One-Pot Synthesis of Aryl Phosphoramidate Derivatives of AZT/d4T as Anti-HIV Prodrugs

Abstract

S y n t h e s i s o f A r y l P h o s p h o r a m i d a t e D e r i v a t i v e s o f A Z T / d 4 TAbstract: Arbuzov reaction of aryl phosphorodichloridite with mixture of one equivalent of AZT or d4T and one equivalent of tertbutyl alcohol led to the corresponding AZT/d4T aryl H-phosphonate diesters, and the following reaction of the H-phosphonate diesters with amino acid methyl esters in the presence of N-chlorosuccinimide (NCS) produced membrane-soluble anti-HIV prodrugs AZT/d4T aryl phosphoramidate derivatives in good yields. Some dideoxynucleosides (ddNs) including 3¢-azido-2¢,3¢-dideoxythymidine (AZT, zidovudine) 1 and 2¢,3¢-dideoxy-2¢,3¢-didehydrothymidine (d4T, stavudine) 2 have emerged as efficient drugs against human immunodeficiency virus (HIV), 3 and the anti-retroviral effects of these compounds involve their conversion, through cellular enzymes, to the corresponding 5¢-triphosphates (ddNTPs) which interact with HIV-associated reverse transcriptase. Specifically, the first phosphorylation step for d4T is the rate-limiting step, whereas for AZT phosphorylation to the nucleoside 5¢-diphosphate is the slowest step in the phosphorylation pathway. 4 However, in many cases the unnatural ddNs have poor affinity for nucleoside kinases, 5 one possibility to improve the efficiency of ddNs could be to bypass the phosphorylation steps. Unfortunately, these polar nucleotides are not able to cross the cell membrane efficiently, 6 and they are readily dephosphorylated in extracellar fluids and on cell surfaces by nonspecific phosphohydrolases. 7 Hence, the strategies of temporarily masking or reducing the phosphate negative charges of nucleoside 5¢-monophosphates (NMPs) with neutral substituents to prodrugs are used, these prodrugs would be freed to the corresponding NMPs once inside the cell. McGuigan et al. have developed the phosphoramidate diester prodrugs of 2¢,3¢-dideoxy-2¢,3¢-didehydroadenosine (d4A), 2¢,3¢-dideoxyadenosine (ddA) 8a and d4T, 8b-d which exhibited greatly enhanced activity against HIV compared to their parent nucleoside analogues in vitro, and full activity was retained in kinase-deficient cell lines. 8 So the development of nucleoside prodrugs capable of undergoing intracellular activation to the corresponding nucleotides has become an area of intense interest. 9 Recently, we have developed the methods for synthesis of the nucleoside phosphporamidate monoesters using nucleoside di-, triphosphates 10,11 or fluorenylmethyl nucleoside 5¢-H-phosphonates 12 as starting materials. The typical synthesis of the nucleoside phosphoramidate diesters is from sequential reactions of phosphoryl chloride with phenol or p-substituted phenol, amino acid methyl ester and nucleoside, 13 however, the intermediates aryl chlorophosphoramidates are of low reactivity, the following reaction with nucleosides usually needs a long reaction time, and sometimes yields are very low. Here, we would like to report a new method for synthesis of aryl phosphoramidate derivatives.Atherton-Todd reaction is a convenient and effic...