Tetrahydropyranyl cation intermediate

Such exploded transition states, in which the reaction centre carries a pronounced positive charge, are the commonest type of nucleophilic displacements at acetal centres. The reactions are unambiguously bimolecular, but in quantitative measures of transition state structure they differ only slightly from those of true 5 nI reactions. Thus, the value of for the hydrolysis of aryloxytetrahydropyrans, in which the solvent-equilibrated tetrahydropyranyl cation is a true intermediate, is —1.18, modestly but significantly more negative than the value of —0.82 obtained for hydrolysis of methoxymethyl esters of the type CH30CH20C0R. ... 

This formal [2h-2h-2] alkyne/alkene/carbonyl cycloaddition proceeds through the opening of the cyclopropyl carbene intermediate 1-4 by the carbonyl group to form oxonium cation 1-5, which undergoes nucleophilic attack by the vinylgold intermediates in a Prins-type cyclization to give tetrahydropyranyl cation 1-6. 

The mechanism for this reaction is analogous to its intramolecular version [Ref. 223 in Chap. 1], and starts with the formation of the cyclopropyl carbene I-ll (Scheme 2.6). Direct attack of the aldehyde to the cyclopropyl gold(I) intermediate 1-11 leads to the oxonium cation 1-20, which suffers Prins-type cycUzation to give tetrahydropyranyl cation 1-21. Intermediate 1-21 can undergo metalation to yield bicycle 1-16 or can evolve by a fragmentation reaction to form 1,3-diene 1-17. 

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