Showing papers by "George M. Sheldrick published in 1989"
TL;DR: In this paper, the authors present the application of several homo-and heteronuclear 1D and 2D-NMR techniques to assign the 1HNMR chemical shifts of the dominant conformation of didemnin B (2; three different conformations in (D6)DMSO solution in the ratio 8:1:1):1) and its conformational analysis, as well as the solution conformation, and the conformations were refined by restrained molecular-dynamics calculations using the GROMOS program and by MOMO, a novel
Abstract: We present the application of several homo- and heteronuclear 1D- and 2D-NMR techniques to assign the 1H-NMR chemical shifts of the dominant conformation of didemnin B (2; three different conformations in (D6)DMSO solution in the ratio 8:1:1) and its conformational analysis, as well as the solution conformation of didemnin A (1). The conformations were refined by restrained molecular-dynamics calculations using the GROMOS program and by MOMO, a novel personal-computer-based interactive molecular-graphics and molecular-mechanics package, using experimental distances (via a H…H pseudo potential function) as restraints. The solution structures of 1 and 2 obtained by GROMOS and MOMO calculations were compared with each other and related to the recently solved crystal structure of 2. Focusing on the main conformer, the two kinds of the distance-restrained conformational calculations for 2 yielded a ‘solution structure’ close to the crystal structure. Almost all of the 40 restrained H…H distances coincided (within the estimated standard deviations) with those observed in the crystal structure. One more hydrogen bond was detected in solution involving the lactoyl OH group (disordered in the crystal structure) and the dimethyltyrosine (Me2Tyr5) carbonyl O-atom. The macrocyclic ring system in the modeled solution structure of 1 exhibited a topology close to those of the solution and crystal structures of 2. The main difference between 1 and 2 could be traced back to a significant change in the Ψ angle of the N-methyl-D-leucine (MeLeu7) residue. In 1, the N-methyl moiety of MeLeu7 points inward within the macrocyclic ring toward the 1st and Hip region. We also tested the suitability of structures obtained from NMR data as ‘search fragments’ in the ‘Patterson search approach’ of crystal-structure analysis. It proved possible to resolve the crystal structure of 2 a posteriori with the Patterson search program PATSEE, in this way.
44 citations
34 citations
TL;DR: In this article, the titanadiazaphosphetidines Ph2P[µ-N(SiMe3)]2TiCl3MeCN (1) and NPPh2N(NiMe3)2]-NSiCl2 (2) were shown to be monoclinic and triclinic, space group P21/n, respectively.
Abstract: Depending on the reaction conditions, Ph2P(NSiMe3)[N(SiMe3)2]reacts with TiCl4 to form the titanadiazaphosphetidines Ph2P[µ-N(SiMe3)]2TiCl3MeCN (1) and Ph2P[µ-N(SiMe3)]2TiCl2,[NPPh2N(SiMe3)2](2). Compound (1) dimerises slowly in solution with elimination of Me3SiCl to yield a tricyclic system containing a central Ti2N2 ring (3). X-Ray analyses of these compounds show (1) and (3) to be monoclinic, space group P21/n, while (2) is triclinic, space group P. Compound (2) has a Ti–N–P angle of 170.3(7)° and a short exocyclic Ti–N bond of 179.2(9) pm.
14 citations
TL;DR: In this paper, the crystal structures of the lithium derivatives (CMe3)2SiFP(C6H2Me3), Li(THF)3 (2), and CMe3 2SiFLi(TMEDA)PC6H 2Me3(3) were determined.
Abstract: Di-tert-butyldifluorsilne reacts with lithiated 2,4,6-trimethylphenylphosphane to give (CMe3)2SiFPHC6H2Me3 (1). A lithium derivative is formed in the reaction of 1 with nC4H9Li. The crystal structures of the lithium derivatives (CMe3)2SiFP(C6H2Me3)Li(THF)3 (2) and (CMe3)2SiFLi(TMEDA)PC6H2Me3(3) were determined. The phosphorus atom is planar in 2 and pyramidal in 3. A (SiPLiF)-four-membered ring is formed in 3. LiF-elimination leads to the formation of the (SiP)-four-membered ring (4). The ring system of 4 is completely asymmetrical, indicating that the sterical limits of the dimerisation of the intermediate silaphosphene have been reached. Both 2 and 3 react with CMe3SiF3 to give the substituted compound (CMe3)2SiFP(SiF2CMe3)C6H2 Me3 (5).
13 citations
TL;DR: The reaction of LiPBuT 2 with InI 3 yields the monomeric compound In(PBu t 2 ) 3, while the action of BuT 2 PH on InEt 3 affords the dimeric phosphide as discussed by the authors.
12 citations
TL;DR: In this article, a suspension of 4.00g (21.6 mmol) of 1 in diethyl ether (50 mL) and triethylamine (2.9 mL) cooled to -40°C was treated with a solution of acetyl cyanide [lo] and the mixture vigorously stirred for 2 h between -40\" and 10°C. After extraction with ice-water the aqueous phase was extracted at 0°C with 50 mL of CH2CI2, the organic phases combined, dried with MgS04, and the solvent removed
Abstract: 2 : A suspension of 4.00g (21.6 mmol) of 1 in diethyl ether (50 mL) and triethylamine (2.9 mL, 21.6 mmol) cooled to -40°C was treated with a solution of acetyl cyanide [lo] (ISOg, 21.6 mmol) in diethyl ether (30 mL) and the mixture vigorously stirred for 2 h between -40\" and 10°C. After extraction with ice-water the aqueous phase was extracted at 0°C with 50 mL of CH2CI2, the organic phases combined, dried with MgS04, and the solvent removed in a rotary evaporator at 0°C. The residue was rapidly dissolved at room temperature in 100 mL of ether/petroleum ether ( I / ] ) and 20 mL of C H I C I ~ and recrystallized at -30°C. The bright yellow crystals of 2 were dried at -20°C. Yield 3.98 g (81%); correct elemental analysis (C,H,N). 'H NMR(~OOMHZ,CDCI,,~~OK):~=~.~~(S,~H),~.I~(~,~H),~.~~(~, IH), 7.47 (t. 2H), 7.58 (2d, 4H), 8.90 (N-H, IH). \"C NMR (400 MHz, CDCI,, 230 K):6= 19.3, 116.9, 126.7, 127.0, 127.7, 128.7, 136.7, 140.1, 145.4, 170.8. IR (Nujol): 5=3245, 1755, 1223 cm' .
