Dedicated to Prof. Dr. Wilhelm Fleischhacker on the occasion of his 60th birthday (8.X1.91)
~ ~~The chemical syntheses of the phosphorothioate of (2'-5')adenylate dimer (see 6a, 6b) and trimer (see 1 la, 1 lb, 12a, 12b) as well as of their 5'-monophosphates (see 15a, 15b, 16a, 16b) using the phosphoramidite method are described. The resulting diastereoisomer mixtures were separated into the pure components by chromatographical means. All synthetic intermediates were characterized by TLC, elemental analysis, and UV and 'H-NMR spectra.1. Introduction. -Establishment of an antiviral state in cells results from a multitude of biochemical changes induced by interferons and involving several proteins in a cascade of reactions [2-41. One of the proteins is the enzyme 2'-5'A synthetase, which is activated upon binding to double-stranded RNA [5] and converts then ATP into a series of (2'-5')-linked adenylate oligonucleotides containing a 5'-terminal triphosphate function [6]. Such oligomers carrying a 5'-di-or 5'-triphosphate and consisting of three or more monomer units bind to, and subsequently reversibly activate, an endogenous or sometimes interferon-induced endoribonuclease [7] [8]. The activated endoribonuclease, RNase L, cleaves then messenger and ribosomal RNA's [9], resulting in inhibition of translation. This effect, however, is only transitory, since the 2'-5'A molecules are rapidly cleaved by cellular phosphodiesterases [lo] leading to loss of antiviral activity. In order to suppress the digestion of the 2'-5'A oligomers, several synthetic modifications of the native structure were accomplished at the aglycone [ll-201, the sugar [21-371, and the phosphate moiety [38][39][40][41][42][43][44][45]. It is well established, mainly due to the pioneering work of Eckstein [46], that phosphorothioate analogues of oligonucleotides are valuable models to study certain stereochemical aspects of enzyme-catalyzed phosphoryl and nucleotidyl transfer reactions.The first synthesis of phosphorothioate analogues of (2'-5')oligoadenylates were performed by Nelson, Bach, and Verheyden [42] applying the phosphite triester approach in conjunction with sulfur oxidation. Formation of chiral phosphorothioate functions led to diastereoisomeric mixtures of which the dimer cores could be separated chromato-