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Abstract Acyclic ketone‐derived oxocarbenium ions are involved as intermediates in numerous reactions that provide valuable products, however, they have thus far eluded efforts aimed at asymmetric catalysis. We report that a readily accessible chiral carboxylic acid catalyst exerts control over asymmetric cyclizations of acyclic ketone‐derived trisubstituted oxocarbenium ions, thereby providing access to highly enantioenriched dihydropyran products containing a tetrasubstituted stereogenic center. The high acidity of the carboxylic acid catalyst, which exceeds that of the well‐known chiral phosphoric acid catalyst TRIP, is largely derived from stabilization of the carboxylate conjugate base through intramolecular anion‐binding to a thiourea site.
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Abstract In the presence of a thiourea–carboxylic acid catalyst, N-9-fluorenyltryptamines undergo highly enantioselective Pictet–Spengler reactions with a range of aldehydes. The reaction works particularly well with aromatic aldehydes, tolerating electronically diverse substituents in all ring positions. Electron-deficient tryptamines are viable substrates. Removal of the fluorenyl protecting group is readily accomplished without deterioration of product ee.more » « less
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This Short Review provides an analysis of the state-of-the-art in catalytic enantioselective oxa-Pictet–Spengler cyclizations. Also discussed are other catalytic reactions providing access to enantioenriched isochromans and tetrahydropyrano[3,4-b]indoles. Context is provided and remaining challenges are highlighted.more » « less
