Substitution of acyclic sugar acetals. Is the acid-catalyzed substitution of erythro and threo l,2,3,4,5-penta-O-acetyl-l-arabinose S-ethyl monothioacetals anchimerically assisted?

Abstract

The rate constants for the inversion and for the '4C-acetoxy exchange of the diastereomerjc 1,2,3,4,5-penta-0-acetyl S-ethyl monothioacetals show that the substitution is not anchimerically assisted by the Cz-acetoxy group. The substitution of acyclic,poly-0-acetyl sugar monof acetals is best explained by the formation of the acyclic intermediate ' CGX-R (X = S or 0)in the rate-determining step. /Canadian Journal of Chemistry. Volume 45, 1473Volume 45, (1967 We have recently reported the rate constants for the substitution of one group bonded to C1 of poly-0-acetyl acyclic sugar acetals when the reaction was carried out in acetic acid solutions containing perchloric acid (1, 2). In all cases an acetoxy group from the solvent replaced a group bonded to the anomeric carbon. The activated species was considered to be the substrate conjugate acid (formed by a rapid acid-base equilibrium), and the rate-determining step n-as considered-to be a first-order dissociation of the conjugate acid to give a carbonium-sulfonium (or oxonium) ion. For the same electron donor (X), the relative substitution rates of Y are CHaC02 > CaH,O > CH30. T h e diethyl oxygen acetals substitute faster than the diethyl sulfur acetals, and acyclic sugar acetals substitute faster than pyranose acetals. Factors which influence the observed difference~ in the rate constants include the relative basic strength of the substrates and the stability of the cationic intermediate given by the dissociation of HY. Groups bonded to the acetal carbon (R1 and R2) have a large influence. Those groups which inductively stabilize an oxonium ion produce enhanced rates. Diethyl acetonal hydrolyzed lo7 times faster than diethyl formal. The overall range in the rates observed (5, 6) exceeded lo9.Kreevoy and Taft ( 5 ) were able to correlate the rate of hydrolysis of diethyl acetals and ketals with Taft's u* values (7). When a term which took into account the contribution of hyperconjugation was included, the plot of log k / k o (where k is the rate constant for the acetal or ketal and ko is the rate constant for a standard (in this case diethyl acetonal)) versus XU* was linear.By far the largest differences in the rate constants for the substitution of acyclic sugar oxygen and sulfur acetals has been observed when a 2-deoxy derivative (Ie and IIIe in ref.2) was included in a substrate group. These deoxy derivatives react about lo3 times faster than the other compounds in the substrate group. Since the only difference between these substrates is that Ie Can. J. Chem. Downloaded from www.nrcresearchpress.com by 34.214.212.150 on 05/11/18For personal use only.