To compare the crystalline-state conformations of MαNP acid esters with those existing in the solution state, X-ray crystallographic analyses of 22 MαNP esters and two MαNP acids were carried out. It was found that all 27 conformers observed in the solid state have structures in which the 2-CH3 group and the 2′-H of the naphthyl group are synperiplanar and are almost always located in the same plane. In addition, the CH3-O7-C2-C1 moiety in all cases has an antiperiplanar structure within the MαNP plane. From further analyses, 22 conformers adopt the so-called syn structure, in which the O7-C2-C1-O6 moiety is synperiplanar. On the other hand, the remaining five conformers have the so-called anti structure. With regard to the rotational conformation around C1″-O5, all conformers have structures similar to the synperiplanar conformation. In the so-called syn conformers, the interatomic distance d(H8′-O6) is distributed between 2.48 Å and 2.96 Å, while the distance d(H8′-O7) varies between 2.26 Å and 2.56 Å, indicating the weak bifurcated hydrogen bond between O6-H8′-O7 as the intramolecular stabilizing force. The crystalline-state conformations of MαNP esters are thus similar to those in solution as determined by 1H NMR spectroscopy, implying that the intramolecular forces make a dominant contribution to the conformations of MαNP esters. The X-ray crystallographic analyses of MαNP acids indicated that the O6-H(acid)-O7 bifurcated hydrogen bond also serves as an intermolecular force to stabilize the so-called syn conformation. A similar bifurcated hydrogen bond [O6-H(alcohol)-O7] was observed in a MαNP ester containing a tertiary alcohol group. It should be emphasized that despite the existence of the so-called syn and anti conformations in the crystalline state, the absolute configurations of all MαNP esters determined here by X-ray crystallography all agree with those obtained by the 1H NMR anisotropy method.
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