Cope rearrangements oxocarbenium ions

The mechanism involves 2-oxonia-Cope rearrangements via (Z)-oxocarbenium ion intermediates, as shown in Scheme 7. It has been shown that racemization can be suppressed by conditions or structural features which slow the Cope rearrangement in either forward or reverse directions, since the [3,3]-sigmatropic rearrangement must occur many times before reaction via the less favoured (Z)-oxocarbenium ion becomes important.20... 

The oxonia-Cope rearrangement is also a key aspect of the Mukaiyama-Michael cascade reaction developed by Rychnovsky, which results in the formation of tetrahydropyran rings with high levels of stereoselectivity. As demonstrated in the example below, the Lewis acid promoted conjugate addition of enol ether 190 results in an intermediate oxocarbenium ion, formation of which is followed by Cope rearrangement. 

Mixed acetals (6), bearing allyl and silyl enolate moieties, are efficiently cyclized to tetrahydropyranones (7) in the presence of (la) and DTBMP [52]. A plausible mechanism is as follows (i) the treatment of (6) with (la) forms oxocarbenium ion (8), (ii) (8) is reversibly converted into another oxocarbenium ion (9) by the 2-oxonia Cope rearrangement, and (iii) intramolecular addition of the silyl enolate moiety to the carbenium ion center in (9) gives (7). The moderate stereoselectivity concerning the substituent R is attributable to competing cyclizations from chair and boat conformations in the last step (Scheme 9.11). 

Certain O-homoallyl-oxocarbenium ions are known to undergo 1,3-allyl transfer by the 2-oxonia Cope rearrangement as described in Scheme 9.11. Recently, Rychnovsky and coworkers have demonstrated that the (la)-promoted reaction of enantioenriched mixed acetal (42) with Bu3SnH affords the rearranged product (43) with a high level of chirality transfer [84]. The key step of this reaction is the 2-oxonia Cope rearrangement of oxocarbenium ion (44)-(45) (Scheme 9.27). Under similar conditions, acetal (46) is efficiently transformed into the bicyclic product (47) by intramolecular trap of the rearranged carbenium ion intermediate (48) with the... 

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