Oxidopyrylium ion

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],... 

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). 

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... 

The substitution pattern of 3-oxidopyrylium ions derived from 6-acetoxy-6/7-pyran-3-ones influences the stereoselectivity of their dimerisation to doubly bridged cyclooct-2-en-l,6-diones (Scheme 22) <07S2360>. 

Pyrones 3-6 are regarded as synthetically equivalent to oxidopyrylium ions 3a-6a (Scheme 3) because of the polarizable carbonyl group. Most... 

Hendrickson pioneered the use of pyranulose acetates with the basic structure 40 as a precursor for oxidopyrylium ion 41, Eq. 26 [53]. Thermolysis of 40 in the presence of unhindered, electron-deficient olefins and acetylenes led to the formation of oxabicyclo [3.2.1] compounds. 

Two key methods for the converison of furan to seven-membered carbocycles have emerged, 14+31 reaction with oxyallyl cations and [5+2] reactions of oxidopyrylium ions. The intermolecular 14+31 methodology has found considerable application (Scheme 36). 

---