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Major Isomer (major + isomer)
Selected AbstractsDivalent Samarium Triflate Mediated Stereoselective Pinacol Coupling of Planar Chiral Phosphanyl and Phosphoryl FerrocenecarbaldehydeEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 5 2009Shin-ichi Fukuzawa Abstract The pinacol coupling reaction of (Rp)-2-diphenylphosphanyl ferrocenecarbaldehyde (1) was smoothly mediated by divalent samarium triflate to give (R,R)-diol 2a predominantly, whereas the use of samarium(II) iodide resulted in low selectivity as described in the previous literature. In contrast, the coupling reaction of (Rp)-2-diphenylphosphoryl ferrocenecarbaldehyde (3) with Sm(OTf)2 gave the (S,S)-diol as the major isomer, which was the opposite stereochemistry of that obtained in the reaction with 1. The rhodium complexes of diphosphanes 2a were good catalysts for the asymmetric hydrogenation of ,-acetamidocinnamic acid, and the product was obtained quantitatively with up to 92,%,ee. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Etorphine-Related Ferrocenyl-Substituted Morphinan AlkaloidsHELVETICA CHIMICA ACTA, Issue 10 2003Gerhard Laus The two diastereoisomeric ferrocenyl-substituted orvinols 2 and 3 were prepared. The modified alkaloids are still able to interact with opioid receptors (see Table). The ferrocene moiety allows highly selective and sensitive electrochemical detection. The X-ray crystal structure of the major isomer 2 was determined. The combination of a metallocene and a morphinan alkaloid holds promise for useful antitumor activity. [source] Highly Selective Stereodivergent Synthesis of Separable Amide Rotamers, by Using Pd Chemistry, and Their Thermodynamic BehaviorCHEMISTRY - A EUROPEAN JOURNAL, Issue 20 2009Nobutaka Ototake Abstract Going separate ways: By using ,-allyl,palladium chemistry (path,A) and PdII -catalyzed Claisen chemistry (path,B), a highly selective stereodivergent synthesis of separable amide rotamers was achieved (see scheme). By using Pd chemistry, a highly selective stereodivergent synthesis of separable amide rotamers was achieved. Allylation of 2,4,6-tri- tert -butylanilides using a ,-allyl,Pd catalyst gave N-allylated anilides with moderate-to-excellent Z -rotamer selectivity (Z/E=3 to >50). The Z -rotamer selectivity depends considerably on the substituent on the anilide substrates. On the other hand, the E rotamers of N-allylated 2,4,6-tri- tert -butylanilides were obtained with almost complete selectivity (E/Z>50) through O-allylation of 2,4,6-tri- tert -butylanilides and the subsequent PdII -catalyzed Claisen rearrangement of the resulting O-allyl imidate. The prepared anilide rotamers changed to equilibrium mixtures in which the E rotamer was the major isomer when heated in toluene. [source] Stereoselective Allylation of Ketones: Explanation for the Unusual Inversion of the Induced Stereochemistry in the Auxiliary-Mediated Crotylation and Pentenylation of Butanone by DFT CalculationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 7 2009Abstract The solvent makes the difference: While attack at the Re face of the intermediate oxocarbenium ion becomes less energy demanding with increasing size of R,, it is the solvent effect that makes this attack the main trajectory, leading to inversed stereochemistry (see picture). Auxiliary-mediated domino crotylations and pentenylations of butanone yield homoallylic ethers with two newly formed stereogenic centers. With our norpseudoephedrine-derived auxiliary, we observed the formation of anti isomers exclusively, and the nature of the major isomer was independent of the substrate double bond geometry. Interestingly, there is a switch in induced selectivity when going from crotylation to pentenylation. Here, we present the computational rationalization for this behavior by identification of the relevant transition states (TSs), the energies of which were determined by using the B3LYP/6-31+G(d) level of theory in combination with the PCM/UAKS method to include the effects exerted by the solvent dichloromethane. To quickly narrow down the number of potentially relevant TSs from the whole set of 288 