X-Ray-Diffraction Study and Determination of Absolute Configuration of the Anticancer Drug S(–) Cyclophosphamide (Endoxan, Cytoxan, NSC-26271)

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Abstract
(-) Cyclophosphamide (1) crystallized from tetrachlorom ethane in the triclinic space group P1 with cell dimensions a = 10.500 (4) Å, b = 10.490 (4) Å, c = 10.761 (4) A, α = 110.0 (2) ° , β = 11 0.0(2)°, γ = 1 0 8.9(2)°. Three molecules are contained in the unit cell. The X-ray analysis was based on diffractometer measurement of 2635 independent reflections and the structure was solved by Patterson and direct methods. The final R and R w factors after full-matrix least-squares refinement are 0.0717 and 0.0677, respectively. The absolute configuration is S based on Hamilton’s R-factor ratio test. The oxazaphosphorinane ring exists in a chair form with the bis-β-chloroethyl-amino group in equatorial position and about perpendicular to and bisecting . The structure is similar to that found in racemic 1 except a different conformation about one ethyl-amino N-C bond.

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