Allyl esters regioselective oxidation

O-Deprotection. Free-radical-initiated bromination followed by treatment with water removes a benzyl ester. A similar method is also effective to cleave allyl ethers. Regioselective oxidations. An alkane-1,2-diol can be selectively oxidized to give the ketol (instead of the hydroxy aldehyde) via NBS reaction of the dibutylstan-nylene acetal. ... 

Regioselective Oxidation Allyl and Homoallyl Ethers and Esters... 

Cu(OTf)2 in the presence of the ligand ( )-N-((naphthalen-7-yl)methylene) benzenamine and C6H5NHNH2 catalyses selective oxidation of benzylic C(sp )-H bonds to C(sp )-0 bonds with t-butyl perbenzoate (TBPB) in acetone. Cu(MeCN)4PF5 catalyses asymmetric allylic oxidation of acyclic olefins by TBPB in acetone in the presence of spiro bisoxazoline ligands (8) the product allyl esters are formed with excellent regioselectivity (>20 1 in most cases) and up to 67% ee. °°... 

Palladium is a useful catalyst in several reactions which lead to keto-esters. p-Keto-esters react with allylic carbonates, with catalysis by palladium, in a decarboxylative allylation reaction. y-Keto-esters are prepared, in reasonable yield, by the palladium-catalysed regioselective oxidation of Py-unsaturated esters. y-Keto-esters are obtained in good yield by the palladium-catalysed Reformatsky reaction of ethyl bromoacetate and acid chlorides. Derivatives of y-ketopimelic acid are formed by the rhodium-carbonyl-catalysed reaction of derivatives of acrylic acid with carbon monoxide. A mild method for the conversion of propiolic esters into P-keto-esters via thiol addition has been reported (Scheme 63) it has been used in a formal synthesis of ( )-thienamycin. 

Regiospecific oxidation of internal olefins bearing neighbouring oxygen functions by means of palladium catalysts can be achieved. The method is an effective way of preparing synthetically useful y-keto-esters or 1,4-diketones. Later work showed that the process could also be applied to allylic and homo-allylic ethers and esters (Scheme Similarly, regioselective oxidation of a -unsaturated esters furnishes p-keto-esters, and this has been elegantly applied in the -lactam field (Scheme 8). ... 

Allyllic ether 53 is oxidized regioselectively to the /3-alkoxy ketone 54, which is converted into the a,/i-unsaturated ketone 55 and used for annulation[99]. The ester of homoallylic alcohol 56 is oxidized mainlv to the 7-acetoxy ketone 57[99]. 

Selenium-mediated allylic oxidations producing allylic alcohols have been discussed above however, in some cases oxidation proceeds further to give the a, -unsaturated carbonyl compounds directly, or mixtures of alcoholic and ketonic products. That the regioselectivity observed in these allylic oxidation reactions closely resembles that found in classical selenium dioxide oxidations is in accord with initial formation of the intermediate allylic alcohol before in situ oxidation to the carbonyl compound. This process was studied by Rapoport and was explained mechanistically as an elimination of the intermediate allylic selenite ester via a cyclic transition state, analogous to Ssi (rather than 5n20 solvolysis (Scheme 21). Of the two possible transition states (78) and (79), the cyclic alternative (78) was preferred tecause oxidation exclusively yields trans aldehydes. 

Electrochemical oxidations have been previously mentioned.122 A novel application is the regioselective epoxidation (80% conversion 90% selectivity) at the 6,7-bond of geranyl and neryl esters and phenyl sulphones.214 Another is the one-step conversion of olefins into allylic alcohols via electrooxidative-oxyselenylation-deselenylation, e.g. (97)—>(99). This method has been applied to the synthesis of (100) which can be converted (70%) into marmelolactone [(101) from quince], and... 

The triester (38) was needed for an investigation into intramolecular pericyclic reactions between electron-rich (a) and electron-poor (b) double bonds.A Wittig disconnection on double bond (b) (nearer the centre of the molecule) demands an a-dicarbonyl compound whichever way we write it. The keto diester (39) can easily be made from a malonate ester by a-oxidation, so this route is preferred. Further disconnection of phosphonium salt (40) suggests allylic alcohol (41) as intermediate and hence regioselective reduction (Chapter 14) of a,/3-unsaturated aldehyde (42) (Chapter 19). 

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