Chapter 27, Problem 26P

Interpretation Introduction

Interpretation:

The reason for the stereo-selectivity of the coupling reaction of $\text{2,6-dichloropurine}$ with $\text{1,2,3,4-tetra-O-acetyl-α+β-arabinofuranose}$ when heated in presence of $p\text{-toluenesulfonic acid}$ is to be explained.

Concept Introduction:

$p\text{-toluenesulfonic acid}$ is a relatively strong organic acid, which can protonate an acyl oxygen and converts it into a better leaving group.

If resonance stabilization of the carbocation can result in the formation of a ring by the conversion of the ${\text{sp}}^2$ hybridized carbocation to an ${\text{sp}}^3$ carbon atom, the incoming nucleophile can attack it only from the side opposite the ring.