Oxidopyridinium ion

Conceptually, a [5+2] cycloaddition involves the reaction of a five-atom species with a two-atom species. The most commonly encountered examples of such [5+2] cycloadditions are the reactions of oxidopyrylium and oxidopyridinium ions with various alkenes and alkynes. Originating more than 40 years ago with the investigations by Wood and Hurd, and continuing with contributions from Weeks, I tritzky, Sammes, Wender, and Williams, this class of [5+2] cycloadditions has serviced a number of synthetic objectives. One of the most complex examples of this process has figured in the first total synthesis of phorbol (Eq. 42), a remarkably demanding prob-... 

Treatment of the iV-methylated product (115) of 3-hydroxypyridine with an ion exchange resin generates the betaine (116 Scheme 97) (71JCS(C)874). Reduction of 1-substituted 3-oxidopyridinium betaines with sodium borohydride gives hexahydro derivatives in good yields <8lH(l6)l883). 

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