A conformationally locked analogue of the anti-HIV agent stavudine. An important correlation between pseudorotation and maximum amplitude

The synthesis and biological evaluation of a bicyclo[3.1.0]hexene nucleoside designed as a conformational mimic of the anti-HIV agent stavudine (1, D4T) is described. The unsaturated methanocarbocyclic pseudosugar of N-MCD4T (2) was constructed from an iodo-substituted precursor by a DBU-catalyzed olefination reaction. Mitsunobu coupling with N³-benzoylthymine afforded the desired target after deprotection. Both D4T and N-MCD4T are in the North (N) hemisphere of the pseudorotational cycle but 70° away from a perfect N (P = 0°) conformation toward the East and West hemispheres, respectively. Despite this large difference, the double bond reduces the puckering amplitude (ν_max) of N-MCD4T to 6.81°, and the superposition of both structures showed a RMS deviation of only 0.039 Å. The combined structural analysis of P and ν_max shows that while the value of P may differ substantially, the low ν_max resolves the differences and becomes the dominant pseudorotational parameter. N-MCD4T is active against HIV-1 and HIV-2 in CEM, MT-2, and MT-4 cells, and while it is somewhat less potent than D4T, it also appears to be less toxic. The triphosphate (N-MCD4TTP) inhibits HIV reverse transcriptase with a 10-fold higher IC₅₀ than D4TTP. By virtue of its carbocyclic nature, N-MCD4T (2) is a more robust molecule stable to conditions that would cleave D4T.