Carbocations oxyallyl

Oxyallyl carbocations can be generated efficiently using diethylzinc together with polybromoketones. These intermediates can be trapped with fitran-alcohols to give good yields of polysubstituted 8-oxabicyclo[3.2.1]oct-6-en-3-ones <96S31, 96JCS(P1)1101>. A route to... 

In 1998, the first deliberate attempt to trap the oxyallyl cation with a nucleophile during a thermal Nazarov reaction was reported by West and coworkers [23]. Coining the term interrupted Nazarov reaction, they described the interception of the oxyallyl intermediate of a Nazarov reaction by a tethered alkene (Scheme 3.20). Upon submission of divinyl ketones 87 to the LA, a conrotatory ring closure generates the oxyallyl cation intermediate 89. The alkene moiety adds selectively to the oxyallyl cation intermediate to form bicycle 90. The stabilized tertiary carbocation is then trapped by the enolate oxygen to give intermediate 91, which, under acidic aqueous conditions, is converted to the final hemiketal product 88. The reaction provides an efficient and stereoselective method to prepare tricychc compounds 88 in yields ranging from 42 to 89% [24]. 

West and coworkers [28] reported the efficient formation of bicyclic bridgehead ketones 110 via the electrocyclization/formal intermolecular [3+2]cycloaddition of dienones 108 with allyltrimethylsUane 109 (Scheme 3.24). LA-mediated electrocyclization of 108 provides oxyallyl intermediate 111, which reacts with allylsilane 109 to give intermediate 112. At this point, ring closure provides the bridgehead ketone 110. In some cases, a minor product 113 was observed, the result of the elimination of the TMS group from the same silicon-stabilized carbocation 112. 

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