Mannose, stereoselective allylation

Addition of a Lewis acid has been noted to improve the stereoselectivity (Table 12, entries 1-3). High stereocontrol is observed for products which can equilibrate via enolization (Table 12, entries 6-10) and for the sterically demanding sulfone derivative (Table 12, entry 4). Application of this methodology to EPC synthesis is illustrated by the use of lactones prepared in 8 to 9 steps from mannose (Table 12, entries 11-18)1 4. The stereochemical information within the reactive allyl ether moiety is faithfully transmitted and, remarkably, the Z geometry of the olefins is retained during formation of the vinylcyclopentanes. These more sterically hindered substrates may give mixtures of 5- and 7-membered rings (Table 12, entries 11,12, and 14-16). 

An influence of pH of the reaction medium on steric course was observed by Warwel and Fessner [114]. Thus, allylation of L-arabinose with ethyl 2-(bromomethyl)acrylate at pH 1-2, afforded KDO with full stereoselectivity [114], The same reaction conditions used for the allylation of D-mannose furnished KDN as the only isomer [114],... 

The key-step in the synthesis of 5-(7-methyl-D-mannose from L-quebrachitol was the glycol cleavage of the isopropyUdene derivative 15 (Scheme 6). L-Tagatose was prepared by a similar, but extended reaction sequence incorporating a Mitsunobu inversion at the future C-4. L-Olucose has been synthesized in many steps from cycloheptatriene (Scheme 7) via the meso-intermediate 16 which was desymmetrizized by enzymic monoacetylation. Transformation to the allylic alcohol 17 involved inter alia stereoselective oxygenation and cis-hydroxylation. Ozonolysis, followed by... 

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