C. Piccinnileopardi

A Novel Type of Bis-quinolizidine Alkaloid From the Sponge Petrosia-seriata

Abstract The structure of petrosin, the major alkaloid isolated from the sponge Petrosia seriata , has been determined by X-ray diffraction analysis. Petrosin is the first member of a new class of bis-quinolizidine alkaloids containing a C-16 macrocycle.

The electric field contribution to the screening constant in 13C NMR and its dependence on the longitudinal bond polarizabilities. the syn/anti anisochrony problem in amides

Abstract The electric field contribution to the screening constant in 13 C NMR depends on the charge borne by the observed carbon atom. This charge can be estimated either by quantum mechanics or by classical methods. It appears that each of these approaches is difficult to handle for complex molecules. The classical approach has the merit of explicitly showing the influence of the various bonds connected to the carbon under study. Nevertheless, the great dependence of the calculated value on the longitudinal bond polarizabilities appears to be a limiting factor because these polarizabilities are not known with sufficient precision. This problem is discussed in detail, and the influence of the choice of a set of polarizability values is illustrated for the itsyn/anti anisochrony of the α (to nitrogen) carbon atoms in N,N -dialkylamides. The influence of other parameters, such as field gradient and dipole position, is also examined. The accuracy of the electric field contribution to the screening constant is then critically discussed. The origin of the syn/anti anisochrony does not appear to be due to the electric field effect. Other possible contributions are discussed but no definitive conclusion can be drawn.

Eight-membered heterocycles containing two phosphorus atoms. X-ray diffraction and nuclear magnetic resonance studies of 2,6-dimethyl-1,3-dioxa-2,6-diphosphacyclo-octane 2,6-disulphides and 2,6-diselenides

trans- and cis-2,6-dimethyl-1,3-dioxa-2,6-diphosphacyclo-octane 2,6-disulphides (1) and (2), and trans- and cis-2,6-diselenides (3) and (4), have been investigated by means of 1H (2D,J-resolved), 13C, and 31P n.m.r. spectroscopy and X-ray diffraction. In the solid state the cis-isomers adopt a crown conformation and the trans-isomers a boat-chair conformation. The orientations of P–X (X = S or Se) and P–Me bonds are quasi-axial and quasi-equatorial, respectively. Room-temperature n.m.r. data can be interpreted on the basis of the X-ray data. A fast enantiomerization between two boat-chair conformations of opposite chirality characterizes the trans-isomers, whereas the cis-isomers exist mainly in a crown conformation similar to that observed in the crystal.

An eight-membered ring containing two phosphorus atoms. trans isomer of 2,6-dimethyl-1,3-dioxa-2,6-diphosphacyclooctane 2,6-disulphide

C6H]402P2S 2 is monoclinic, P2]/n, with Z = 4, a = 9.954(2), b = 9.632(1), c = 12.165(1)A, fl = 102.31 (1) °, D c = 1.42, O m -1-40 (2) Mg m -3. 2012 independent reflexions were collected on a Syntex computer-automated diffractometer. The structure was solved by direct methods using the MULTAN system. A final R value of 0.037 was obtained for 1770 reflexions with I > 2.5o(1). The eight-membered ring adopts an asymmetric boat-chair conformation. The bond lengths and bond angles are compared to those obtained on cyclic 2-methyl-2-thioxo-l,3-dioxa compounds of various ring size.

1,3,2,5-Dioxadiphosphorinanes. Six-membered ring compounds containing two different phosphorus atoms

The reaction of bis(hydroxymethyl)phenylphosphane withtert-butyldichlorophosphane leads to the formation ofcis-2-tert-butyl-5-phenyl-1,3,2,5-dioxadiphosphorinane (1). Thecis configuration was definitively proven by the X-ray diffraction study of the 2, 5-dithioxo derivative (2), which was obtained from (1) by a stereoselective reaction. The diselenoxo compound (3) was also prepared. These three new compounds were subjected to a complete NMR study (1H,13C,31P, and77Se).

Chemical studies of marine invertebrates. Part 46. Confirmation of the molecular structure of sigmosceptrellin-B by X-ray diffraction analysis

The structure of sigmosceptrellin-B (3), a norsesterterpene peroxide isolated from the sponge Sigmosceptrella laevis, had previously been proposed on the basis of chemical correlations. This structure has now been independently confirmed by an X-ray diffraction analysis on the crystalline derivative (1S,4aR,5S,6R,8aR)-1-hydroxymethyl-5-{2-[(3R,6S)-3-(1R)-(2-hydroxy-1-methylethyl)-6-methyl-1,2-dioxan-6-yl]ethyl}-5,6,8a-trimethylbicyclo[4.4.0]decan-1-ol (6). This paper discusses the conformational parameters of the 1,2-dioxan ring of (6).