Solution 1H and 13C NMR of new chiral 1,4-oxazepinium heterocycles and their intermediates from the reaction of 2,4-pentanedione with α-L-amino acids and (R)-(—)-2-phenylglycinol

The reaction of 2,4-pentanedione (1) with (R)-(—)-2-phenylglycine methyl ester (2), (R)-(—)-2-phenylglycinol (3) and the proteinogenic amino acids (2S,3R)-(—)-2-amino-3-hydroxybutyric acid (L-threonine) (4) and (R)-(—)-2-amino-3-mercaptopropionic acid (L-cysteine) (5) methyl esters was investigated. The corresponding enamines 6, 7 and 8 were isolated and characterized spectroscopically whereas 9, which is unstable, was transformed in situ into 13. Treatment of 7, 8 and 9 with boron trifluoride etherate afforded the new [1,4]oxazepines 10, 11 and [1,4]thiazepine (12) as their BF3O− salts. The structures of the enamines and their corresponding seven-membered heterocycles were assessed by 1D and 2D NMR spectroscopy. Variable-temperature experiments revealed different molecular mobility behavior among these heterocycles. Copyright © 2003 John Wiley & Sons, Ltd.