ABSTRACT
The classical problem in carbohydrate chemistry of the stereocontrolled synthesis of β-mannopyranosides was largely solved by the introduction of the 4,6-O-benzylidene acetal controlled method in the mid-1990s.1 The main drawback to this method,
Experimental .......................................................................................................... 176 General Methods ............................................................................................... 176 Phenyl 2,3,4,6-tetra-O-acetyl-1-thio-α-d-mannopyranoside ............................ 176 Phenyl 1-thio-α-d-mannopyranoside ................................................................ 177 Phenyl 4,6-O-benzylidene-1-thio-α-d-mannopyranoside ................................. 177
References .............................................................................................................. 180
which has been widely applied in the synthesis of a variety of oligosaccharides, glycoconjugates, and natural products,2 is the need for careful control in the introduction of the 4,6-O-benzylidene acetal so as to limit the formation of the corresponding over-reaction product-the 2,3:4,6-di-O-benzylidene acetal. Chromatographic puri-cation has typically been necessary to obtain the pure monoacetal thereby limiting reaction scale. This situation was remedied in 2005 by the group of Mallet, who described a protocol for 4,6-O-benzylidenation employing uoroboric acid as catalyst that enabled purication of the product by simple crystallization.3 The present method describes the use of the Mallet conditions, along with the preparation of the starting tetraol, for the facile production of multigram quantities of phenyl 4,6-O-benzylidene-1-thio-α-d-mannopyranoside thereby overcoming the main obstacle to the use of the benzylidene acetal-directed β-mannosylation on a larger scale.
