Cycloalkene oxides isomerization

The growing interest in enantioselective isomerization of meso oxiranes to allylic alcohols arises from the ready availabihty of starting materials and the synthetic value of the homochiral products. First apphed to simple meso cycloalkene oxides, this methodology has been successfully exteuded to fuuctioualized meso oxiranes, and even to the kinetic resolution of racemic oxiranes, demonstrating its potential in accessing highly advanced synthons. 

Isomerization of substituted styrene oxides allows the synthesis of aldehydes in high yields726 [Eq. (5.275)]. Cycloalkene oxides do not react under these conditions, whereas 2,2,3-trimethyloxirane gives isopropyl methyl ketone (85% yield). Isomerization of oxiranes to carbonyl compounds is mechanistically similar to the pinacol rearrangement involving either the formation of an intermediate carbocation or a concerted mechanism may also be operative. Glycidic esters are transformed to a-hydroxy-/3,y-unsaturated esters in the presence of Nafion-H727 [Eq. (5.276)]. 

Oxidation with lead tetraacetate is a far less selective process.490,491 Studied mainly in the oxidation of cycloalkenes, it gives stereoisomeric 1,2-diol diacetates, but side reactions (allylic acetoxylation, skeletal rearrangement) often occur. A change in reaction conditions in the oxidation of cyclopentadiene allows the synthesis of different isomeric mono- and diesters.492... 

Despite earlier observations,624 625 ozonolysis of cyclic olefins in MeOH or in mixed solvents (ethers or esters and MeOH) followed by isomerization in the presence of Lindlar Pd and hydrogen does not give directly dicarboxylic acids. Instead, mainly the corresponding dialdehydes are formed.626 The best method of synthesis of dicarboxylic acids is ozonolysis of cycloalkenes in a mixture of acetic acid and formic acid followed by further oxidation with oxygen 626... 

Cyclic olefins preferably undergo twofold carbonylation, even without the presence of buffer compounds such as sodium acetate. With methanol serving as the solvent and under comparable reaction conditions to those mentioned above, cycloalkenes give product mixtures consisting of methyl rrans-2-methoxy-cycloalkanecarboxylates, dimethyl cw-cycloalkane-l,2-dicarboxylates and dimethyl cw-cycloalkane-1,3-dicarboxylates (eq. (3) and Table 1). The formation of 1,3-diesters may be explained by an intermediate reaction sequence which involves an oxidative elimination and a subsequent readdition of the Pd species, thus giving rise to an isomerization [9]. 

The relative reactivities of different alkenes towards 9-borabicyclo[3,3,l]nonane (9-BBN) and di-isoamylborane have been studied. Di-isoamylborane is the more selective for cycloalkenes, the relative rates of hydroboration of cyclop>entene, cyclo-heptene, and cis-cyclo-octene being 1.0 18.6 68.6, respectively. Hydroboration of substituted cyclo-octenes using diborane is complicated by rapid isomerization of the initially formed alkyl-borane, leading to mixtures of alcohols on oxidation. However, 9-alkyl-9-borabicyclo[3,3,l]nonanes are thermally stable, and hydroboronation of 1-alkylcyclo-octenes using 9-BBN leads sp>ecifically to trans-2-alkylcyclo-octanols after oxidation. ... 

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