Robert A. Pascal

Synthesis and X-ray crystallographic characterization of a (1,3,5)cyclophane with three amide N-H groups surrounding a central cavity. A neutral host for anion complexation

Abstract A short synthesis of 2.15.28-trioxo-3,16,29-triaza-6,19,32-trithia-[7.7.7](1,3,5)cyclophane(1) is described. An X-ray crystallographic analysis shows that the cyclophane exists in an extended conformation with three amide N-H groups directed approximately inward toward the central cavity.

Nitrogen-chlorine donor-acceptor interactions dominate the structure of crystalline cyanuric chloride

Abstract Cyanuric chloride (2,4,6-trichloro-1,3,5-triazine) crystallizes as notably slippery, colorless plates. X-ray analysis reveals a lattice formed from π-stacked layers of planar molecular arrays, in which all in-plane intermolecular interactions are short, linear Nue5f8Cl contacts (3.100 and 3.113 A), and the interlayer separation is 3.26 A.

Albatrossidine: A large, easily synthesized molecular cleft

Abstract 2,6-Bis(heptaphenyl-2-naphthyl)pyridine ( 1 ) was prepared in three steps from 2,6-bis(phenylethynyl)pyridine, and its X-ray structure was determined. The pyridine nitrogen lies at the base of a broad, chiral molecular cleft created by the perphenylnaphthyl “wings” of 1 .

Effects of thiastearic acids on growth and on dihydrosterculic acid and other phospholipid fatty acyl groups of Leishmania promastigotes.

Thiastearic acid positional isomers (8, 9, 10, 11) were examined for their ability to inhibit population growth and the biosynthesis of a phosphatidylethanolamine cyclopropane fatty acyl group, cis-9,10-methyleneoctadecanoic acid (dihydrosterculic acid), by promastigotes of Leishmania species. Thiastearic acids are candidate chemotherapeutic agents, since cyclopropane fatty acids are not formed by vertebrate cells. 8- and 10-thiastearic acids strongly inhibited the growth of strains containing the most dihydrosterculic acid (Leishmania tropica and Leishmania donovani; 25-35% phosphatidylethanolamine fatty acyl groups) and less strongly inhibited strains containing no dihydrosterculic acid (Leishmania major). The 11-thiastearic acid was less effective and 9-thiastearic acid ineffective. Strains containing 1-15% dihydrosterculic acid (L. donovani, Leishmania braziliensis, Leishmania aethiopica and Leishmania mexicana mexicana) were with few exceptions not inhibited by any of the isomers. All the thiastearic acid isomers caused a dose-dependent loss of dihydrosterculic acid. This was accompanied by a loss of phosphatidylethanolamine in the case of dihydrosterculic acid-rich leishmanial strains exposed to the 8- and 10-isomers. The 8- and 10-thiastearic acids also caused a loss of C18 unsaturated fatty acyl groups and increases in palmitic and stearic acids in the phosphatidylethanolamine and phosphatidylcholine of the dihydrosterculic acid-rich and dihydrosterculic acid-free leishmanial strains. 11-Thiastearic acid was much less effective and 9-thiastearic acid ineffective. These changes were not evident in those strins which contained 1-15% dihydrosterculic acid and whose growth was not inhibited by the thiastearic acid isomers. It is concluded that thiastearic acid isomers may inhibit both dihydrosterculic acid biosynthesis and fatty acid desaturation, with the 9-isomer having the highest specificity for dihydrosterculic acid biosynthesis. Population growth of promastigotes of Leishmania species in culture is not dependent upon dihydrosterculic acid biosynthesis but is dependent upon fatty acid desaturation.

The solid state structure of 9,11,20,22-tetraphenyltetrabenzo[a,c,l,n]pentacene-10,21-dione: A longitudinally twisted molecular ribbon

An X-ray crystallographic analysis of the title compound (1) shows that this fused-ring polycyclic molecule is greatly distorted from planarity. The structure is centrosymmetric, and the mean plane of the center ring is twisted by approximately 48° relative to the plane determined by the terminal atoms of the pentacene nucleus.

Molecular structures of host-guest complexes with Rebek's Diacid

Abstract The crystal and molecular structures of 1:1 complexes of Rebeks diacid (1) with pyrazine (2) and quinoxaline (3) have been determined by single crystal X-ray diffraction. The unit cells of the crystalline 1:2 and 1:3 each contain two crystallographically independent molecules of their respective complexes; all four complexes exhibit nearly symmetric, syn, two-point binding of the diamine guests by the convergent carboxyl groups of 1. These data confirm the original structural proposal of Rebek et al. (J. Am. Chem. Soc. 1987, 109, 2426–2431), and stand in contrast to the central conclusion (one-point binding) of Jorgensen et al.s computational study of the complex of Rebeks diacid and pyrazine (J. Am. Chem. Soc. 1989, 111, 755–757). In addition, the X-ray structure of Rebeks diacid hydrochloride (1:HCl) has been determined. In this complex, compound 1 is protonated on the acridine nitrogen, and the chloride counterion participates in a “three-point binding”, T-shaped, hydrogen-bonded network within the diacid cleft.

Curious reduction products from a strained polycyclic quinone

Abstract Sodium borohydride reduction of 9,11,20,22-tetraphenyltetrabenzo[ a,c,l,n ]pentacene-10,21-dione ( 1 ), a strained polycyclic quinone, followed by an aqueous workup does not give the conventional hydroquinone 2 . Instead, reduction of an adjacent aromatic ring leads to the cis -9,22-dihydro compound 3 in which steric crowding is substantially relieved.

Synthesis and structure of 1,7-dichlorodibenzo[ef,kl]heptalene-4,10-dione, a saddle-shaped polycyclic aromatic compound

Abstract We describe the synthesis and X-ray crystal structure of the title compound, which is the first fully unsaturated derivative of dibenzo[ef,kl]heptalene to be prepared. The X-ray structure reveals a saddle-shaped molecule possessing approximate C 2 symmetry.

A cyclophane bridged by an inwardly pyramidalized olefin

Abstract 2,8,17-Trithia[4 5,12 ][9]metacyclophan-4-ene, a cyclophane with a tertiary olefin strapped across the face of a benzene ring, has been prepared and its X-ray structure determined. The olefin is pyramidalized toward the benzene ring. The X-ray structure of the corresponding saturated phane ( in -2,8,17-trithia[4 5,12 ][9] metacyclophane, which was synthesized previously) has also been determined. The structure and reactivity of the in -olefin are discussed in the context of other structures and the results of computational studies.

Synthesis of sulfur-substituted phosphatidylethanolamines and inhibition of protozoan cyclopropane fatty acid synthase

Abstract A phosphalidylethanolamine(it1) containing two 9- thiastearyl groups was prepared by chemical synthesis. This compound and its methyl sulfonium derivative were found to be inhibitors of the cyclopropane fatty acid synthase from parasitic protozoan Crithidia fasciculata.