Relative reactivity of hydroxyl groups in inositol derivatives: role of metal ion chelation
O-Alkylation of myo-inositol derivatives containing more than one hydroxyl group via their alkali metal alkoxides (sodium or lithium) preferentially occurs at a hydroxyl group having a vicinal cis-oxygen atom. In general the observed selectivity is relatively higher for lithium alkoxides than for the corresponding sodium alkoxide. The observed regioselectivity is also dependent on other factors such as the solvent and reaction temperature. A perusal of the results presented in this article as well as those available in the literature suggests that chelation of metal ions by inositol derivatives plays a significant role in the observed regioselectivity. Steric factors associated with the axial or equatorial disposition of the reacting hydroxyl groups do not contribute much to the outcome of these O-alkylation reactions. These results could serve as guidelines in planning synthetic strategies involving other carbohydrates and their derivatives.
Devaraj, Subramanian,Jagdhane, Rajendra C.,Shashidhar, Mysore S.
experimental part
p. 1159 - 1166
(2009/10/04)
Chelation-Assisted C-O Bond Cleavage of Ortho Esters. A Convenient Synthesis of myo-Inositol Derivatives Having Free Hydroxy Group(s) at Specific Position(s)
Reactions of ortho esters of myo-inositol 8 or 10 with 1-2 equiv of Grignard reagents in benzene-ether yield regio- and stereoselectively the corresponding ring opening products having a free hydroxy group at C-1. The regioselectivity is rationalized owing to the presence of the 2-methoxy group which will serve as an auxiliary to form a chelation complex 12 with magnesium. Inositol derivatives having two free hydroxy group at C-1 and C-3 positions can be achieved from reactions of 6 or 8 with excess Grignard reagents or under more drastic conditions. The reaction of 8b with excess LiAlH4/AlCl3, on the other hand, yields the corresponding 1,5-diol 19.