2',3'-Dideoxy-3'-C,2'-N-[(3R,5R)-5-ethoxy-carbonyl-2-methyl-1, 2-isoxazolidine]-ribothymidine

The title compound, C15H21N3O7, is a thymidine derivative with a tetrahydroisoxazole (THI) ring fused to the sugar ring at the 2'- and 3'-C atoms. The thymine base is in an anti conformation [chi = -122.5 (3) degrees] while the ribose moiety has C2'-exo,C3'-endo (3T2) conformation with a highly flattened sugar ring [P = -5.8, tau m = 13.1 degrees]. The molecule contains a pseudo-twofold axis on the C2'-C3' bond with the ethyl ester group acting as the base and the N-methyl group acting as the 5'-hydroxyl group in a nucleoside. The THI ring fused to the furanose has a C3'-endo,C4'-exo (3T4) conformation [P = 4.18, tau m = 19.3 degrees]. The ethyl ester moiety is disordered with respect to two conformations, the population of the major form being 87.4 (8)% and that of the minor form 12.6 (8)%.