Carboxylates, conformation, lanthanide shift

The theoretical values of (Sz) and D/(Sz) are available as given in Tables 10.1 and 10.2. A plot of A b/(Sz) versus D/(Sz) for lanthanide-DPA complexes is shown in Fig. 10.7. It is clear from the figure that CY values are in keeping with the model. On the other hand the carboxyl carbon which is in a position to the coordinating atoms does not conform to the model probably due to the operations of other shift mechanisms. Other nuclei in the ligand DPA conform to the model. It is useful to note that a maximum in structural imperfections in the lanthanide series will be shown by nuclei for which 9 = 45° where the derivative of (3 cos2 9 — 1) has a maximum. This may also contribute to the shifts of carboxyl carbon. 

The stereochemistry at C-4 in 3-methylenecephams derived from these chemical reductions was determined from detailed NMR studies on their sulfoxides and computer calculations based on lanthanide-induced NMR shifts (Ochiai et ai, 1972d,e). Both procedures led to the conclusion that the proton at position 4 was in the p orientation. Methyl 7-phenoxyace-tamido-3-methylene-cepham-4-carboxylate (16) was oxidized with m-chloroperbenzoic acid to give the p-sulfoxide (17S), and with N,N-di-chlorourethane to give the a-sulfoxide (17R). The anisotropic effect of each sulfoxide on its respective C-4 proton was examined. The proton at C-4 was found to be shielded by 0.06 ppm in the a-sulfoxide (R) and deshielded by 0.09 ppm in the p-sulfoxide (5). Dreiding models of the two conformational possibilities (A) and (B) for both sulfoxides suggest... 

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