Efficient Synthesis of Specifically Deuterated Nucleosides: Preparation of 4'-Deuteriothymidine

Abstract

A very straightforward synthesis of 4'-deuteriothymidine (I) from inex- pensive thymidine (2) is described. The key step involvesdriple deprotonation of the ester prepared by selective oxidation of unprotected thymidine (and esterific- ation) and deuteration from the a-face to produce the desired 4'-deuterio derivative (5) which is then taken on to 1. Thus one can prepare the 4'-deuteriothymidine (1) in only five steps from thymidine (2) in an overall yield of 16%. Chemical nucleases, synthetic or naturally occurring, usually cleave DNA by oxidizing the sugar unit.3 The first step generally involves the abstraction of a hydrogen atom from the 2'-deoxyribose unit, although the site of the attack varies. Specifically isotopically-labelled nucleotides are valuable tools for studying the mechanism of cleavage by a chemical nuclease. In pa~ticular, 4'-labelled nuclwtides have been used in the study of the chemical nucleases that first abstract the hydrogen atom from the 4'-position. Neocarzinostatin exhibited kinetic isotope effects of 2 to 5 when reacted with 4'-labelled nucleotides4 while the deuterium atom on C4' was transferred to reduced ~alicheamycin.~ For a project aimed at determining the mechanism of the cleavage of nucleic acids by the system ortho-phenanthroline, copper, hydrogen peroxide? we needed to prepare a small DNA fragment that incorporated specifically deuterated nucleosides. For this reason, we required a sample of thymidine which was specifically deuterated at the 4'-position, 1. This compound is known in the literature but the reported preparation which uses an enzyme based method seemed inappropriate for our purpose.7 Also the best chemical method among several developed8 for the production of specifically deuterium-labelled nucleosides, the work of Townsend and coworkers?b did not appear to be particularly applicable for the synthesis of 1. Therefore we developed a new route to 1 which begins with thymidine (2) itself. We described herein this approach, which should also be applicable to the preparation of other normal and 2'-deoxynucleosides.