Epoxides catalyzed

Recently (79MI50500) Sharpless and coworkers have shown that r-butyl hydroperoxide (TBHP) epoxidations, catalyzed by molybdenum or vanadium compounds, offer advantages over peroxy acids with regard to safety, cost and, sometimes, selectivity, e.g. Scheme 73, although this is not always the case (Scheme 74). The oxidation of propene by 1-phenylethyl hydroperoxide is an important industrial route to methyloxirane (propylene oxide) (79MI5501). 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

In another study that appeared prior to the advent of CASTing, the traditional combination of epPCR and DNA shuffling was used to enhance the enantioselectivity of the hydrolytic kinetic resolution of p-nitro phenyl glycidyl ether and other epoxides catalyzed by the EH from Agrobacterium radiobacter [59]. Several mutants were obtained with up to 13-fold improved enantioselectivity. The amino acid exchanges took place around the active site. 

A modified Payne rearrangement of amino epoxides catalyzed by Lewis acid or induced by base, represents an interesting but a limited method for the synthesis of fonctionalized aziridines of high enantiopurity. The limitations are primarily due to the accessibility of the starting materials (Scheme 6) [15]. 

Asymmetric Ring Opening of Some Terminal Epoxides Catalyzed by Dimeric Type Novel Chiral Co(Salen) Complexes... 

Fig. 1. UV-Vis absorption spectra of the precatalyst Scheme 2. Possible working model for the HKR chiral Co(salen) and monomer and dimer complex. of terminal epoxides catalyzed by C0-AIX3...

The hydrolytic kinetic resolution of terminal epoxides catalyzed by the monomer la and dimer lb... 

The carcinogenicity of PAH with relativelyTigh IP, such as benzo[c]phenanthrene, benz[a]anthracene, chrysene, 5-methyl chrysene and dibenz[a,h]anthracene (Table I), can be related to the formation of bay-region diol epoxides catalyzed by monooxygenase enzymes (j>). However, the most potent carcinogenic PAH have IP < ca. 7.35 eV. 

Fig. 16. Proposed reaction mechanism for the olefin ketonation and epoxidation catalyzed by platinum-blues.

Several general reviews describe the state of the art of peroxide epoxidation catalyzed by TM compounds at about a decade ago [2A. Later on, specialized reviews dealt with particular peroxides ofCr, Mo, andW [5], V [6], and with epoxidation reactions catalyzed by methyltrioxorhenium (MTO) [7] that involve Re peroxo complexes as species responsible for the oxygen transfer. 

We also note that some 2,2-disubstituted oxiranes have toxicological significance, as exemplified by 2,2-dimethyloxirane (2-methyl-l, 2-epoxypropane, 10.43, R = Me). This compound is the toxic metabolite of 2-methyl-prop-1-ene (isobutene), a gaseous alkene widely used as a monomer in the industrial production of adhesives, plastics, and other polymers. Interestingly, detoxification of this epoxide catalyzed by liver epoxide hydrolase was high in the human, intermediate in the rat, and low in the mouse [125], These activities were inversely correlated with the epoxide levels measured in vitro in liver tissue of these species. 

Interestingly, there is a marked species difference in the in vitro hydrolysis of carbamazepine 10,11-epoxide, such that the reaction was observed only in liver microsomes from humans but not in liver microsomal or cytosolic preparations from dogs, rabbits, hamsters, rats, or mice [181][196], Thus, carbamazepine appears to be a very poor substrate for EH, in analogy with the simpler analogues 10.129 (X = RN, RCH, or RCH=C). The human enzyme is exceptional in this respect, but not, however, in the steric course of the reaction. The diol formed (10.131, X = H2NCON) is mostly the trans-(10.S, 11. S )-enaniiomer [196], In other words, the product enantioselectivity of the hydration of carbamazepine epoxide catalyzed by human EH is the same as that of di benzol a,oxide catalyzed by rabbit microsomal EH, discussed above. 

Scheme 5. HKR of terminal epoxides catalyzed by polymeric salen catalysts 26-32.

Table 2. HKR of terminal and meso epoxide catalyzed by the immobilized chiral Co(III)...

Table 6 Stirred batch reactor ARO of epoxides catalyzed by a dimeric (R,R)-Cr(III)(salen) complex 65 immobilized in a supported ionic liquid phase compared to the dimeric complex impregnated on silica 64-silica and the reported homogeneous reactions with the monomeric Cr(salen) complex 63...

Tokuoka, E. Kotani, S. Matsimaga, H. Ishizuka, T. Hashimoto, S. Nakajima, M. (2005) Asymmetric ring opening of meso-epoxides catalyzed by the chiral phosphine oxide BINAPO., Tetrahedron Asymmetry 16 2391-2392. 

Gigante, B. Corma, A. Garcia, H. Sabater, M. J. (2000) Assessment of the negative factors responsible for the decrease in the enantioselectivity for the ring opening of epoxides catalyzed by chiral supported Cr(III)-salen complexes Catalysis Lett. 68 113-119. 

Kureshy, R. I. Singh, S. Khan, N. H. Abdi, S. H. R . Agrawal S. Jasra R. V. (2006) Enantioselective aminolytic kinetic resolution (AKR) of epoxides catalyzed by recyclable polymeric Cr(III) salen complexes Tetrahedron Asymmetry 17 1638-1643. 

Scheme 6.22 Substrate-dependent cyclization of enynyl epoxides catalyzed by ruthenium.

Cyclooctene epoxidation catalyzed by supported sulfonated metalloporphyrins. 

Alkenes. Alkenes are, in general, metaholically stable. The majority of alkene-containing drugs do not exhibit significant rapid metabolism at the double bond. There are some isolated examples of alkene-containing compounds that undergo epoxidation, catalyzed by mixed-function oxidase, or that add water across the double bond to give an alcohol. 

Table 1 Polymerization of terminal epoxides catalyzed by aluminum-porphyrin complexes...

Hori, K., Tamura, M., Tani, K., Nishiwaki, N., Ariga, M. and Tohda, Y. Asymmetric Epoxidation Catalyzed by Novel Azacrown Ether-type Chiral Quaternary Ammonium Salts under Phase-transfer Catalytic Conditions. Tetrahedron Lett. 2006, 47, 3115-3118. 

Asymmetric epoxidation catalyzed by novel azacrown ether-type chiral... 

ASYMMETRIC EPOXIDATION CATALYZED BY NOVEL AZACROWN ETHER-TYPE CHIRAL QUATERNARY AMMONIUM SALTS UNDER PHASE-TRANSFER CATALYTIC CONDITIONS... 

Ni(II) complexes of cyclam and oxocyclam derivatives catalyze the epoxidation of cyclohexene and various aryl-substituted alkenes with PhIO and NaOCl as oxidants, respectively. In the epoxidation catalyzed by the Ni(II) cyclam complex using PhIO as a terminal oxidant, the high-valent nickel- complexes (e.g., LNiin-0, LNi=0, LNiin-0-... 

Figure 5.14 Routes of oxidative metabolism of aflatoxin showing activation to an epoxide catalyzed by CYP1A2 and CYP3A4 and conjugation with glutathione catalyzed by a-GST. The 8,9-exoepoxide is the enantiomer, which binds extensively to DNA and is carcinogenic. The endoepoxide binds less readily. Abbreviation. a-GST, a-glutathione transferase.

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