uncapped flask for 24 h at room temperature, a brucine inclusion compound of 94% ee of +yi5a was obtained in quantitative yield, which upon decomposition gave 94% ee of - -)-15 a (1.0 g). Repeating inclusion formation of the 94 % ee containing (ri- )-75a (1.0 g) Avith 72(2.1 g) one more time yielded 100 % ee of (+)-15a [1.0 g, [a]jj -1- 15.9° (c 1.0 in MeOH)]. The process of the complete conversion of racemic cyanohydrin to one enantiomer consists of racemization of cyanohydrin through the equilibrium in Equation 1 and selective inclusion of one enantiomer in brucine... 

Preparation.—From Alcohols or other Halides. Factors affecting the formation of isomerically and optically pure alkyl halides from saturated aliphatic alcohols have been discussed by Hudson and co-workers. - Reactions with thionyl chloride give reduced amounts of rearrangement products if pyridine hydrochloride is added, and isomerically pure chlorides RCl from almost all alcohols ROH if HMPT or DMF is the solvent. In the latter medium (58) is a presumed intermediate, as is the case in the reaction of alcohols with Vilsmeier reagents (59, X = Cl or Br). Such species produce halides via inversion of configuration from secondary alcohols, presumably according to Scheme 24. In the related reaction of PCI3 with unhindered primary alcohols in DMF to produce alkyl... 

When one needs to determine the optical purity of a compound that is not amenable to salt formation (i.e., not a carboxylic acid or amine), analysis by NMR becomes slightly more difficult. It is frequently necessary to determine the enantiomeric excesses of chiral secondary alcohols, for example. In these cases, derivatization of the alcohol through covalent attachment of an optically pure auxiliary provides the mixture of diastereomers for analysis. This requires reacting a (usually small, a few milligrams) sample of sample alcohol with the optically pure derivatizing agent. Sometimes, purification of the products is necessary. In the example shown below, a chiral secondary alcohol is reacted with (5)-2-methoxyphenylacetic acid [(5)-MPA] using dicyclohexylcarbodiimide (DCC) to form diastereomeric esters. After workup, the NMR spectrum of product mixture is acquired, and the res-... 

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