14 220MHz NMRスペクトルの天然物化学への応用 : その1.コンタクトシフトとアルカロイド

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Proton n.m.r. contact and pseudocontact shifts using nickel, cobalt, europium acetylacetonate have been applied to stereochemical problems of nicotine, tropanes, papaverine, cinchonine, cinchonidine and brucine. I) Chemical shifts and relative contact shifts for nicotine are shown in Fig. 1 and Table 1. A triplet signal at δ 3,11ppm shifted down and up-field on addition of Ni(AA)_2 and Co(AA)_2 respectively. This behavior is similar to that of the methyl's in β-picolins spectrum, thus the signal was assigned to C-2 hydrogen. Two structures, an envelop form (I) and a half chair form (II), are conceivable for the pyrrolidine ring of nicotine and the observed pseudo-contact shifts of triplet signal at δ 3.27 and the quartet signal at δ 2.34ppm made it possible to assign these signals to quasi-equatorial and -axial protons of I respectively. II) Scheme 1 summerizes the paramagnetic shifts for piperidines. The caluculated geometric factors, 3cos^2θ-1/r^3, of various conformers of piperidine are shown in scheme 2. The chemical shifts and the relative paramagnetic shifts for tropanes are summerized in Table 2. On the basis of these results the conformation of homatropine has been discussed. On addition of Co(AA)_2 the large downfield shifts of C-2 and C-4 equatorial protones and C-3 proton signals demonstrated that the conformation of the tropane ring must be in semi-planar form such as it is shown in Scheme 2(e). The paramagnetic shifts of proton n.m.r. of some quinoline, isoquinoline and indole alkaloids have been briefly discussed.
天然有機化合物討論会の論文
1970-10-01

14 220MHz NMRスペクトルの天然物化学への応用 : その1.コンタクトシフトとアルカロイド

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