POSIT is a computer program package for the determination of the approximate structure of small organic molecules using known molecular fragments. The orientation and the translation vector of this fragment are determined and refined by a special R-value analysis. In contrast to other molecular replacement methods, only a few low-order strong reflections are needed to start the structure analysis. Therefore, this method is suitable for the determination of crystal structures of small rigid or semi-rigid organic molecules without heavy atoms from powder diffraction data. CsH6N202, Mr=126"ll, monoclinic, P2~/c, a=4.5137(7), b=10.9888 (12) for 165 approximate structure factors obtained from a powder diffraction measurement. The molecules were found in the diketo form. They are linked by a two-dimensional network of hydrogen bonds forming layers parallel to the yz plane.

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The title compound 9-vinyladenine crystallizes in the monoclinic space group C2/c with 16molecules in the unit cell and the lattice parameters a = 13.682, b = 10.260, c = = 21.674 A, /? = 97.72". The crystal structure has been determined by direct methods and refined by full matrix least squares calculations up to the discrepancy factor R = 0.045.The two symmetry independent molecules differ in the orientation of the vinyl group relative to the adenine frame.Die Titelverbindung 9-Vinyladenin kristallisiert, in der monoklinen Raumgruppe C 2 / c mit 16 Molekulen in der Elementarzelle und den Gitterkonstanten a = 13,682, b = 10,250, e = 21,674 A, /I = 97,72". Die Kristallstruktur wurde mit direkten Methoden bestimmt und mit der Methode der kleinsten Quadrate bis zu einem R-Wert von 0,045 verfeinert.Die zwei symmetrieunabhangigen Molekule unterscheiden sich in der Orientierung der Vinylgruppe relativ zum Adeninkfirper.

The 1 titljor constituent of the alkaloid fraction of the leaves of Puraei~lluras wiu[rophyllu I'IERRE has found to be the new paravallarine-type steroidal alkaloid 20-epi-kibataline. The structure of this steroidal alkaloid has been determined by X-ray analysis. The corilpouncl crystallizes in the inonoclinic space group 1'21 with the lattice parameters

For the title compound a guaianolide structure is shown by X‐ray analysis. Only the relative configuration of the molecule could be established.