Chromic configurations

It seems likely that a complete interpretation of the catalytic activities of these oxides must take account not only of the electron configurations of the ions, but also of the role of defects, including those which give rise to semiconductivity the importance of the latter has already been demonstrated in the cases of zinc oxide and chromic oxide. 

It can be seen from molecular models that two diastereoisomers are possible for the ferric enterobactin complex, A-cis and A-cis. These are not mirror images because of the optical activity of the ligand. The similarity of the roles played by the ferrichromes and enterobactin lent additional speculative interest to the preferred absolute configuration of the iron complex (20). The structural studies of the tris catechol complexes (vide infra) and the spectroscopic properties of the chromic... 

Finally, proof was obtained that the aliphatic portion of the aspidocarpine molecule was identical with the corresponding portion of aspido-spermine by oxidation of the two compounds with chromic acid in dilute sulfuric acid. In both cases, the aromatic ring was destroyed and there was isolated the a-keto amide (LI), mp 224°-225°, [a]D —132° (chloroform). The identity of this oxidation product shows not only that aspidocarpine (XLIV) and aspidospermine (II) have an identical skeleton but also that they have the same absolute configuration at all four centers. 

It should be recalled here that the result of the chromic acid-ammonia-degrada-tion is the same with Pr as with Pfr86). This means that peripheric substituents — including the thioether bond at C-31 — and the steric configurations at these positions are not changed during the phototransformation of Pr to Pfr and vice versa. 

The simple model complex, tris(cateeholato)chromate(III) has been prepared, and complete resolution of the optical isomers was achieved at pH 13. The known crystal structure of [Cr(cat)3]3- and arguments similar to those for the hydroxamate chromium complexes lead to the assignments of absolute configuration of the CD spectra. It was found that the CD spectra of A-[Cr(cat)3f and [Cr(ent)3f " are essentially identical, and the mirror image of chromic desferriferrichrome (Fig. 28), which shows that enterobactin has a predominant A-cis absolute configuration 147). Unfortunately the usual oxidation sensitivity of the catechol dianion is substantially increased in the chromium complexes, which precludes their use as biological probes l47). 

The diosphenol (214) has been prepared from 3a-bisnorcholanic acid (217), itself obtained from ergosterol. The amino-group is introduced with retention of configuration by means of the Curtius reaction and methylated to give 3a-acetoxy-20a-dimethylamino-5/5-pregnane (216). After hydrolysis and oxidation with chromic acid, the ketone (215) is obtained. Aerial oxidation of (215) in the presence of potassium t-butoxide in t-butyl alcohol gives (214). 

The stereochemical interrelationships among all four of the 1-hydroxymethylpyrrolizidines isoretronecanol, lindelofidine, trachelan-thamidine, and labumine, have been determined, insofar as the Cj, Cg relative configurations are concerned, by the chromic acid oxidation studies of Labenskil, Serova, and Men shikov (163). Repeating the oxidation of isoretronecanol originally reported by Adams and Hamlin (63), they were able to isolate two diastereoisomeric l-p3nrrolizidinecarboxylic aci in approximately equal amounts the isoretronecanolic acid of Adams and Hamlin, m.p. 228-229° (dec.) [a] —71.4° (ethanol), (picrate,... 

As a result of the chromic acid oxidation the location of the second hydroxyl function in etioline appeared to be at C-16. The oxo product of 102 showed the absorption of a five- and a six-membered ring ketone in its IR spectrum. Compound 102 failed to cyclize to the spirosolane and therefore the a-orientation was assigned to the hydroxyl function at C-16. Biogenetic consideration indicated the 25S configuration for etioline. The structure 102, (25S)-22,26-epiminocholesta-5,22(i T)-diene-3jS,16a-diol, was proposed for the alkaloid. 

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