A stereochemical comparison of some addition reactions to methyl 4,6- O-benzylidene-3-deoxy-3- C-ethyl-α- D-hexopyranosid-2-uloses

Abstract

Methyl 4,6- O-benzylidene-3-deoxy-3- C-ethyl-α- D- ribo-hexopyranosid-2-ulose ( 2) reacts with lithium aluminium hydride, methylmagnesium iodide, and phenylmagnesium bromide to give products having only the allo configuration. The C-3 epimer of 2, methyl 4,6- O-benzylidene-3-deoxy-3- C-ethyl-α- D- arabino-hexopyranosid-2-ulose (7), reacts with lithium aluminium hydride to give only a product having a gluco configuration. With methylmagnesium iodide and phenylmagnesium bromide, 7 gives products having gluco and manno configurations. The 2- C-methyl-glucopyranoside preponderates in the former reaction, and the 2- C-phenyl-mannopyranoside is the major product in the latter reaction. The reaction between 2 and diazomethane gives a spiro-epoxide having the allo configuration as the major product ( i.e., the same configuration as the 2-MeMgI reaction product), whereas the 7-diazomethane reaction gives a spiro-epoxide having the manno configuration ( i.e., the same configuration as the minor product from the 7-MeMgI reaction) as the major product. With the thionyl chloride-pyridine reagent, 3-deoxy-3- C-ethyl-2- C-methyl-α- D-hexopyranosides are dehydrated endocyclically when H-3 and HO-2 are antiparallel and exocyclically when H-3 and HO-2 have a gauche relationship.