Acid-catalyzed conversion of 2- O-(2-hydroxypropyl)- d-glucose derivatives into 1,2- O-(1-methyl-1,2-ethanediyl)- d-glucose acetals. Studies related to O-(2-hydroxypropyl)cellulose

Abstract

The acid-catalyzed solvolysis of methyl 3,5,6-tri- O-benzyl-2- O-(2-hydroxypropyl)-α- d-glucofuranoside ( 1) in chloroform involves a neighboring-group attack on C-1 by the hydroxypropyl substituent, and opening of the furanoside ring to yield a diastereomeric pair of 3,5,6-tri- O-benzyl-1- O-methyl-1,2- O-(1-methyl-1,2-ethanediyl)- d-glucose acetals ( 2 and 3). The latter, which differ in configuration at C-8, represent a resolution of the enantiomeric forms of the original 2- O-(2-hydroxypropyl) group. In a succeeding reaction, the 1-methoxyl group of each acetal undergoes an intramolecular displacement by O-4, leading to the formation of the corresponding bicyclic acetals, i.e., the two diastereomers ( 4 and 5) of 3,5,6-tri- O-benzyl-1,2- O-(1-methyl-1,2-ethanediyl)-α- d-glucofuranose. Solvolysis of 6, the β anomer of 1, proceeds in an analogous manner, although more rapidly, to yield a corresponding pair of acyclic-aldose acetals ( 7 and 8), as well as bicyclic acetals 4 and 5. Similar results are observed for solvolysis in the 2- O-(2-hydroxyethyl) series, whereas the reaction of the 2- O-(2,3-epoxypropyl) counterpart of 1 (or 6) with hydrogen chloride affords the corresponding chloromethyl analogs of 4 and 5. In all of these series, one of each diastereomeric pair of products is more stable than the other, and reasons for this are considered. Evidence based on n.m.r.-spectral data and steric factors is presented to show that the configuration of the chiral center C-8 of 2, 4, and 7 is (S), whereas it is (R) in 3, 5, and 8. Also, conformational characteristics of the various solvolysis products are assessed, and mechanisms possibly involved in their formation are discussed.