Epoxidation isotope labeling experiment

The reaction of iV-(2,4-dinitrophenyl)amino acids with base in aqueous dioxane has been shown to give benzimidazole iV-oxides (7). The rate-determining step is likely to be formation of an iV-alkylidene-2-nitrosoaniline intermediate (6), which is followed by rapid cyclization and decarboxylation.19 The loss of carbon dioxide from perbenzoate anions has been investigated by mass spectrometry and electronic structure calculations. The results, including isotopic labelling experiments, support a mechanism involving initial intramolecular nucleophilic attack at either the ortho- or ipso-ring positions. They also indicate that epoxides may be intermediates en route to the phenoxide products.20 There has also been a theoretical study of the formation of trichlorinated dibenzo-/ -dioxins by reaction of 2,4,5-trichlorophenolate ions with 2,4-dichlorophenol.21... 

Oxiranes (or epoxides) are conveniently formed by delivery of electrophilic oxygen to one of the faces of an alkene. This is achieved with a peroxy acid (peracid) in a single-step reaction (Scheme 4.2) that necessarily delivers oxygen syn to the double bond. Isotope labelling experiments have shown that the oxygen atom that is transferred to the alkene is the one that is further from the carbonyl group of the peroxy acid. 

Subsequently, the isotope labeling experiment using water, H2 O (95% O), was performed during olefin epoxidation using czs-stilbene. cis-Stilbene is a useful substrate when seeking mechanistic information from isotope labeling... 

The isotopic 180 labelling experiments for the incorporation of oxygen into the epoxide, support mainly an Ag-O complex as the reactive species, because the incorporation of 180 labelled oxygen from H2180 into the epoxide is consistent with an independent Ag-O intermediate which undergoes isotopic exchange with 180 enriched water (ref. 10)... 

There are as yet no conclusions nor even a consensus hypothesis as to which types of chlorine substitution patterns govern ease of metabolism by enzymes. Some researchers favor the hypothesis that chlorine substitution at the 4,4 or combinations at the 3,5 3 5 positions of the biphenyl molecule block ease of enzymatic epoxidation of easily accessible vicinal carbons (14, 35). Other researchers suggest that 2,2 or 6,6 substitutions or some combination reduce coplanarlty of the biphenyl rings thereby causing a steric hlnderance to enzymatic activity or transfer across membranes (14, 36). Our data on Individual chloroblphenyla In biota does not yet encompass a wide enough range of chloroblphenyl structures to test these hypotheses which must be tested rigorously with Isotopically labeled chloroblphenyla In carefully controlled experiments In any event. 

Isotopic 0 labeling experiments suggested that the formation of 5-HPETE occurs via radical trapping of oxygen. The 5-HPETE is then transformed into the (5S, 6S)-epoxide LTA4 by a loss of water, involving stereospecific elimination of a C-10 hydrogen from 5-HPETE. ... 

Denmark has recently disclosed results that help address the question of whether dioxirane (A) or Criegee-type (B) intermediates are involved in ketone-catalyzed epoxidation reactions (Fig. 14) [98]. [ 0]-Labeling experiments using ketone 40 showed 80% of the expected isotope label was incorporated into the epoxide product, providing compelling evidence that dioxiranes are indeed the active oxidizing species [99]. 

The reaction of peroxyacids with alkenes is illustrated by the experiment that reacts cyclopentene with peroxyacetic acid (83). There are two isolated products cyclopentene oxide (88) in 57% yield and acetic acid (89). The yield is only 57% because the remainder of the 100% is unreacted alkene. Labeling experiments (replacing the natural isotope with the less abundant isotope 1 0) show that the oxygen atom of the epoxide in 88 comes from the OH oxygen in the peroxyacid (labeled in red in 83 and 88). 

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