Showing papers by "Drake S. Eggleston published in 1990"
TL;DR: In this article, a two-step procedure consisting of an asymmetric epoxidation mediated by a poly-L-leucine polymer, followed by a previously unreported Baeyer-Villiger oxidation was found to depend on the aryl substituent.
55 citations
8 citations
TL;DR: In this article, X-ray diffraction analysis reveals 5-hydroxywarfarin crystallizes as the open isomer in contrast to the more than 20 warfarin and analog structures which occur ascis or trans hemiketal forms in the solid.
Abstract: Addition of a 5-hydroxy substituent to warfarin [3-(1-phenyl-3-oxobutyl)-4-hydroxycoumarin] shifts the solution equilibrium in chloroform to favor the open isomeric form over the two cyclic diastereomeric hemiketals. X-ray diffraction analysis reveals 5-hydroxywarfarin crystallizes as the open isomer in contrast to the more than 20 warfarin and analog structures which occur ascis ortrans hemiketal forms in the solid. The two peri hydroxyl groups in the structure of 3-(1-phenyl-3-oxobutyl)-4,5-dihydroxycoumarin are intramolecularly H-bonded. Methyl kctal derivatives of 5-hydroxywarfarin and a close analog effectively model the minor cyclic hemiketal forms in solution. Structures of bothcis andtrans cyclic methyl ketals of 3-(4-oxopent-2-yl)-4,5-dihydroxy-coumarin have been determined and the aryl hydroxyls H-bond to the dihydropyranyl ring oxygens of the cyclic ketals. Nuclear magnetic resonance studies suggest that these intramolecular H-bonds persist in chloroform solution. Infrared spectroscopy on the series of compounds in KBr pellets is consistent with the crystallographically determined structures and H-bonding schemes.
8 citations
TL;DR: In this article, a dilithio reagent was generated in situ by the metalation of 2,2′-dibromobiphenyl with n-butyl lithium.
4 citations
TL;DR: The crystal structures of 2-methyl-5-oxo-4phenyl-3,4-dihydro-2Hpyrano are described in this paper.
Abstract: The crystal structures of (_)-cis-2-methyl-5-oxo-4phenyl-3,4-dihydro-2H,5H-pyrano[3,2-c][1]benzopyran-2-yl acetate [C21H1805, Mr = 350"37, monoclinic, P2Jn, a = 12.091 (4), b = 8.288 (3), c = 17.840 (5)A, fl=106.34(2) ° , V=1715(2) A 3, Z=4, Dx= 1.356 g cm -3, A(Mo K~) = 0.7107 A, /z = 0.904 cm-~, F(000) = 736, T = 295 K, R = 0.050 for 2767 observations with I>_ 3tr(/)] and (6R,12S)-(-)6,8-dimethyl-6,12-methano-6H, 12H, 13H-[1]benzopyran[4,3-d][1,3]benzodioxocin-13-0ne [C20Hi604, Mr = 320.36, tetragonal, P43, a = 10.788 (4), c = 13.587 (9) A, V= 1581 (2) A 3, Z = 4, Ox = 1.345 g cm -3, a(Mo K~) = 0.7107 ,~, /z = 0.873 cm-l, F(000) = 672, T = 295 K, R = 0.049 for 1425 observations with I___ 2-5tr(/)] are described. They are acyl and aryl ketals of warfarin, respectively, and contain an embedded dihydropyran ring. The molecules were studied as part of a series of axial 2-O-substituted-2-methyl-3,4-dihydro-2Hpyran structures which show (hemi)ketal C---O bond-length variations indentified through factor analysis with the systematic geometrical changes associated with a spontaneous elimination (El-like) reaction from the ketal leading to 2-methyl-4Hpyran. As in a-tetrahydropyranyl acetals, the C--O lengths in dihydropyranyl ketals can be expressed as a function of the electron-withdrawing ability of the substituent conjugate base, and the slopes of the relationships for the two systems are similar. Corresponding endocyclic C--O lengths are about 0.052 A longer in these model dihydropyranyl ketals.
4 citations