Enzyme-catalyzed preparation ofp-2_deo~-D-glucosides ami galaccosides including disaccharides has been achieved using the corresponding glyc& .m substrates. Major advances in the chemical synthesis in terms of efficiency and selectivityl. of oligosaccharides have been made during the last decade To circumvent the unavoidable lengthy synthetic routes to these polyfunctional substances, enzyme-catalyzed syntheses have emerged as a valuable strategic choice. TWO solutions can be set up: using either glycosyltransferases2 which are generally highly specific enzymes towards both the glycosyl donor and acceptor, or glycosylhydrolases which are capable of catalyzing glycosyl transfer reactions by inverse hydrolysis or transglycosylation3,4. Good yields and highly predictable results are possible with glycosyltransferases but multienzymatic systems .mvolving cofactors are necessary. From a synthetic viewpoint, the more readily available glycosylhydrolases offer the advantage of a simple catalytic system and accept a broader structural range of alcohol acceptors while retaining the capacity to create stereospecifically glycosidic bonds. However, technical "tricks" have to be set up to compensate for the thermodynamic disadvantage. One is to operate in organic solvents5 to reduce the amount of water. Another would be to select the catalysis of a reaction in which ro water appears in the chemical equilibrium. We now report the results of preliminary experiments which show that pyranoid glycals (1,5-anhydro-2-deoxy-hex-l-enitols) can be used as R-2-deoxyglycosyl donors in transfer reactions on a preparative scale, catalyzed by glycosidases. The stereoselective chemical synthesis of B-2-deoxy-glycosides, a problem of current interest, is only possible through a stereodirecting auxiliary group equatorially disposed at C-2, which is removed in a later step.6 A one-pot enzymatic approach offers an attractive alternative. Glycals, well known inhibitors of most glycosidases, slowly undergo an enzyme-catalyzed addition of water or alcohols7 as shown in experiments designed mostly for mechanistic data, e.g., for defining the orientation of the functional groups involved in the catalytic process. The almond B-glucosidase [E.C.3.2.1.21] and the Aspergillus oryzae B-galactosidase [E.C.3.2.1.23],