Radical Reactions of Carbohydrates

Volume II: Radical Reactions in Carbohydrate Synthesis

Chapter 13: Carboxylic Acid Esters
of N-Hydroxypyridine-2-thione

Chapter 13 is in a portable document file (pdf) and can be viewed by clicking the blue, Chapter 13 button below. The drawing underneath the button shows a proposed mechanism for formation of a carbohydrate radical from reaction of a carboxylic acid ester of N-hydroxypyridine-2-thione. Beside the drawing is a brief description of the mechanism of this reaction. Below the drawing and its description is a summary of Chapter 13.

Chapter 13: Carboxylic Acid Esters of N-Hydroxypyridine-2-thione

This drawing describes the reaction of a carboxylic acid ester of N-hydroxypyridine-2-thione.

Drawing Description

Carboxylic acid esters of N-hydroxypyridine-2-thione (also know as Barton esters) generate carbon-centered radicals in photochemically initiated reactions in which the esters themselves produce the chain-carrying radicals needed for the chain reaction to continue. (A proposed mechanism for radical formation from a Barton ester is given in the drawing on the left, which is Scheme I in Chapter 13.) Radical formation from a Barton ester is followed by loss of carbon dioxide to generate a carbohydrate radical.

Summary of Chapter 13

Radical reaction of a carboxylic acid ester of N-hydroxypyridine-2-thione causes the weak N–O bond to fragment to produce acyloxy and 2-pyridylthiyl radicals. The acyloxy radical then loses carbon dioxide to form a carbon-centered, carbohydrate radical. Carbon-centered radicals produced in this way abstract hydrogen atoms from highly reactive donors, such as thiols, and they combine with 2-pyridylthiyl radicals formed in the initial N–O bond homolysis. These carbohydrate radicals also can add to α,β-unsaturated esters and amides, to compounds with phenylsulfonyl- or nitro-substituted multiple bonds, and to protonated aromatic amines. Photolysis of acetylated N-hydroxypyridine-2-thione provides a source of methyl radicals that react readily with tellurium-containing carbohydrates to produce carbohydrate radicals.