Regioselective Deprotection ofTert-Butyldimethylsilyl Ethers by Boron Trichloride

Abstract

Several fully tert-butyldimethylsilyl protected mono- saccharides reacted with boron trichloride in tetrahydrofuran to effect a regioselective cleavage at the sites of primary alcohols. An application of this methodology was illustrated in an efficient approach to the synthesis of L-gulose from D-gluconolactone. Selective protection and deprotection of polyfunctional molecules present a critical challenge in organic synthe- sis. Particularly in carbohydrate chemistry, many prob- lems result from how to distinguish multiple hydroxyl groups of similar reactivity. A variety of trialkylsilyl groups have been developed for the protection of free hy- droxyl groups due to their easy preparation and selective attachment. 1-3 In order to remove the silyl protection of a primary alcohol selectively in the presence of secondary and tertiary ones, most methods require distinguishable si- lyl groups for different hydroxyl functionalities. 4 This process often increases the difficulty of preparation and the number of synthetic steps. 4 It would be desirable that one can use only one silyl group for protection of several hydroxyl groups in a molecule, and the subsequent depro- tection can be conducted in a regioselective manner. Al- though effective procedures have been used in some specific examples, 5 there are only a few reports on such selective deprotection of carbohydrate molecules. 6 In the course of L-gulose synthesis, we found that BCl 3 was effective to cleave primary tert-butyldimethylsilyl (TBDMS) ether without affecting the same groups at secondary positions.