Synthesis and Stability of Nucleoside 3′,5′‐Cyclic Phosphate Triesters Masked with Enzymatically and Thermally Labile Phosphate Protecting Groups

Abstract

Appropriately protected structurally modified nucleoside 3′,5′‐cyclic monophosphates are known to show antiviral activity. For this reason, a straightforward synthesis of nucleoside 3′,5′‐cyclic phosphates protected with three different enzymatically removable groups, viz. 3‐acetyloxy‐2,2‐bis(ethoxycarbonyl)propyl (in 1 and 4), 4‐acetylthio‐2,2‐dimethyl‐3‐oxobutyl (in 2), and 4‐(tert‐butyldisulfanyl)‐2,2‐dimethyl‐3‐oxobutyl (in 3) groups, is described. Removal of these protecting groups at pH 7.5 and 37 °C was monitored by reverse‐phase HPLC.

4‐(Acetylthio)‐2,2‐dimethyl‐3‐oxobutyl and 4‐(tert‐Butyldisulfanyl)‐2,2‐dimethyl‐3‐oxobutyl as Protecting Groups for Nucleoside 5′‐Phosphoramidates Derived from L‐Alanine Methyl Ester

“…Accordingly, treatment of diphenyl phosphite with commercially available 2′,3′‐ O ‐isopropylideneuridine in pyridine at room temperature, followed by addition of a mixture of H 2 O and Et 3 N gave the triethylammonium salt of 2′,3′‐ O ‐isopropylideneuridine 5′‐( H ‐phosphonate) ( 3 ; Scheme ). Compound 3 was esterified with 1‐( tert ‐butyldisulfanyl)‐4‐hydroxy‐3,3‐dimethylbutan‐2‐one ( 4 )12 or S ‐(4‐hydroxy‐3,3‐dimethyl‐2‐oxobutyl) ethanethioate ( 5 )11 in pyridine using pivaloyl chloride as activator. The product, the 4‐(acetylthio)‐2,2‐dimethyl‐3‐oxobutyl or 4‐( tert ‐butyldisulfanyl)‐2,2‐dimethyl‐3‐oxobutyl ester of 2′,3′‐ O ‐isopropylidene 5′‐( H ‐phosphonate), was not isolated, but subjected to oxidative amination with L ‐alanine methyl ester in the presence of CCl 4 and Et 3 N in a mixture of MeCN and pyridine.…”

“…The yield (10 %) was, however, lower than that obtained using diphenyl phosphite as the phosphitylating agent. Attempts to prepare phosphoramidate 1 by oxidative coupling,12,14 i.e., by iodination of the 5′‐[4‐(acetylthio)‐2,2‐dimethyl‐3‐oxobutyl H ‐phosphonate] and subsequent displacement of iodine from the iodophophate with L ‐alanine methyl ester, failed.…”

“…We now report on the thermally and enzymatically labile 2,2‐disubstituted 4‐(acylthio)‐3‐oxobutyl group11 and the reductively cleavable 4‐( tert ‐butyldisulfanyl)‐2,2‐dimethyl‐3‐oxobutyl group12 as alternative protecting groups for nucleoside 5′‐phosphoramidates. For this purpose, 2′,3′‐ O ‐isopropylideneuridine 5′‐{ O ‐[4‐(acetylthio)‐2,2‐dimethyl‐3‐oxobutyl]‐ N ‐[(1 S )‐2‐oxo‐2‐methoxy‐1‐methylethyl]phosphoramidate} ( 1 ) and 2′,3′‐ O ‐isopropylideneuridine 5′‐{ O ‐[4‐( tert ‐butyldisulfanyl)‐2,2‐dimethyl‐3‐oxobutyl]‐ N ‐[(1 S )‐2‐oxo‐2‐methoxy‐1‐methylethyl]phosphoramidate} ( 2 ) were prepared as model compounds (Figure 1).…”