A facile synthesis of 4,6-O-benzylidene-D-glycals via 1,5-anhydro-4,6-O-benzylidene-D-hex-1-en-3-ulose.

Fused triheterocyclic ring systems of the 5:6:6 type are much more numerous and diverse than the 5:5:6 systems. However, in this case, the review period extends only from 1995 to 2006, with occasional references to earlier work. The emphasis in the survey is on synthetic routes; the ring systems are grouped according to structural similarities, for example, the number and variety of heteroatoms in each ring, whether the rings are ortho- or peri-fused, and whether or not the systems contain ring junction heteroatoms. Brief reference is made, at the appropriate points in the text, to natural products and compounds of potential pharmaceutical importance or interest.

Three Heterocyclic Rings Fused (5-6-6)

4,6-O-Benzylidenation of D-galactal with PhCH(OCH3)2 catalyzed by bromodimethylsulfonium bromide leads to methyl 2-dexoy-4,6-O-benzylidene galactopyranoside efficiently, which serves as a key intermediate to the ready preparation of 2,3- and 2,6-dideoxy galactopyranosides.

Bromodimethylsulfonium Bromide Catalyzed Synthesis of Methyl 2‐Dexoy‐4,6‐O‐benzylidene Galactopyranoside from Galactal and the Rapid Route to 2,3‐ and 2,6‐Dideoxygalactopyranoses

This chapter discusses the chemical reactions that cause functional group transformations within monosaccharides, either for the purpose of elaboration into oligosaccharides or for the synthesis of a derivative of the monosaccharide. Such derivatives include the deoxy and amino deoxy sugars. The major advances have been concerned with the discovery or invention, as some have called it, of chemoselective reagents for oxidation of the various hydroxyl groups within a carbohydrate. The well-established Jones' (CrO 3 , H 2 SO 4 ) and Collins' (CrO 3 , pyridine) reagents soon gave way to milder and more easily handled oxidizing agents, namely pyridinium chlorochromate and pyridinium dichromate. These reagents oxidize both primary and secondary alcohols and can produce either the aldehyde or the acid from a primary alcohol. Another major class of oxidizing agents is based on dimethyl sulfoxide. Since the initial report by Pfitzner and Moffatt (dimethyl sulfoxide, dicyclohexylcarbodiimide), a string of reagents has been developed that activate the dimethyl sulfoxide for oxidation of, in particular, secondary alcohols. The reduction of an aldose to an alditol is now more safely done with sodium borohydride than with the more toxic sodium amalgam. With the advent of sodium borohydride and lithium aluminium hydride, many of the reductions performed on carbohydrates became easier and, with sodium borohydride, safer. A modern Kiliani-Fischer synthesis now uses sodium borohydride for reduction of the lactone to the aldose and, if necessary, further reduction to the alditol. Alkenes derived from carbohydrates are rarely found naturally and usually result from some chemical transformation in the laboratory, e.g., dehydration, dehydrohalogenation, or de-dihydroxylation. Anomeric alkenes, termed “glycols,” are unique and their chemistry is discussed separately. Carbocycles form the core of several important classes of biologically important molecules, e.g., the cyclitols.

The Reactions of Monosaccharides

The two-step synthesis of 4,6-O-benzylidene glucal, in 59% overall yield, from phenyl 1-seleno-α- d -mannopyranoside is described.

Rajdeep K. Nijjar

Papers

A new entry to 4,6-O-benzylidene glucal from phenyl 1-seleno-α-D-mannopyranoside