Pyranosides. formation

In the formation of glycosides by the Fischer process (equilibration in the presence of an acid catalyst), fucopyranosides are apparently formed more rapidly than galactopyranosides. In the presence of mineral acids, fucose attains equilibrium in boiling methanol within 6-8 h, whereas a reaction time of 20 h is customary for D-galactose. In reactions catalyzed by cation-exchange resins, maximum pyranoside formation is attained by 8 h for L-fucose,35a whereas D-galactose requires35612-24 h. 

Only with o-lyxose was there any evidence of pyranoside formation early in the reaction. A slight reverse reaction, furanoside to pentose, was also noted for this sugar. This behavior is in agreement with the low conformational stability of the D-lyxofuranosides. However, no initial formation of D-arabinopyranosides was noted. 

Bu2SnO, toluene, reflux, 4 h Pd(Ph3P)4, THE, CH2=CHCH(OAc)2, rt, 1 h, 80-89% yield. In pyranoside protection, selectivity for 1,3-dioxane formation is generally observed, but dioxolanes are often formed. 

In addition, NaOMe, and NaNH2, have also been employed. Applieation of phase-transfer conditions with tetra-n-butylammonium iodide showed marked improvement for the epoxide formation. Furthermore, many complex substituted sulfur ylides have been synthesized and utilized. For instance, stabilized ylide 20 was prepared and treated with a-D-a/lo-pyranoside 19 to furnish a-D-cyclopropanyl-pyranoside 21. Other examples of substituted sulfur ylides include 22-25, among which aminosulfoxonium ylide 25, sometimes known as Johnson s ylide, belongs to another category. The aminosulfoxonium ylides possess the configurational stability and thermal stability not enjoyed by the sulfonium and sulfoxonium ylides, thereby are more suitable for asymmetric synthesis. 

The same anhydride of mannitol has been obtained recently by the deamination of 1-amino-l-desoxy-D-mannitol.8 It is interesting to note that this method of formation of anhydro rings is of fairly general application. Thus 1,4-anhydro-D-sorbitol is obtained from 1-amino-l-desoxy-D-sorbitol. Similarly, methyl 2,3-anhydro-4,6-benzylidene-a-D-manno-pyranoside and methyl 2,3-anhydro-4,6-benzylidene-a-D-allopyranoside9... 

On the grounds that furanosides anomerise and hydrolyse very much more readily than do the corresponding pyranosides. Bishop eind Cooper assumed that the first step in the glycosidation process is the methanol-ysis of the furanose form of the free sugar, and they visualised, without evidence, a unimolecular process proceeding by way of a stabilised cyclic ion (1). In support of this they observed 5) that for xylose, lyxose and ribose the furanoside formation rates (3,1,12 respectively) correlated with the furanoside contents at equilibrium (see Table 3) and hence, presum-... 

The selectivity for the formation of the p-D-pyranoside (equatorial) stereoisomer in the reaction originated from a thermodynamic control. Indeed, starting from D-glucose, after 24 h at room temperature, a mixture of the four possible a,p-furanosides 46 and 47, and a,p-pyranosides stereoisomers 48 and 45a, in which... 

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