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101.
Ming YuBrian L. Pagenkopf 《Tetrahedron》2003,59(16):2765-2771
The conversion of glucal derived donor-acceptor cyclopropanes to acetals by TiCl4 mediated ring opening and alcohol trapping is described. Good selectivity for the alpha anomer and yields near 90% are consistently observed with aliphatic, benzylic and allylic alcohols, phenols and thiophenols. The utility of this method to prepare oligosaccharides through intermediate O,S-acetals is illustrated. 相似文献
102.
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104.
Dipl.‐Chem. Tobias F. Schneider Dipl.‐Chem. Johannes Kaschel Shahid I. Awan Dr. Birger Dittrich Dr. Daniel B. Werz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(37):11276-11288
The synthesis of oligocyclic oligoacetals using five‐membered rings as repetitive unit is described. Furan was used as the starting material, which is converted by a three‐step procedure consisting of twofold cyclopropanation, reduction, and oxidative ring enlargement into a tricyclic bis(enol ether). A repetition of this synthetic procedure leads to the formation of extended oligoacetal systems. Insights into the structures were gained by X‐ray crystallographic investigations and revealed helical arrangements of the subunits in the solid‐state. DFT (B3LYP) calculations have been carried out to elucidate the transition state of the ring enlargement and the flexibility of the annelated oligocyclic systems. Strain energies and topologies of potential cyclically condensed oligoacetals are predicted. 相似文献
105.
含有推-拉电子(DA)体系的张力环化合物,例如DA环丙烷,是非常有用的合成砌块,被应用于天然产物全合成以及合成具有生物活性的分子的研究中。 近年来,本课题组利用手性铜、镍等配合物为催化剂,一方面发展了一系列高效合成手性DA环丙烷的新方法;另一方面陆续实现了仲胺、醇、硝酮、氮杂亚胺叶立德、烯醇硅醚、吲哚等多种亲核试剂与DA环丙烷的高对映选择性开环/环化反应。 本文结合我们课题组工作综述了DA环丙烷化合物的不对称合成、开环/环化反应以及动力学拆分方面的主要研究进展并进行了展望。 相似文献
106.
Dr. Patrick M. Jüstel Alexandra Stan Cedric D. Pignot Dr. Armin R. Ofial 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(64):15928-15935
The kinetics of the ring-opening reactions of thiophenolates with geminal bis(acceptor)-substituted cyclopropanes in DMSO at 20 °C was monitored by photometric methods. The determined second-order rate constants of the SN2 reactions followed linear relationships with Mayr nucleophilicity parameters (N/sN) and Brønsted basicities (pKaH) of the thiophenolates as well as with Hammett substituent parameters (σ) for groups attached to the thiophenolates. Phenyl-substituted cyclopropanes reacted by up to a factor of 15 faster than their unsubstituted analogues, in accord with the known activating effect of adjacent π-systems in SN2 reactions. Variation of the electronic properties of substituents at the phenyl groups of the cyclopropanes gave rise to parabolic Hammett relationships. Thus, the inherent SN2 reactivity of electrophilic cyclopropanes is activated by electron-rich π-systems because of the more advanced C1−C2 bond polarization in the transition state. On the other hand, electron-poor π-systems also lower the energetic barriers for the attack of anionic nucleophiles owing to attractive electrostatic interactions. 相似文献
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108.
The (19)F NMR spectra of a series of fluorinated cyclopropanes, most of which contain chlorine or bromine on the ring, have been observed and analyzed. A scheme has been developed to assign the resonances and the molecular stereochemistry, based on substituent effects, comparison of related molecules, and isotope shifts induced by the halogens. Replacement of fluorine by chlorine shifts cis fluorine resonances to lower field, and bromine has an even greater downfield influence. However, the shift effect of bromine compared to chlorine on gem or trans fluorines is variable. The magnitude of the isotope shifts is found to be regularly related to the geometrical relationship of the halogen to the observed fluorine and thus makes a significant contribution to the fluorine assignments. The three-bond spin-spin coupling constants between fluorine atoms in cyclopropanes display unusual behavior and are not helpful for assignment of the fluorine resonances. The signs of the coupling constants have been investigated by spin-tickling experiments, and the previously developed relation of the coupling constant sign to its temperature dependence has been found to be violated for some molecules. 相似文献
109.
Davide Moi Maria Chiara Cabua Viktoria Velichko Andrea Cocco Annalisa Chiappone Rita Mocci Stefania Porcu Monica Piras Stefano Bianco Fabio Pesciaioli Francesco Secci 《Molecules (Basel, Switzerland)》2022,27(22)
The straightforward, continuous-flow synthesis of cyclopropyl carbaldehydes and ketones has been developed starting from 2-hydroxycyclobutanones and aryl thiols. This acid-catalyzed mediated procedure allows access to the multigram and easily scalable synthesis of cyclopropyl adducts under mild conditions, using reusable Amberlyst-35 as a catalyst. The resins, suitably ground and used for filling steel columns, have been characterized via TGA, ATR, SEM and BET analyses to describe the physical–chemical properties of the packed bed and the continuous-flow system in detail. To highlight the synthetic versatility of the arylthiocyclopropyl carbonyl compounds, a series of selective oxidation reactions have been performed to access sulfoxide and sulfone carbaldehyde cyclopropanes, oxiranes and carboxylic acid derivatives. 相似文献
110.
Hillary Straub Pavel Ryabchuk Marina Rubina Michael Rubin 《Molecules (Basel, Switzerland)》2022,27(20)
Enantiomerically enriched cyclopropyl ethers, amines, and cyclopropylazole derivatives possessing three stereogenic carbon atoms in a small cycle are obtained via the diastereoselective, formal nucleophilic substitution of chiral, non-racemic bromocyclopropanes. The key feature of this methodology is the utilization of the chiral center of the cyclopropene intermediate, which governs the configuration of the two adjacent stereocenters that are successively installed via 1,4-addition/epimerization sequence. 相似文献