The usual mechanistic sequence involves oxidation of AdoHcy at C3′ followed by elimination of L-homocysteine, Michael addition of water, and reduction to give adenosine. A 6′-fluorohomovinyladenosine analogue (EDDFHA) undergoes hydration of the 5′,6′ double bond (hydrolytic activity) at a more rapid rate than oxidation at C3′. Three 4′,5′-didehydro-5′-deoxy-5′-fluoro nucleoside analogues were prepared from 3′-deoxy-and 3′-(chloro and fluoro)-3′-deoxyadenosine via generation of the vinyl fluorides by thermolysis of 5′-fluoro-5′-thioether sulfoxides. The 3′-deoxy analogues of 6′-halohomovinyladenosines were prepared by Wittig extension with a 3′-deoxy-5′-carboxaldehyde and halodestannylation of vinyl stannanes. The 3′-hydroxyl group appears to be essential for binding to AdoHcy hydrolase. No hydrolytic activity at C5′ or C6′ was observed with the nonoxidizable 3′-deoxy or 3′-(chloro or fluoro) analogues in contrast with their 3′-hydroxy counterparts (ZDDFA and EDDFHA). These 3′-modified analogues cannot reduce enzyme-bound NAD + to NADH and do not produce time-dependent inhibition of AdoHcy hydrolase, but are weak competitive inhibitors (K i ) 150-200 µM).

Moffatt oxidation of 2',3'-O-isopropylideneuridine (1a) and treatment of the crude 5'-aldehyde with formylmethylene-stabilized Wittig reagent gave the vinylogously extended 7'-aldehyde2a. Condensation of 2a with ethoxycarbonyl- or dibromomethylene phosphorane reagents gave the conjugated dienes 6a and 4a, respectively. Deacetonization gave diene ester 7a [5'(E),7'(E); with s-trans conformation] and dibromodiene 5a [5'(E)], respectively. Analogously, 2',3'-O-isopropylideneadenosine (1b) was Wittig-extended into the conjugated dibromodiene 5b [5'(E)] and dienoic ester 7b [5'(E),7'(E)]. Furthermore, palladium-catalyzed coupling of the vinyl 6'(E)-stannanes 14 with (E) and (Z) ethyl 3-iodoacrylate gave stereodefined access to dienoic esters 7 (E,E) and 16 (E,Z). Incubation of AdoHcy hydrolase with 100 microM of 5b resulted in partial inhibition of the enzyme without any apparent change in the enzyme's nicotinamide adenine dinucleoside (NAD(+)) content. In contrast, 7b and 16b produced time- and concentration-dependent inactivation of S-adenosyl-L-homocysteine (AdoHcy) hydrolase producing significant decreases in the enzyme's NAD(+) content. However, 7b and 16b upon incubation with AdoHcy hydrolase were not metabolized suggesting that these compounds are type I mechanism-based inhibitors. No specific antiviral activity was noted for 5a,b, 7a,b, and 16a,b against any of the viruses tested; dibromodiene 5b proved cytotoxic at a concentration > or =6.7 microM and cytostatic at > or =11 microM, while dienoic esters 16a,b showed activity against both varicella-zoster virus (at 10 microM, 16a) and cytomegalovirus (at 10 microM, 16a; 18 microM, 16b).