Synthesis of Conformationally Restricted Carbocyclic Nucleosides: The Role of 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)MCdAdo), or a southern(S)‐type conformation, i.e. S‐type 2′‐deoxy‐methanocarba‐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 aristeromycin (10). Further comparison between the rates of deamination of these 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 furanose O(4′) atom, helped differentiate between the roles of the O(4′) anomeric effect and sugar conformation in controlling the rates of deamination by ADA. Differences in rates of deamination as large as 10000 can be attributed 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 preference for N‐type substrates is not arbitrary; it is rather the direct consequence of the conformationally dependent O(4′) anomeric effect, which is more efficient in N‐type conformers in promoting the formation of a covalent hydrate at the active site of the enzyme. The formation of a covalent hydrate at the active site of ADA precedes deamination. A new and efficient synthesis of the important carbobicyclic template 14a, a useful intermediate for the synthesis of (N)MCdAdo (8) and other conformationally restricted nucleosides, is also reported.