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1.
Sandeep Kanwar 《Tetrahedron letters》2005,46(8):1329-1332
Chiral oxocarbenium ions have been exploited to carry out highly diastereoselective and enantioselective acetate aldol addition reactions. The chiral auxiliary has been optimized to give the product with good diastereoselectivity. 相似文献
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The novel styryl lactone analog (±)-3-deoxyisoaltholactone was synthesized in 5 steps from dihydrofuran. An intramolecular vinylsilane–oxocarbenium condensation thus afforded the central cis-fused bicyclic ether array in near quantitative yield as a single stereoisomer. 相似文献
4.
A nickel(II)-catalyzed asymmetric alkylation of acyclic oxocarbenium ions generated in situ from corresponding acetals with carboxylic acid derivatives to prepare β-alkoxyl carbonyl moieties with diverse α-substituents has been disclosed. The method exhibited broad scope of acetals and carboxylic acid derivatives with excellent enantioselectivity and good functional group compatibility, and can be conducted in a gram-scale without obvious loss of efficiency. 相似文献
5.
The first asymmetric chiral N-triflylphosphoramide-catalyzed ionic [2+4] cycloaddition reaction of unsaturated acetals is described. This reaction proceeds through the intermediacy of a vinyl oxocarbenium/chiral anion pair, and the chiral N-triflylphosphoramide anion controls the stereoselectivity of the cycloaddition step. Moderate enantioselectivities (up to 80:20 e.r.) have been obtained when α,β-unsaturated dioxolanes were employed as the dienophiles. These reactions demonstrate strong dependence on the counterion coordinating properties and solvent polarity, a behavior characteristic of oxocarbenium ions. 相似文献
6.
We have examined several methods for the stereoselective formation of carbon-carbon bonds between contiguous rings where a stereogenic center is already present. The approaches investigated were a [1,3] oxygen to carbon rearrangement of cyclic vinyl acetals, an intermolecular enolsilane addition into an in situ generated oxocarbenium ion, an intramolecular conjugate addition of tethered alkoxy enones, and epimerization of several α-pyranyl cycloalkanones. These routes have been found to be complementary in several cases and have enabled formation of both the trans:anti and cis:anti stereoisomers in good to excellent yields and varying diastereoselectivities. We have proven C2-C2′ relative stereochemistry of 1-2 via a chemical correlation. 相似文献
7.
Robert J. Hinkle Yajing Lian Nichole D. Litvinas Alex T. Jenkins Daniel C. Burnette 《Tetrahedron》2005,61(49):11679-11685
For BiBr3 initiated tandem cyclization-additions of very reactive silyl ketene acetal nucleophiles with δ-silyloxy aldehydes to afford 2,6-disubstituted THP products, diastereoselectivities range from 5-6:1 (trans-/cis-). The selectivity for axial attack on the intermediate oxocarbenium ion is inversely proportional to π-nucleophilicity. We have utilized this chemistry to convert a common starting material to both cis- and trans-diastereomers of an acid first isolated from the secretions of Viverra civetta. 相似文献
8.
Vitaly A. Shcherbinin Ekaterina S. Spesivaya Valery V. Konshin 《Tetrahedron letters》2019,60(14):957-960
The reactions of levulinic acid and its pseudo esters, which are readily available from carbohydrate-containing raw materials, with various C-nucleophiles are described. Different reaction pathways were demonstrated depending on the nature of the leaving groups and nucleophiles. For 5-methyl-5-methoxydihydrofuran-2(3H)-one, the reaction was found to proceed by a route giving hept-6-enoic acid derivatives. In contrast, the reaction of 2-methyl-5-oxotetrahydrofuran-2-yl acetate proceeds via intramolecular cyclization to form γ-valerolactone derivatives. 相似文献
9.
An efficient method for the synthesis of O-substituted mandelic acids containing alkenyl, alkynyl, methoxycarbonyl, or phenacyl fragments via the Lewis acid-catalyzed reaction of 1,3-dioxolan-4-ones with different C-nucleophiles is proposed. 相似文献
10.
Xiaohan Chen Ran Zhao Ziqiang Liu Shutao Sun Yingang Ma Qingyun Liu Xia Sun Lei Liu 《中国化学快报》2021,32(7):2305-2308
Chiral α-substituted 1,3-dihydroisobenzofurans are key scaffolds in a number of bioactive natural products and synthetic pharmaceuticals. However, catalytic asymmetric approaches have been rarely developed. Here, a redox deracemization technology is adopted to address the catalytic asymmetric synthesis. A broad range of α-aryl substituted 1,3-dihydroisobenzofurans are effectively deracemized in high efficiency with excellent ee. α-Alkynyl substituted ethers were also compatible with the deracemization technology. 相似文献