Corey resonance

So either, or some combination of both, the oxygen-halogen van der Waals repulsion and the Corey resonance appeared to be responsible for the observed progression in the axial-equatorial equilibria as the halogen was changed from bromine to chlorine to fluorine. 

When we look at the axial bromine compound, the Corey resonance causes a lengthening of the C-Br and C=0 bonds, and a shortening of the C-C bond. These effects can best be seen by comparing the axial values with those in the equatorial. The... 

Recently, a variety of (3-silylated carboxonium ions have been prepared and characterized by NMR spectroscopy.541 Kira et al.631 used the Corey hydride transfer method, whereas Olah, Prakash, and co-workers applied triphenylmethyl tetrakis (pentafluorophenyl)borate to silylate esters,632 ketones, enones, and carbonates633 [Eq. (3.91)]. The ions thus produced are resonance hybrids of oxocarbenium (327b) and carboxonium (327a) ions with the latter as the major contributors. Calculated (DFT/IGLO) 29 Si NMR chemical shifts agree well with the experimental data. 

Corey and Zhu96 examined the sterochemistry of the oligomers (up to Si4) obtained by dehydrocoupling of PhMeSiH2 with the Cp2ZrCl2/BuLi catalyst. They used the distribution and intensities of the H NMR resonances due to the Si-Me groups in the trimer and tetramer to estimate the isomeric composition. In the case of the trimer, the 11 resonances anticipated for a statistical distribution of the 2 meso and 1 racemic isomers were all observed, including one with twice the intensity of the other 10. The case was less clear cut for the tetramer, but the cluster of Me resonances did not have the intensity distribution expected for a statistical distribution of stereoisomers.96... 

Barton deoxygenation, 548, 549 [Functionalized carbocycles] bicyclic compounds, 551 caiba-D-fructofuranose, 547 Corey lactone, 549 Cp2TiCl catalyst, 553, 563 cyclic transition state, 548, 549 cyclohexanes, 554 cyclopentanes, 546 1,2-dialkylcyclopentanes, 548 electron spin resonance, 552 eneynes, 556... 

GMP 5 - guanosine monophosphate, 5 - CMP 5 - cytosine monophosphate. HPLC high - pressure liquid chromatography NMR nuclear magnetic resonance, s = singlet, d doublet, t triplet, q = quartet, m multiplet, NOE nuclear Overhauser effect, CD circular dichroism. tn trimethylene diamine, dien diethylene-triamine. CPK models Corey - Pauling - Koltun atomic models. 

Synthesis of (+)-lactacystin Lactacystin 7 is one of the most important biologically active pyrrolidinone-based natural products found in nature. It was isolated from the culture broth of a Streptomyces in 1991, and since that time, it has generated an enormous interest as a consequence of its highly selective and irreversible inhibition of the 20S pro-teasome. Its synthesis was first reported by Corey and Reichard, and an alternative more recent approach is based on the preparation of an oxazoline 8 that is produced via formation of a 2-ethynyl-allyl epoxide 10 via Sharpless asymmetric epoxidation of the allyl alcohol 9 (Scheme 34.3). The reaction was carried out in the presence of catalytic amounts of calcium hydride and silica gel (whose role is not described in the paper) at —40°C to — 18°C obtaining the desired product 10 in 89% yield and >95% ee determined by integration of the F resonances of the corresponding Mosher ester derivative. 

The echoes of Corey s seminal discovery with sulfonium and sulfoxonium ylides have found considerable resonance in modern investigations of enan-tioselective cyclopropanation reactions. Shibasaki showed that treatment of pyrrole amides such as 126 with trimethylsulfoxonium ylide was effectively promoted by the La-Li-(biphenyldiolate) catalyst 127 to give the cyclopropane adduct 128 in 98% ee (Equation 20) [78]. Key to the success of the transformation was the use of Nal as an additive. The investigators speculated that exchange of Li for Na occurs in 127 to some extent to give an active cyclopropanation catalyst that is more enantioselective. The observation underscored the versatility of such bifunctional heterobimetallic catalysts, the properties of which can be fine-tuned in a multitude of ways by variation of the ligand and the nature of the metals employed [79]. 

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