The crystal structure has been determined from three-dimensional X-ray data. The space group is P2~2~21 with four molecules in a unit cell of dimensions a= 21.466 (6), b = 8"804 (4) and e= 5.819 (2) A. The structure was solved by direct methods and refined with anisotropic temperature factors by fullmatrix least-squares procedures. The final R value for 1154 independent reflexions is 0-074. The molecules related by the twofold screw axis are linked by N-H.• -S hydrogen bonds to form a helicoidal packing arrangement. Other hydrogen bonds are present.

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