10 citations
TL;DR: The absolute configurations of the known juglomycins A (1) and B (2) have been elucidated by single crystal X-ray structure analysis of 1 and its 6,8-dibromo derivative (1b) as discussed by the authors.
Abstract: The absolute configurations of the known juglomycins A (1) and B (2) have been elucidated by single crystal X-ray structure analysis of 1 and its 6,8-dibromo derivative (1b). The structure of 2 has been corrected; it differs from 1 in its configuration at C-4', and not at C-3' as previously assumed. Relationships with the closely related isochromanquinone antibiotics are discussed.
9 citations
TL;DR: The reaction of (CF3)2P(Cl)NSiMe3 and Me3SiNVCl3 in CH2Cl2 leads to volatile [(CF3]2PN]2NVCl2; the X-ray structure analysis shows a planar six-membered NVNPNP-ring with C2v molecular symmetry as discussed by the authors.
Abstract: The reaction of (CF3)2P(Cl)NSiMe3 and Me3SiNVCl3 in CH2Cl2 leads to volatile [(CF3)2PN]2NVCl2; the X-ray structure analysis shows a planar six-membered NVNPNP-ring with C2v molecular symmetry.
9 citations
TL;DR: In this article, the crystal structure of the four-membered (SiP) ring III has been determined and the SiP spin coupling constant (84.12 Hz) in II is remarkably large.
5 citations
TL;DR: In this article, the chiral centers of four diastereomeric bisoxiranes were established by chemical correlation with compounds of known stereochemistry and by single crystal X-ray structure determination of the benzoyl derivative.
Abstract: Upon complete epoxidation of the vitamin D3 4-phenyltriazoline-3,5-dione adducts2 and3 withMCPBA, four diastereomeric bisoxiranes7, 8, 9, and10 were generated. The stereochemistry of all the chiral centers in these compounds has been established by chemical correlation with compounds of known stereochemistry and by single crystal X-ray structure determination of the benzoyl derivative10 b.
2 citations
TL;DR: In this article, the authors derive quinoxalino [2,3-b:2',3'-b] from dioxalino and derive diquinox alino [ 2,3b: 2',3'b] for hexahydro.
Abstract: Photolyse UV d'hexahydro-5,5a,6,11,11a,12 tetramethyl-5,6,11,12 quinoxalino [2,3-b] quinoxaline en derive de diquinoxalino [2,3-b:2',3'-b] quinoxaline
TL;DR: In this article, the endocyclic acetal bond was investigated to determine the relative configuration, which could not be established unambiguously by NMR, but the structure was determined by the analysis of 1592 observed reflections.
Abstract: Methyl { 1-[ ( 1 'R)1 '-methoxybutyl]-2,3,4-triO-acetyl-fl-D-glucopyranosid }uronate, ClsH2sOt 1, Mr =420.41 , orthorhombic, P212121, a = 8 . 6 0 4 ( 3 ) , b = 13.500 (2), c = 18.664 (4) A, V = 2167.9 A 3, z = 4, D x = 1.288 Mg m -3, 2(Mo Ks) = 0.71069 A, /~ = 0-10 mm -1, F(000) = 896, T = 298 K, R = 0.055 for 1592 observed reflections. The structure was investigated to determine the relative configuration, which could not be established unambiguously by NMR. The endocyclic acetal bond is longer than the exocyclic by
TL;DR: In this paper, the first triazatrimetallabenzene derivative, [Cp∗Ta(Cl)N]3, as well as eight-membered phosphazene rings containing two vanadium atoms are reported.
Abstract: Syntheses and structures of novel inorganic heterocycles containing two or three transition metal atoms are reported. Examples include the first triazatrimetallabenzene derivative, [Cp∗Ta(Cl)N]3, as well as eight-membered phosphazene rings containing two vanadium atoms.
TL;DR: In this paper, the endocyclic acetal bond was investigated to determine the relative configuration, which could not be established unambiguously by NMR, but the structure was determined by the analysis of 1592 observed reflections.
Abstract: Methyl { 1-[ ( 1 'R)1 '-methoxybutyl]-2,3,4-triO-acetyl-fl-D-glucopyranosid }uronate, ClsH2sOt 1, Mr =420.41 , orthorhombic, P212121, a = 8 . 6 0 4 ( 3 ) , b = 13.500 (2), c = 18.664 (4) A, V = 2167.9 A 3, z = 4, D x = 1.288 Mg m -3, 2(Mo Ks) = 0.71069 A, /~ = 0-10 mm -1, F(000) = 896, T = 298 K, R = 0.055 for 1592 observed reflections. The structure was investigated to determine the relative configuration, which could not be established unambiguously by NMR. The endocyclic acetal bond is longer than the exocyclic by