Fragmentation of Cyclic Oxonium Intermediates

In addition to the formation and reactions of carbonyl ylides discussed in the previous section, carbenoids also react intramolecularly with ethereal oxygen atoms to generate oxonium intermediates. When the ether is part of a ring as in substrates 63 a-b, the intramolecular addition of rhodium carbenoids produces bicyclic oxonium intermediates, which generated [5.2.1] oxabicycles 64a-b upon rearrangement by a [2,3]-sigmatropic pathway, Eq. 44 [74]. 

West has studied another system that generated oxonium intermediates which underwent fragmentation by a similar pathway, Eq. 45 [75]. 

For substrates lacking olefinic migrating groups, [1,2]-shifts occur instead to give oxabicydic products, Eq. 46. 

Molander reported formation of [3.2.1] and [3.3.1] oxabicydic compounds by applying Trost s trimethylenemethane dianion chemistry with dicarbonyl substrates, Eq. 47 [76]. The inherent symmetry of the dianion limits this methodology to those compounds with relatively simple and symmetrical structures. 

In 1979, Chan reported the synthesis of oxabicyclo [3.2.1] and [3.3.1] compounds using the bis-silylated enol ethers of ketoesters 65, which cyclized with dicarbonyl compounds and their tetrahydrofuran or tetrahydropyran derivatives in the presence of titanium tetrachloride, Eq. 48 [77]. 

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