Solid-phase extraction on C18 silica as a purification strategy in the solution synthesis of a...

A novel strategy for the purification of carbohydrate-based chemical libraries synthesized in solution was developed. Purification of reaction products was accomplished by means of solid-phase extraction enabled by protecting the 2-, 3-, 4-, and 6-hydroxyl groups of a galactose derivative 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 quantitatively 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 those of solid-phase synthesis (facile product isolation permitting the use of large excesses of reagents). To demonstrate the utility of the hydrophobic recovery-procedure, tetra-O-lauroyl-beta-D-galactopyranose-1-thiol was subjected to high-yielding reactions with a panel of Michael-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-galactosides as mixtures of four diastereomers after removal of protecting groups. At each step, the product was separated from the reagents and their by-products by simple adsorption onto C18 silica, washing with MeOH and elution of product with pentane. After completion of the combinatorial 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 thus 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 Ki value of 1.7 microM.