PREPARATION OF C-NUCLEOSIDES BY DEHYDRATION OF PENTAHYDROXYPENTYL-HETEROCYCLES: STERIC COURSE AND MECHANISM

Article abstract of DOI:10.1016/S0008-6215(00)88054-4

The acid-catalysed dehydration of pentahydroxypentyl-heterocycles can take place between C-1' and C-4' to yield C-nucleosides having furanoid structures, or between C-1' and C-5' to yield pyranoid compounds.Each reaction can yield one of the two possible anomers or a mixture of both.We have dehydrated pentahydroxypentyl-heterocycles having D-gluco, D-manno, and D-galacto configurations.When the reactions were kinetically controlled, the 6,6-dimethyl-2-(pentitol-1-yl)-4,5,6,7-tetrahydroindol-4-ones having the D-gluco or D-manno configurations yielded 2-α-D-arabinofuranosyl-6,6-dimethyl-4,5,6,7-tetrahydroindol-4-one, but they yielded 2-α-D-arabinopyranosyl-6,6-dimethyl-4,5,6,7-tetrahydroindol-4-one under conditions of thermodynamic control.Dehydration of 3-acetyl-2-methyl-5-(D-galacto-pentitol-1-yl)-1-propylpyrrole gave a mixture of 3-acetyl-5-(α- and β-D-lyxopyranosyl)-2-methyl-1-propylpyrrole.These results are consistent with a mechanism involving an intermediate C-1' carbocation.