and 864 TSs for the crotylation and pentenylation, respectively, we employed a screening process based on B3LYP//AM1 energies. The predicted selectivities are in good agreement with experimentally determined ones. Furthermore, the obtained results also facilitate an explanation of the selectivities obtained in hexenylations and heptenylations. Finally, activation energies were determined that account for the significantly longer reaction times than those for the domino allylation with unsubstituted trimethylallylsilane. [source] Carboazidation of Chiral Allylsilanes: Experimental and Theoretical InvestigationsCHEMISTRY - A EUROPEAN JOURNAL, Issue 9 2008Laurent Chabaud Dr. Abstract The carboazidation of chiral allylsilanes has been investigated by varying the nature of the substituents at the silicon center and on the carbon framework. The influence of temperature and the nature of the sulfonyl azide, as well as the stereochemistry of the remote stereogenic center, on the 1,2-diastereocontrol of the process were considered. Good to excellent levels of diastereocontrol were generally observed, with the syn -,-azidosilane always being formed as the major isomer. An illustration of the value of this methodology has been provided with a short and efficient synthesis of an analogue of castanospermine. EPR spectroscopy was carried out on various ,-silyl radicals providing useful information about their conformations in the ground state. Based on this experimental evidence and DFT calculations, reactant-like transition state models were finally proposed that rationalize the observed 1,2-stereoinduction. [source] Regioselectivity of Friedländer Quinoline Syntheses,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 10 2008Claas Lüder Diedrich Abstract Optically active, bicyclic ketones were submitted to Friedländer quinoline syntheses with 2-aminobenzaldehyde to yield regioisomeric linear or angular products. When starting from trans -configured ketones, the linear products are the major isomers (ratios ranging from 76:24 to >98/2). With cis -configured ketones the angular products are predominantly formed, although with lower regioselectivity.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Structures of the Reactive Intermediates in Organocatalysis with Diarylprolinol EthersHELVETICA CHIMICA ACTA, Issue 7 2009Abstract Structures of the reactive intermediates (enamines and iminium ions) of organocatalysis with diarylprolinol derivatives have been determined. To this end, diarylprolinol methyl and silyl ethers, 1, and aldehydes, PhCH2CHO, tBuCH2CHO, PhCH=CHCHO, are condensed to the corresponding enamines, A and 3 (Scheme,2), and cinnamoylidene iminium salts, B and 4 (Scheme,3). These are isolated and fully characterized by melting/decomposition points, [,]D, elemental analysis, IR and NMR spectroscopy, and high-resolution mass spectrometry (HR-MS). Salts with BF4, PF6, SbF6, and the weakly coordinating Al[OC(CF3)3]4 anion were prepared. X-Ray crystal structures of an enamine and of six iminium salts have been obtained and are described herein (Figs.,2 and 4,8, and Tables,2 and 7) and in a previous preliminary communication (Helv. Chim. Acta2008, 91, 1999). According to the NMR spectra (in CDCl3, (D6)DMSO, (D6)acetone, or CD3OD; Table,1), the major isomers 4 of the iminium salts have (E)-configuration of the exocyclic NC(1,) bond, but there are up to 11% of the (Z)-isomer present in these solutions (Fig.,1). In all crystal structures, the iminium ions have (E)-configuration, and the conformation around the exocyclic N-CC-O bond is synclinal-exo (cf.C and L), with one of the phenyl groups over the pyrrolidine ring, and the RO group over the , -system. One of the meta -substituents (Me in 4b, CF3 in 4c and 4e) on a 3,5-disubstituted phenyl group is also located in the space above the , -system. DFT Calculations at various levels of theory (Tables,3,6) confirm that the experimentally determined structures (cf. Fig.,10) are by far (up to 8.3,kcal/mol) the most stable ones. Implications of the results with respect to the mechanism of organocatalysis by diarylprolinol derivatives are discussed. [source] | ||||||||||