Abstract
A simple approach was developed for the solvent-free regioselective functionalization of carbohydrate polyols with 4-toluesulfonyl (tosyl) group, allowing the easy and quick activation of a saccharide site with a tosylate leaving group. The method is based on the use of catalytic dibutyltin oxide and tetrabuylammonium bromide (TBAB), and a moderate excess of N,N-diisopropylethyl amine (DIPEA) and tosyl chloride (TsCl), leading to the selective functionalization at 75 °C of a secondary equatorial hydroxy function flanked by an axial one in a pyranoside. The procedure is endowed with several advantages, such as the use of cheap reagents, experimental simplicity, and the need for reduced reaction times in comparison with other known approaches.
Highlights
Landolfi, A.; Vessella, G.; Iadonisi, A.Carbohydrates are well-appreciated precursors in organic synthesis owing to both their broad availability at a low cost and their dense functionalization with a large number of defined stereo-centers
Initial experiments were designed to test the feasible applicability of solvent-free conditions in order to achieve fast regioselective tosylation at the secondary site of sugar polyols with a free primary function
The optimization conducted on the benzylation procedure evidenced that the liquid diisopropylethyl amine (DIPEA) is by far the best base for these solvent-free reactions, and a stoichiometric excess of 3–4 equivalents, unable to dissolve the reagents, was sufficient to achieve high-yielding reactions
Summary
Over the last few years, there has been a growing interest in the development of solvent-free approaches focused on synthetic carbohydrate chemistry [29], and numerous contributions were provided by our laboratory, with the proposal of non-conventional and experimentally simple procedures for the regioselective protection of sugars with benzyl [30,31,32,33], allyl [31], silyl [34,35], trityl [35,36] and acetal groups [37], and the possible sequential application thereof in one-pot procedures [34,36,38] In this frame, the mentioned benzylation/allylation procedure [31] represented the first example at the use of catalytic dibuyltin oxide for stannylene-mediated alkylations, after about 30 years of stoichiometric applications. The study described is aimed at further extending the synthetic scope of solvent-free methodologies with the regioselective installation of a tosylate leaving group at the secondary positions of sugars
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