59 CD vs NMR : Elucidation of the Stereochemistry of Marine Terpenoids by Use of The Modified Mosher's Method
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Part I. Lobatriene (1) has been obtained from the Okinawan soft coral of Sinularia species. The attempts to determine the stereochemical relationship of the substituents on its A and B rings by means of NMR spectroscopy have been found to be fruitless, because several unexpected NOE's appear between the protons on both the rings in the NOESY spectrum possible due to free rotation of the rings around the single bond connecting them. Therefore, chemical transformations of 1 have been carried out to produce the glycol (5) and the secondary alcohol (8). The application of the CD method in the presence of Eu(fod)_3 as well as the modified Mosher's method has resulted in the elucidation of the absolute configuration of C-17. Use of the modified Mosher's method1 to 1 for 8 has led to S configuration of C-4. These findings have revealed the absolute configurations of all the chiral centers of lobatriene as shown in structure 1. Part II. Isoclavukerin A (1) has been isolated from the Okinawan soft coral of Clavularia species. The relative stereochemistry of its substituents has been established by comparison of its spectral properties with those of clavukerin A (2), the structure including absolute configuration of which has been firmly established by Kitagawa's group. Finally, the absolute configuration of isoclavukerin A has been determine by application of the modified Mosher's method to the alcohol (4a) formed by autoxidaiton of 1, although the benzoate chirality method15 happened to be inapplicable to its benzoate (5) because of antipararell nature of the benzoate group and the olefinic group of 5.
