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-CH 3 group and the 2Ј-H of the naphthyl group are synperiplanar and are almost always located in the same plane. In addition, the CH 3 -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 C1ЈЈ-O5, all conformers have structures similar to the synperiplanar conformation. In the so-called syn conformers, the interatomic distance d(H8Ј-O6) is distributed between 2.48 Å and 2.96 Å, while the distance d(H8Ј-O7) varies between 2.26 Å and 2.56 Å, indicating the weak bifurcated hydrogen
The enantioresolution of racemic alcohols as esters of 2-methoxy-2-(1-naphthyl)propionic acid (MalphaNP acid 1) and the determination of their absolute configurations on the basis of (1)H NMR anisotropy effect are described. The enantiopure MalphaNP acid (S)-(+)-1 was allowed to react with racemic 2-alkanols and 1-octyn-3-ol, yielding diastereomeric mixtures of esters, which were easily separated by HPLC on silica gel. To determine the absolute configurations of the first-eluted diastereomeric esters by the (1)H NMR anisotropy method, the general scheme was proposed. Separated esters were reduced with LiAlH(4) or hydrolyzed with KOH/EtOH to recover enantiopure alcohols.
Novel tetraterpenes 1 and 3 have been isolated from an Okinawan soft coral Sarcophyton glaucum, and their structures have been deduced by means of spectroscopy and chemical transformation. Their carbon framework, presumably formed by Diels-Alder reaction of two cembranoids, has been found to be the same as that of methyl sartortuoate and methyl isosartortuoate.
This paper is the first to report the structures of crystalline diastereomeric salts 8 and 9 prepared from (R)-2-methoxy-2-(1-naphthyl)propanoic acid [(R)-MaNP acid, (R)-1] and (S)-1 with (R)-1phenylethylamine [(R)-PEA, (R)-7], respectively. These crystal structures helped elucidate a novel chiral recognition mechanism characteristic of MaNP salts. The less-soluble diastereomeric salt 8 prepared from (R)-1 and (R)-7 formed an ammonium-carboxylate ion pair by means of a methoxy-assisted salt bridge and an aromatic CH/p interaction. The more-soluble diastereomeric salt 9 prepared from (S)-1 and (R)-7 formed an ion pair by a methoxy-assisted salt bridge in which the 1-naphthyl and phenyl groups did not overlap. Instead, salt 9 formed a close ion pair by means of a salt bridge, a CH/O hydrogen bond, and a p/p interaction. These crystal structures suggest that the molecular length from the MaNP plane containing the carboxy and methoxy groups is critical to the crystallisation of diastereomeric salts. The crystal packing in both salts was investigated with regard to the weak interactions (i.e., salt bridges, NH/O and CH/O hydrogen bonds, and aromatic CH/p, CH/p, and p/p interactions). Finally, diastereomeric amides 11 and 12 were prepared from (S)-2-methoxy-2-(2-naphthyl)propanoic acid [(S)-MbNP acid, (S)-2] and (R)-2 with (S)-1-(1-naphthyl)ethylamine [(S)-10]. The solution-phase structures of the MbNP amides, and their separation, was investigated by NMR spectroscopy and high-performance liquid chromatography (HPLC). The less-stereochemically demanding and longer 2-naphthyl group made the MbNP amide more flexible and less polar than the MaNP amide. Acid 2 was more efficient than acid 1 in separating amides 11 and 12.
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