Synthesis of conformationally restricted carbocyclic nucleosides: The roleof the O(4 ')-atom in the key hydration step of adenosine deaminase

Conformationally restricted carbocyclic nucleosides with either a northern( N)-type conformation. i.e., N-type 2'-deoxy-methanocarba-adenosine 8 ((N)MC dAdo), or a southern(S)-type conformation, i.e. S-type 2'-deoxy-methanocarb a-adenosine 9, ((S)MCdAdo), were used as substrates for adenosine deaminase (ADA) to assess the enzyme's preference for a fixed conformation relative to the flexible conformation represented by the carbocyclic nucleoside aris teromycin (10). Further comparison between the rates of deamination of thes e compounds with those of the two natural substrates adenosine (Ado; 1) and 2'-deoxyadenosine (dAdo; 2), as well as with that of the conformationally locked nucleoside LNA-Ado (11), which, like the natural substrates, has a f uranose O(4') atom, helped differentiate between the roles of the O(4') ano meric effect and sugar conformation in controlling the rates of deamination by ADA. Differences in rates of deamination as large as 10000 can be attri buted to the combined effect of the O(4') atom and the enzyme's preference for an N-type conformation. The hypothesis proposed is that ADA's preferenc e For N-type substrates is not arbitrary; it is rather the direct consequen ce of the conformationally dependent O(4') anomeric effect, which is more e fficient in N-type conformers in promoting the formation of a covalent hydr ate at the active site of the enzyme. The formation of a covalent hydrate a t the active site of ADA precedes deamination. A new and efficient synthesi s of the important carbobicyclic template 14a, a useful intermediate for th e synthesis of (N)MCdAdo (8) and other conformationally restricted nucleosi des, is also reported.