Propionic acids, alkoxy-, esters

Further work on the preparation of cryt/iro-2-alkyl-3-hydroxy-esters (140) by various condensations between propionic acid derivatives and aldehydes has been reported " the use of zirconium enolates seems to be particularly efficacious. Rules for predicting the stereochemical outcome of condensations between lithium enolates of esters and ketones and a-alkoxy-aldehydes have also been delineated. Pure erythro-isomer (140) can also be obtained in some cases by reduction of the corresponding jS-keto-ester with zinc borohydride. In related work it has been found that sodium borohydride in isopropanol reduces t-butyl a-alkoxy-j8-keto-esters to the corresponding -hydroxy compounds with erythro-threo ratios of between 2 1 and 20 1 in favour of the eryt/iro-isomer. In an extension of his previous work, Frdter has reported that dianions derived from cyclohexanol (141) can be alkylated with 95% stereoselectivity, to give (142). When the starting alcohol (141) is optically pure, a sequence of alkylation and oxidation leads to 2-ethoxycarbonylcyclohexanones with 76% enantiomeric enrichments. 

These tri(alkoxy)titanium enolates, which have low Lewis acidity, are known to react chemoselective-ly with an aldehyde group in the presence of a ketone (equation 4). Other uses described by Reetz et al. include the diastereofacially selective additions of ketone and ester enolates to chiral a-alkoxy aldehydes with nonchelation control. - For example, aldol addition of the tri(isopropoxy)titanium enolate of pro-piophenone to the aldehyde (24) leads to only the two syn diastereomers, with the nonchelation adduct (25) favored (equation 5) i.e. Felkin-Anh selectivity is operating. In the case of aldol addition of t-butyl propionate to the same aldehyde (equation 6), highest stereoselectivity for the isomer (26) is obtained using the tri(diethylamino)titanium enolate. Very high levels of nonchelation stereoselectivity can also be obtained in the aldol addition to chiral a-siloxy or a-benzyloxy ketones if a titanium enolate of low Lewis acidity is employed, as in equation (7). ... 

So far we have been considering the chemical ionization behavior only of various types of hydrocarbons, but now we consider nonhydrocarbons, starting with esters of carboxylic acids. As a typical example, the spectrum of n-heptyl propionate is given in Table IX. For a molecule as polar as a carboxylate ester the concept of a random attack by the reactant ions must be abandoned, and the attack will doubtless occur at the carb-alkoxy group in the molecule. 

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