AbstractIt is demonstrated that an intramolecular hydrogen bridge can inhibit a ring closure reaction by constraining an otherwise flexible molecule in a conformation which cannot cyclize. Thus the β‐secondary amino‐α, β‐unsaturated ketones IXa and IXb could not be ringclosed under any conditions, whereas the analogous tertiary amono compound IXc, which cannot bridge intramolecularly, suffered immediate ringclosure to the tetrahydropyri