A further investigation of the stannic chloride-catalyzed condensation reaction of 1-hexene and 1,2,3,5-tetra-O-acyl-[beta]--ribofuranoses

Abstract

The reaction of 1-hexene with either 1-O-acetyl-2,3,5-tri-O-benzoyl-[beta]--ribofuranose (5b) or 1,2,3,5-tetra-O-acetyl-[beta]--ribofuranose (5a) in the presence of stannic chloride leads to the formation of a complex mixture of products. By a combination of 1H-n.m.r. and mass spectroscopy, the products were shown to be anomeric and diastereomeric mixtures of the 8,9,11-tri-O-acyl-protected derivatives of 7,10-anhydro-1,2,3,4,5,6-hexadeoxy--allo(altro)-undec-4-enitol (1) and 7,10-anhydro-5-chloro-1,2,3,4,5,6-hexadeoxy--allo(altro)-undecitol (2). The [alpha] anomer of 1 was the predominant anomer, whereas the [alpha] and [beta] anomers of 2 were present in approximately equal amounts. It was found that 2 was not formed when trimethylsilyl trifluoromethanesulfonate was used as the catalyst instead of stannic chloride. The acyl-protected sugar 3,6-anhydro-2-deoxy--allo(altro)-heptose (3), prepared by ozonolysis of 1, reacted with tert-butoxycarbonylmethyltriphenylphosphorane to give tert-butyl trans-5,8-anhydro-6,7,9-tri-O-acetyl-2,3,4-trideoxy--allo(altro)-non-2-enanate (4). The basicity of the ylide was sufficient to cause anomerization and resulted in an [alpha],[beta] ratio of 5:1 in the product, 4.