Interrupted 4+3 cycloaddition

Apart from acting as effective Lewis base catalysts, the quinuclidine structure of cinchona alkaloids can also participate in the other cycloaddition reaction by a different catalytic mechanism. Calter et al. described an interesting asymmetric interrupted Feist-Benary reaction between ethyl bromopyruvates and cyclohexadione. They proposed that the protonated cinchona alkaloid would perform as a Bronsted acid to form hydrogen-bonding interaction with a-ketoester moiety, rendering it more electrophilic toward attack by either the enol or enolate of cydohexandione. Then intramolecular alkylation would afford the formal [3 + 2] cycloadduct (Scheme 10.12) [16]. 

The ability of the anthracene ring system to undergo concerted cycloadditions is also well established (see Section 6.1). This tendency toward addition results because little, if any, resonance stabilization is lost on interruption of aromatic conjugation at... 

The trapping of the oxyallyl cation by the alkene in a formal [3+2] fashion is not the only cycloaddition possible by this method. The oxyallyl cation, in the presence of a 1,3-butadiene, undergoes a [4+3] cycloaddition. West and coworkers [27] reported cases of cascade-interrupted Nazarov/intramolecular [4+3] cycloadditions in 1999, a representative example of which is depicted in Scheme 3.23. Thus, tetraenone 104 was subjected to FeClj at low temperature to produce a mixture of stereoisomeric tetracyclic products 105 and 106 in 98% yield. The initially formed oxyallyl cation 107 generates these two diastereomeric products by way of putative endo and exo-mode cycloaddition transition states, 107-endo and 107-exo, respectively. 

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