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Conformational analysis of N-acetyldaunomycin in solution. A transient 1H nuclear Overhauser effect study of the glycosidic linkage geometry

The preferred conformation of the sugar with respect to the aglycone moiety has been determined for N-acetyldaunomycin in solution, by transient nuclear Overhauser experiments (n.O.e.). The time development of proton n.O.e.s. has been measured after the selective inversion of single resonances and the whole curves have been analysed through a non-linear least-squares procedure. Calculations performed by a ‘two-spin approximation’ allowed values of the cross-relaxation rates (σ) between close protons to be extracted. This method was shown to give a good estimate of σ, from which the interproton distance ratios have been obtained, with an accuracy comparable to that of an X-ray method. The preferred conformation of the glycosidic linkage has also been expressed in terms of the rotational angles φ= H(1′)–C(1′)–O(7)–C(7) and ψ= C(1′)–O(7)–C(7)–H(7), and the results have been compared with those from X-ray analyses and from energy-minimum calculations.