Allylic oxidation 12-membered ring

The synthesis of (23) illustrates how a six-membered ring may bo used to control even more remote chiral centres. Reverse Michael disconnection leaves enone (24), an oxidation product from allylic alcohol (25). The double bond can come from elimination on bromohydrln (26) and hence from (27). 

The thermolysis of acyclic- and/or six- and larger ring sulfoxides to yield olefins and sulfenic acids is well documented . The formation of allylic sulfenic acids and thiosulfinates in the thermolysis of thiirane oxides containing hydrogen on the a-carbon of the ring substituent (which is syn to the S—O bond) has been discussed previously in terms of /i-elimination of hydrogen, which is facilitated by relief of strain in the three-membered ring (Section llI.C.l). 

Electrochemical oxidation in methanol of both a-pinene 17 and p-pinene 18 leads to opening of the four membered ring by cleavage of one allylic carbon-... 

The palladium catalysed substitution reaction of allylic systems has also been utilised in the formation of five membered rings. Intramolecular nucleophilic attack of the amide nitrogen atom on the allylpalladium complex formed in the oxidative addition of the allyl acetate moiety on the catalyst led to the formation of the five membered ring (3.63.). In the presence of a copper(II) salt the intermediate pyrroline derivative oxidized to pyrrole.80... 

Cationic iron-alkene complexes also participate in an unusual cycloaddition process, wherein electron-deficient alkenes are attacked by nucleophilic o -allylic Fp complexes, generating stabilized carb-anions and cationic alkene-iron complexes. Attack of the carbanion on the alkene forming a five-membered ring completes this process (Scheme 13). Oxidative removal of the iron produces useful organic compounds.19-21... 

The Cl carboxylic acid substituent of the kainoids is introduced by oxidation of the aryl substituent derived from the A-benzyl group, a step which is characterised by chemoselectivity problems and only moderate yields. Cyclisation of alternatively substituted amides is less successful, though A-allyl groups can lead to new seven-membered rings (445 gives a mixture of 446 and 447).199 A-methoxybenzyl groups also cyclise, and their aromatic rings are oxidised to carboxylic acids more rapidly. 

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