4‐Hydrazinothieno[2,3‐d]pyrimidines were cyclized with triethyl orthoformate and formic acid to give 1,2,4‐triazolo[4,3‐c]thieno[3,2‐e]pyrimidines and 1,2,4‐triazolo[2,3‐c]thieno[3,2‐e]pyrimidines depending on the reaction conditions employed.

The structure-activity relationships of several pyrimidine nucleosides related to 3'-azido-3'-deoxythymidine (AZT) were determined in human immunodeficiency virus type 1 (HIV-1) infected human peripheral blood mononuclear cells. These studies indicated that nucleosides with a 3'-azido group on the sugar ring exhibited the most potent antiviral activity. Substitution at C-5 with H, CH3, and C2H5 produced derivatives with the highest potency, whereas alkyl functions greater than C2, including bromovinyl substitution reduced the antiviral potency significantly. Changing the 3'-azido function to an amino or iodo group reduced the antiviral activity. Replacement of the uracil ring by cytosine or 5-methylcytosine produced analogues with high potency and low toxicity. Modification of the 5'-hydroxy group markedly reduced the antiviral activity. Similarly, various C-nucleoside analogues related to AZT and 2',3'-dideoxycytidine were inactive and nontoxic. From these systematic studies 3'-azido-2',3'-dideoxyuridine (5a), 3'-azido-5-ethyl-2',3'-dideoxyuridine (5c), and 3'-azido-2',3'-dideoxycytidine (7a) and its 5-methyl analogue (7b) were identified as potent and selective anti-HIV-1 agent in primary human lymphocytes.

In order to study the structure-activity relationships of 2',3'-dideoxypurine nucleosides as potential anti-HIV agents, various 6-substituted purine analogues have been synthesized and examined in virus-infected and uninfected human peripheral blood mononuclear cells. N6-methyl-2',3'-dideoxyadenosine (D2MeA, 7a) was initially synthesized from adenosine via 2',3'-O-bisxanthate 3. As extension of this reaction to other N6-substituted compounds failed, a total synthetic method utilizing 2',3'-dideoxyribose derivative 9 was used for the synthesis of other purine nucleosides. An acid-stable derivative of N6-methyl-2',3'-dideoxyadenosine, 2'-fluoroarabinofuranosyl analogue 32 (D2MeFA), has been synthesized from the appropriate carbohydrate 24 by condensation with N6-methyladenine 23. Among these compounds, N6-methyl derivative (D2MeA) 7a proved to be one of the most potent antiviral agents. The order of potency for the 6-substituted compounds was NHMe greater than NH2 greater than Cl approximately N(Me)2 greater than SMe greater than OH approximately NHEt greater than SH greater than NHBn approximately H. The results suggest that a bulk tolerance effect at the 6-position of the 2',3'-dideoxypurine nucleoside may dictate the antiviral activity of these compounds. Acid-stable analogue 32 (D2MeFA) was found to be 20-fold less potent than the parent compound. Both D2MeA and D2MeFA were resistant to calf intestine adenosine deaminase. The presence of a fluorine atom in the carbohydrate moiety greatly increased stability to acid, making D2MeFA a potential orally active antiviral agent that could be useful for the treatment of retroviral infections in humans.

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