Abstract
The Reaction Product of Phenylalanin and Ninhydrin: Revision of Its Structure. Proton Transfer in an N-Unsubstituted, Highly Stabilized Azomethinylide The formerly proposed structures 6 and 7 for the intense colored reaction product of ninhydrin and phenylalanin are revised in favour of 5a, for which four other tautomeric forms 5b–5e have to be taken into account. Representing a highly stabilized N-unsubstituted azomethinylide 5a can be trapped with N-phenylmaleimide as dipolarophile; 1,3-dipolar cycloaddition and subsequent double bond shift leads to two diastereomeric products 15a and 15b. UV spectrometric data and dynamic 1H NMR measurements reveal an intramolecular proton transfer by hydrogen bonding between 5a and 5b dominating the tautomeric equilibrium.
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