Abacavir methanol 2.5-solvate

The structure of abacavir (systematic name: {(1S,4R)-4-[2-amino-6-(cyclopropylamino)-9H-purin-9-yl]cyclopent-2-en-1-yl}methanol), C14H18N6O·2.5CH3OH, consists of hydrogen-bonded ribbons which are further held together by additional hydrogen bonds involving the hydroxyl group and two N atoms on an adjacent purine. The asymmetric unit also contains 2.5 molecules of methanol solvate which were grossly disordered and were excluded using SQUEEZE subroutine in PLATON [Spek, (2009 ▶). Acta Cryst. D65, 148–155].

The absolute configuration around C9 and C11 of abacavir was assigned as R and S, respectively, based on the synthetic procedures. The large voids in the lattice of abacavir appear to hold methanol solvate molecules but attempts to model the solvent were unsuccessful.

Experimental
Abacavir was prepared according to literature procedure (Vince & Hua, 1990). The compound was dissolved in a minimal amount of hot methanol and the solution was then placed in a chamber saturated with dichloromethane at room temperature, covered and allowed to crystallize for two weeks. The resulting clear colorless rod shaped crystals were washed with cold methanol, dried then collected and a suitable crystal was selected for structural determination.

Refinement
The program PLATON (Spek, 2009) indicated solvent accessible void space of 688.7 Å 3 , corresponding to 179 electrons in a unit cell, equivalent to ten molecules of methanol solvate. Since the solvent molecules were grossly disordered and could not be modeled, their contribution was excluded using the subroutine SQUEEZE. H atoms were placed in idealized positions and treated as riding atoms with distances: O-H = 0.84, N-H 0.88 and C-H in the range 0.95-1.00 Å and U iso (H) = 1.2U eq (parent atom). An absolute structure could not be determined by anoimalous dispersion effects; Friedel pairs (2405) were therefore merged. Fig. 1. The molecular structure of abacavir with atomic lables; thermal displacement ellipsoids have been plotted at 50% probability level.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.