Paraformaldehyde Pentene

Formation of the oxonium ion can be achieved via in situ transacetylation or in situ acid-catalyzed acetalylation of homoallylic alcohols (see Section 1.5.5.2.1.). 2,2-Dimethoxypropane and 4-penten-2-ol react in a titanium(IV) chloride mediated 6-endo process to give cisoid-4-chloro-2,2,6-trimethyltetrahydropyran (6)28. 1-Cyclohexenylacetic acid generates the presumed acyloxonium species 7 in the presence of trifluoroacetic acid and paraformaldehyde, which undergoes ring closure to give the bicyciic lactone 829. 

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