Axial ester

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. 

When racemic 173 (R = Me) was hydrolyzed in the presence of chymo-trypsin, the resulting optically active acid 173 (R = H) exhibited an ORD absorption spectrum characteristic of L-phenylalanine the starting ester possessed an axially oriented carbomethoxy group.338-362 Atropisomerism and conformational asymmetry of a precisely definable nature in a substrate are therefore recognized by chymotrypsin. X-ray diffraction studies confirmed that the chymotrypsin-active isomer has an axial ester moiety in the solid state, and that the ester mutarotates in solution to a CD-inactive isomer, whose ester group is in the equatorial position.363... 

Participation of the C-2 axial ester group cannot be excluded.519 The stereospecific cleavage of l,6-anhydro-3-azido-2-0-benzoyl-3,4-dideoxy-j8-D-xyZo-hexopyranose by antimony pentachloride, followed by the action of methanol, also supposedly proceeds by way of a 1,2-acyloxonium ion.750,751... 

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.

Finally, Mitsunobu reaction of macrocycle 282 with the acid 299 afforded the axial ester, which was hydrogenated using Lindlar catalyst to yield (+)-leucascandrolide A (199) (Scheme 64). 

Substituent polarity effects are apparent in the conformational equilibria of cyclohexyl acetate and monochloro- dichloro-, and trichloro-acetates as evidenced by their H n.m.r. spectra. The inversion barriers decrease, and the proportions of axial ester conformer increase, with successive chlorine substitution of the acetate. The conformations of various trans-1,2-disubstituted cyclohexanes have been examined by n.m.r. and a discussion of the observed inversion barriers in terms of steric and dipole-dipole interactions has been given. A new description of these conformational effects is proposed. 

Coupled P- and H-NMR spectra were simulated using a Laocoon-type spectral-simulation program representative P and H coupling constants derived from this analysis are shown in Table VI. The three-bond coupling constants (particularly V2P 24.5 Hz) for all aryl esters 8a-11a (8, Ar = p methoxyphenyl 9, Ar = phenyl 10, Ar = p-nitrophenyl 11, Ar = 2,4-dini-trophenyl 12, Ar = p-nitrophenyl) are consistent with an axial-ester chair... 

V4P coupling constants ( 0) indicate a gauche conformation of H and relative to phosphorus, confirming the chair conformation assigned to these axial esters (Gorenstein et ai, 1980). 

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