Acetoxy group, chemical shift

The and NMR data of these compounds (Table 3 and 5) are markedly similar and found to resemble those of 9 and 10, with the exception of also exhibiting signals for an acetyl group. The acetoxy group was positioned at C-9 in both compounds 33 and 22 on comparing the NMR chemical shifts of H2-9 4.07... 

Furthermore, a rather bold but chemically more feasible mechanism can account for the proposed migration of the acetoxy group. This is double, symmetry allowed, suprafacial-suprafacial, Claisen-type, 3,3-sigmatropic shift. 

The chemical structure of baogongteng A (1), a new myotic agent from Erycibe obtusifolia, has been determined. High-resolution mass-spectral data indicated the presence of a 3,6-disubstituted tropane skeleton. Infrared and and n.m.r. data indicated the presence of secondary amine, secondary hydroxyl, and acetoxy functions, and the location of the latter group at C-6 was based on comparison of n.m.r. chemical shifts with those of 6B-acetoxy-3a-tropanol. The hydroxyl group was assigned to the 23-position from the n.m.r. spectrum of the N-methylated alkaloid. 

The value of these chemical-shift correlations for determining configuration is best exemplified by consideration of the a- and j3-D-manno-pyranose acetates. The anomers cannot be distinguished by their 2 coupling constants since H-1 and H-2 occupy a gauche relationship in each case. The anomers can be distinguished by the relative chemical shifts of H-1 and H-5. The equatorial H-1 in the a-D-anomer resonates at lower field than the axial H-1 in the -D-anomer. Also the effect of the axial acetoxy group in the a-D-anomer causes H-5 to resonate at 0-2 p.p.m. to lower field than in the -D-anomer in accordance with rule 1(b). 

Lemieux and his co-workers suggested that the methyl protons of axially orientated acetoxy-substituents in pyranose derivatives have lower chemical shifts than those of equatorially orientated groups. 

The chemical shifts of the H-atoms at C(4) [or C(8), resp.], which are geminal to an acetoxy group are neither substantially influenced by the nature nor the configuration of the substituent at the opposite carbon C(8) [or C(4), resp.], see Table 18, parts a and b. However, H-atoms geminal to an iodine atom are sometimes shifted quite remarkably, see Table 18, parts c and d. 

Even in simple cases, the structure determination may present unexpected pitfalls for the unwary, and several methods of determining stereochemistry have failed when applied to limonoids. The molecular conformation is not always readily predictable, and in n.m.r. spectroscopy of limonoids use of the Karplus equation and of solvent shift methods has led to mistaken conclusions 98, 152). More reliable has been the effect of substituents on the chemical shift of nearby methyl groups 105,151). It has been shown that the presence of a hydroxy substituent causes a considerable downfield shift of methyl substituents in a 1,3 diaxial relationship, while an acetoxy or other carbonyl containing substituent may or may not produce a similar shift depending on orientation effects. This can produce good evidence of the stereochemistry, and in suitable cases, of the position of a substituent 98, 152). 

Opposite to many other examples studied earlier in case of alkaloid sulfoxides in question the reaction does not show high stereospecificity and both possible isomers ars formed. They could be separated and assignments of the configurations to both isomers could be given by interpretation of NMR spectra since in each pair of isomers obtained from neothiobinupha-ridine, thiobinupharidine and thionuphlutine only one has the acetoxy group very close to the furan protons which causes different values for the observed chemical shifts in isomeric acetoxyderivatives. 

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