Chemoselective allylic oxidation

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

Considering the excellent chemoselectivity observed in the allylic oxidation of dehydroepiandrosterone (Scheme 16), it was interesting to evaluate the selective allylic alcohol oxidation in the presence of a secondary saturated hydroxyl group using the BiCls/f-BuOOH system. This study was performed using androst-... 

The second synthetic approach to oidiolactone C (61) is summarized in Scheme 20. This route also commences with the ozonolysis of trans-communic acid 180. Now, when this compound was exposed to ozone in excess, keto aldehyde 187 was obtained in 76% yield. The key step in this approach was the y-lactone closure via chemoselective reduction of the lactone moiety on compound 189 through a SN2 mechanism. Compound 189 could be prepared by saponification of the corresponding methyl ester with sodium propanethiolate. Once the primary alcohol is oxidized, the completion of the synthesis of key lactone 103 only requires the allylic oxidation of the C-17 methyl with concomitant closure of the 8-lactone. This conversion was achieved with Se02 in refluxing acetic acid to give 103 in 51% yield. 

As shown in Scheme 3.19, two competing pathways are possible with regard to allylic oxidation. The alkene 1 can either undergo abstraction of an allylic hydrogen and subsequent formation of the allylic alcohol 2 and the enone 3 (path A), respectively, or alternatively epoxidation of the C=C double bond occurs to give derivative 4 (path B). In order to develop a suitable catalytic system for path A, it is of utmost importance to achieve high chemoselectivity in addition to high catalytic... 

Simple Fe3+ salts have rarely been used for catalytic allylic oxidations. Covalent metal nitrates are well known to be strong oxidants which undergo dissociation of the bidentate metal nitrate bond resulting in the formation of the N03 radical as reactive species [105], However, Sahle-Demessie and coworkers were the first who showed the utility of even commercially available Fe(N03)3-9H20 as an oxidation catalyst [106], Turnover and chemoselectivity turned out to be strongly dependent on the alkene substrate and the partial pressure (Scheme 3.20). 

A chromium hexacarbonyl-r-butyl hydroperoxide system has also been developed with the remarkable chemoselective ability to e ect allylic oxidation even in the presence of some secondary alcohols (equations 40 and 41). ... 

For most reagents this pattern is multiplied in cases where several similar possible sites of oxidation exist around a particular double bond. While chemoselectivity and stereoselectivity are often good, poor regioselecdvity is a weakness afflicting many allylic oxidation methods. 

Treatment of 762 with allyl bromide and sodium hydride provides in 82% yield the C2-symmetric pyrrolidine 776. Chemoselective N-oxidation with er butylhydroperoxide in the presence of vanadyl acetylacetonate affords in 75% yield the N-oxide 111 which, when treated with LDA, forms a benzylideneazomethine ylid (having the Z-configuration) that undergoes an intramolecular 1,3-dipolar cycloaddition to afford the e isolable product in 35% yield (Scheme 170). 

There are some important features of reductive dioxygen activation which are apparent from Table II. The chemoselectivity between allylic oxidation and epoxidation is illustrated by the first two entries with no reducing additive epoxidation is strongly suppressed in favor of allylic oxidation to cyclohexenol and cyclohexenone when 0 is the... 

The observed types of reactions include alkene epoxidation, allylic and benzylic oxidation, and alkane hydroxylation. The ligands on the cobalt atom control the chemoselectivity of oxidation. 

Allylic Oxidation. Mn(OAc)3 can serve as a catalyst to promote allylic oxidation of alkenes to the corresponding enones (eq 48). The method shows excellent regioselectivity and chemoselectivity. Depending on the nature of the substrate, the reaction is done under a nitrogen or an oxygen atmosphere. 

The reductive coupling/silylation reaction was extended to more complicated polyenes, such as the triene-substituted cyclopentanol 73, which cyclizes to provide 74 with a 72% yield and 6 1 dr after oxidation (Eq. 10) [44], The reaction is chemoselective the initial insertion occurs into the allyl substituent, which then inserts into the less hindered of the two remaining olefins, leaving the most hindered alkene unreacted. 

The sulfone moiety was reductively removed and the TBS ether was cleaved chemoselectively in the presence of a TPS ether to afford a primary alcohol (Scheme 13). The alcohol was transformed into the corresponding bromide that served as alkylating agent for the deprotonated ethyl 2-(di-ethylphosphono)propionate. Bromination and phosphonate alkylation were performed in a one-pot procedure [33]. The TPS protecting group was removed and the alcohol was then oxidized to afford the aldehyde 68 [42]. An intramolecular HWE reaction under Masamune-Roush conditions provided a macrocycle as a mixture of double bond isomers [43]. The ElZ isomers were separated after the reduction of the a, -unsaturated ester to the allylic alcohol 84. Deprotection of the tertiary alcohol and protection of the prima-... 

The RLi homochiral ligand complexes are seldom used for the base-promoted isomerization of oxiranes into allylic alcohols because their poor chemoselectivity lead to complex mixtures of products. As examples, the treatment of cyclohexene oxide by a 1 1 i-BuLi/(—)-sparteine mixture in ether at low temperature provides a mixture of three different products arising respectively from -deprotonation (75), a-deprotonation (76) and nucleophilic addition (77) (Scheme 32) . When exposed to similar conditions, the disubstituted cyclooctene oxide 78 affords a nearly 1 1 mixture of a- and -deprotonation products (79 and 80) with moderate ee (Scheme 32, entry 1). Further studies have demonstrated that the a//3 ratio depends strongly on the type of ligand used (Scheme 32, entry 1 vs. entry 2) . ... 

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