Cycloadditions of Pyrylium Betaines

Metastable, aromatic pyrylium species undergo cycloadditions with olefins to generate [3.2.1] oxabicydic derivatives. This reaction has been reviewed by Sammes and by Katritzky [51 - 52]. 

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. 

Subsequently, Sammes showed that while simple olefins were unreactive, strained olefins such as norbornadiene, and electron-rich olefins such as vinyl ethers also undergo cycloaddition with 41 [54]. In the latter case, the endo cycloadduct tends to predominate. Sammes also successfully induced the formation of 41 from 40 in the presence of catalytic base at ambient temperatures, thus allowing the cycloaddition to proceed under mild conditions, Eq. 27. 

Intramolecular versions of this cycloaddition were observed starting from 2-substituted and 6-substituted oxypyrans 42 and43, Eqs. 28,29 [55-57]. 

Wender extended the studies of the intramolecular cycloaddition by examining substituted oxidopyrylium intermediates with stereocenters in the tethers [58 a]. Pyran 44 underwent smooth cycloaddition with complete stereoselectivity to give 45 due to the methyl group at Cn assuming an equatorial position in the chair-like conformation of the olefinic side-chain, Eq. 30. The stereocenter at Cn effectively controlled the stereochemistry at C6, C8, and C9. The reaction proceeded with heating or at room temperature with a catalytic amount of base. 

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