3-oxidopyrylium

Like alkenes, methoxyallene undergo [5 + 2]-cycloaddition with the oxidopyrylium ion formed from the precursor 254 and triethylamine. The allenic terminal C=C bond adds from its sterically less encumbered face to afford the [5 + 2]-cycloadduct [186],... 

The corresponding intramolecular allene-oxidopyrylium cycloaddition gave the [5 + 2]-cycloadduct as a single diastereomer. 

Over the past several years, Mascarenas and co-workers (150-153) utilized the oxidopyrylium ion with variously hetero-substituted olefin tethers. Mascarenas has used this methodology in tandem with a Diels-Alder reaction to prepare tricyclic cycloheptanoid substrates. Further, Mascarenas and co-workers (154—156) achieved the synthesis of optically active oxabicyclic[3.2.1]octane derivatives through the addition of a homochiral p-tolylsulfinyl group substituted at the olefin tether. The Mascarenas group has also used this methodology to prepare the THF portion of ( )-nemorensic acid via oxidative cleavage of the substituted a-hydroxyketone moiety (157) (Scheme 4.78). 

Magnus and co-workers (158-162) used the power of the oxidopyrylium cycloaddition to prepare a taxol precursor 288, generating a synthon for the taxol... 

The Williams approach to bicyclo[5.4.0]undecane containing molecules such as the cyathins, striatins, dolastanes, clavularanes, grayanotoxins, tiglianes, and daphnanes also utilizes an oxidopyrylium cycloaddition to generate the ether constrained bicyclic (Scheme 4.81) (164). The object of the Williams study was to prepare the bicyclo[5.4.0]undecanes and investigate the regio- and stereocon-trolled oxidation of the substrate. 

Finally, in a very recent disclosure, Lee et al. (165) approached the total synthesis of arteminolide using a [5 + 2] cycloaddition strategy with an oxidopyrylium ion. Despite its long history of use, Lee was the first to utilize an allene moiety both in an intra- and an intermolecular cycloaddition with oxidopyrylium ions. By utilizing a pyrone cycloaddition precursor (294) similar to those used in the Wender phorbol synthesis, Lee was able to synthesize various ring sizes and... 

When the pyrone 297 was exposed to base in dichloromethane, formation of the oxidopyrylium occurs and subsequent cycloaddition transpires. Lee found that when the allene tether was 3 (n = 0), cycloaddition occurs to form the bicy-clo[5.3.0]decadiene 298 exclusively, rather than the alternative bicyclo[5.2.0] nonadiene product 299. Increasing the tether length by one resulted in formation of the exo-substituted double bond (299) in 45% yield. Increasing the tether length by one carbon, to four, completely retarded the cycloaddition (Scheme 4.83). 

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