Equatorial ester

The taloside derivatives can be acquired starting from 34 and 36 via the inversion of the 2-position, or starting from 37 via the double parallel inversion of 2- and 4-positions, if the equatorial ester group in the 3-position is able to activate the epimerization of the neighboring 2- and 4-positions at the same time (Figure 15). On the other hand, the... 

The axial or equatorial nature of a substituent has a bearing on its reactivity, or ability to interact with its environment. Equatorial substituents are more stable and less reactive than their axial counterparts. For example, equatorial carboxyl groups are stronger acids than axial ones because of the higher stability of the carboxylate ion, whereas equatorial esters are hydrolyzed more slowly than axial ones because they are less accessible to protons or hydroxyl ions during acid- or base-catalyzed hydrolysis. 

Extended reaction times can lead to the production of the 6,6-dibromides in considerable proportions, not only from such compounds as 1,6-anhy-dro-2,3,4-tri-0-benzoyl-/ -D-gulose (equatorial ester group at C-3),42 but also from such less reactive compounds as, for example, 2,3,4-tri-O-acetyl-1,6-anhydro- D-glucose.40... 

King and All butt (60, 62) have described the stereoselective hydrolysis of the dioxolenium ion 98 derived from a trans-decalin. Mild acid hydrolysis of 98 afforded almost entirely the axial ester product 99 with only a trace (<0.5%) of the equatorial ester 100. A similar result was obtained by using mild basic conditions. The authors further established by equilibration studies that the equatorial ester 100 is the most stable isomer, demonstrating that the formation of the axial isomer 99 is subject to kinetic rather than thermodynamic control. Essentially identical results were produced with two other dioxolenium salts derived from steroids. 

Fig. 7. 80.9 MHz "P NMR spectra of ethyl esters of cyclic 2 -deoxy-AMP prepared from equimolar mixtures of unlabeled cyclic 2 -deoxy-AMP and the cyclic 2 -[ 0]deoxy-AMP samples. (Top) 0-Labeled ester from the axial P-anilidate. The approximate chemicai shift of the unlabeled diester is 3.0 ppm, that of the equatorial ester is 4.5 ppm, and that of the axial ester is 6.5 ppm. From Ref. 25.

However, Crich and co-workers reported that there was no evidence to support the remote participation by an 0-3 equatorial ester, by 0-4 axial or equatorial esters, or by 0-6 esters, and concluded that the remote participation by esters at these positions does not occur under typical glycosylation conditions [65]. 

A new synthesis of the antineoplastic compound, podophyllotoxin (57), has been reported. It scores over previous approaches in that the problem of epimerization of the C-2 carboxyl group is avoided by the use of the acetonide protecting group in the key intermediate (56). This controlling element forces (56) into a conformation with axial phenyl and equatorial ester, thus removing the usual driving force for epimerization at C-2. 

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