Inhibitors of Guanine and Adenine/Adenosine Deaminases Based On Ring-Expanded Purine Structural Skeleton

The work described in this dissertation is a continuation of decades of research conducted in this laboratory on ring-expanded heterocycles and nucleosides as inhibitors of guanine- and adenine/adenosine deaminases, the key members of the deaminase family of enzymes of nucleic acid metabolism. The dissertation consists of two parts: Part-I: Synthesis and Biochemical Studies of Ring Expanded Purine Analogues Containing the 5:7-Fused Imidazo[4,5-e][1,4]diazepine Ring System as inhibitors of Guanine Deaminase, and Part-II: Development of Synthetic Strategies for Ring-Expanded Purine Analogues Containing the 5:8-Fused Imidazo[4,5-f][1,4]diazocine Ring System as Potential Inhibitors of Adenine/Adenosine Deaminases. Part-I of the dissertation is based on a hypothesis that azepinomycin, a purported transition state analogue inhibitor of guanase, does not represent the transition state of the enzyme-catalyzed reaction as closely as does iso-azepinomycin, wherein the 6-hydroxy group of azepinomycin has been translocated to the 5-position. This is because the hydrolysis involves the C2 and N3 (imine N) atoms rather than C2 and N1 (lactan N) of guanine. Therefore, an aminol intermediate formed by hydrolysis of an imine precursor would be better represented by iso-azepinomycin than azepinomycin. Based on this hypothesis, and assuming that iso-azepinomycin would bind to guanase at the same active site as azepinomycin, several analogues of iso-azepinomycin were designed and successfully synthesized in order to gain a preliminary understanding of the hydrophobic and hydrophilic sites surrounding the guanase binding site of the ligand. Specifically, the analogues were designed to explore the hydrophobic or hydrophilic pockets, if any, in the vicinity of N1, N3, and N4 nitrogen atoms as well as O5 oxygen atom of iso-azepinomycin. Biochemical inhibition studies of these analogues were performed using a mammalian guanase. Our results indicate that (1) increasing the hydrophobicity near O5 results in a negative effect, (2) translocating the hydrophobicity from N3 to N1 also results in decreased inhibition, (3) increasing the hydrophobicity near N4 produces significant enhancement of inhibition, (4) increasing the hydrophobicity at both N4 and O5 considerably brings down the inhibition, and (5) finally, increasing the hydrophilic character near N3 has a deleterious effect on inhibition. Part-II of this dissertation work is geared toward developing synthetic strategies for ring-expanded heterocycles and nucleosides containing the described 5:8-fused ring systems. Most of the analogues investigated in the Hosmane lab so far include ring-expanded heterocycles and nucleosides containing the 5:7-fused ring systems. Little, if anything, is known about the effect of further increasing the size of the 7-membered diazepine ring to the 8-membered diazocine ring to form the 5:8-fused ring systems. Our molecular modeling studies with a hypothetical compound containing the 5:8-fused imidazo[4,5-f][1,4]diazocine ring system revealed that a number of hydrophobic amino acid residues fall near the zinc co-ordination site of ADA. Therefore, ring expansion from the 7-membered diazepine ring to the 8-membered diazocine ring with an extended alkyl chain is anticipated to enhance hydrophobic interactions with the enzyme. Furthermore, the ring expansion from 7 to 8 does not seem to adversely affect the interactions of the heterocycle with the protein. I have successfully synthesized the target parent 5:8-fused heterocyclic ring system as well as its 1-benzyl and 3-benzyl analogues. In addition, the desired nucleoside precursor for the final ring-annulation has also been successfully synthesized. These analogues pave way into the new territory of ring-expanded heterocyclic and nucleoside analogues containing the 5:8-fused imidazodiazocine ring systems