SOLID-PHASE EXTRACTION ON C18 SILICA AS A PURIFICATION STRATEGY IN THE SOLUTION SYNTHESIS OF A 1-THIO-BETA-D-GALACTOPYRANOSIDE LIBRARY

A novel strategy for the purification of carbohydrate-based chemical l ibraries synthesized in solution was developed. Purification of reacti on products was accomplished by means of solid-phase extraction enable d by protecting the 2-, 3-, 4-, and 6-hydroxyl groups of a galactose d erivative as their hydrophobic O-laurates. The presence of multiple O- laurates allowed adsorption of reaction products onto C18 silica while reagents and by-products were washed away with MeOH. Products were qu antitatively eluted with pentane. Purification of products using solid -phase extraction offers the combined advantages of solution synthesis (normal solution reactivity and ease of reaction monitoring) with tho se of solid-phase synthesis (facile product isolation permitting the u se of large excesses of reagents). To demonstrate the utility of the h ydrophobic recovery-procedure, tetra-O-lauroyl-beta-D-galactopyranose- 1-thiol was subjected to high-yielding reactions with a panel of Micha el-acceptors and an alpha-chloro ketone. The resulting ketone adducts were then either reduced to the alcohols or reductively aminated with a selection of amino acids to give 30 different 1-thio-beta-D-galactos ides as mixtures of four diastereomers after removal of protecting gro ups. At each step, the product was separated from the reagents and the ir by-products by simple adsorption onto C18 silica, washing with MeOH and elution of product with pentane. After completion of the combinat orial chemistry sequence, the O-laurates were cleaved by methanolysis and the product methyl laurate in turn removed from the desired water- soluble products by C18 adsorption. Individual library members were th us conveniently produced on 10-30 mg scales at purity levels of > 90%. One of the 1-thio-beta-D-galactosides thus produced was found to be a competitive inhibitor of the beta-galactosidase from E. coli with K-i value of 1.7 mu M. (C) 1998 Elsevier Science Ltd. All rights reserved .