The Observation of the C―H···Osp3 Hydrogen Bond in Trisialic Acid Lactone and Its Implications for Cooperative Lactonization

The C–H···O hydrogen bond in trisialic acid lactones has been examined by using long-range COSY (LRCOSY) and ab initio calculations. From an analysis of the LRCOSY spectra, 3J correlations of BH9ax/AH8 and CH9ax/BH8 confirmed the existence of hydrogen-bond connections of x+1C9–x+1H9ax···xO8. The theoretical bond energy of the hydrogen bond was estimated to be 1.0 or 1.6 kcal/mol by using two models. The acid-catalyzed cooperative lactonization of oligosialic acids can be understood in terms of the additional C–H···O stabilization in the lactone product. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)