R. Mondelli

Nucleophilic character of alkyl radicals—VII : Substituent effects on the homolytic alkylation of protonated heteroaromatic bases with methyl, primary, secondary and tertiary alkyl radicals

Abstract The relative rates of the homolytic alkylation of protonated 4-substituted pyridines with methyl, n-propyl, n-butyl, sec-butyl and t-butyl radicals, obtained by the silver-catalyzed decarboxylation of acids with ammonium peroxydisulphate, are reported. This is the first time that the reactivities of the main alkyl radicals have been compared using the same reagent model in a homolytic aromatic alkylation. The alkylation takes place exclusively in the 2 position and the chemical reactivity parameters correlate well with the chemical shifts, but not with the Hammett σ m constants because of an enhanced conjugation of the electron-releasing substituents. The influence of the polar characteristics on the t-butyl radical is very striking. The results are discussed in terms of a π-complex transition state.

Indole alkaloids from gambir structure of bambirtannine, oxogambirtannine and dihydrogambirtannine

Abstract On the basis of chemical and spectral evidence, the structures I, II and III were assigned to gambirtanine, dihydrogambirtannine and oxogambirtannine, three new indole alkaloids extracted from the tannin Gambir. The phenol betaine IV (neooxygambirtannine) was obtained during the workup of the extract.

The structure of roxburghines A–E, new indole alkaloids from an Uncaria Sp☆

Abstract The structural elucidation of roxburghines A–E, new diastereoisomeric indole alkaloids C 31 H 32 N 4 O 2 , isolated from the leaves and stems of an Uncaria Sp., is reported. The derivation of the skeleton ( 4 ) from two tryptamine and one monoterpenoid C 10 units is consistent with the current biogenetic theory. Tetrahydroalstonine, one isomer, and dihydrocorynantheine were also isolated.

13C NMR analysis of anthraquinones as models for anthracycline Antibiotics

Abstract 9,10-Anthraquinone (9,10-AQ) and several derivatives have been analyzed by FT 13 C NMR spectroscopy. All the 13 C frequencies have been assigned; the one-bond coupling constants and some long-range interactions are reported. The effect of substituents OH, OMe, OAc, and COMe on the chemical shifts has been studied for monosubstituted and some 1,5- and 1,8-disubstituted compounds. The structure of 9,10-AQ is discussed on the basis of the comparison with the parent hydrocarbons, disubstituted benzenes, and naphthoquinone and benzoquinone. Intramolecular H bonding and tautomeric structures are also discussed.

Structural changes induced in the NH2 group of aniline by substituents: The structure and orientation of para-bromo- and para-nitroaniline [1-15N] in nematic liquid crystal solvents by NMR

Abstract We report the first investigations of the NH2 groups in anilines by NMR using liquid crystal solvents. The NMR spectra of para-nitroaniline[1- 14 N], para-nitroaniline[1 15 N], and para-bromoaniline[1- 15 N] partially oriented in nematic phase have been studied. The molecular reorientation is found to be correlated with NH 2 internal motions. The nonplanar nature of the molecules is shown by both the uncorrected and vibrationally corrected data. The effect of the substituents at the NH 2 pyramid is confirmed to be a flattening by electron-withdrawing substituents, and an accentuation by electron donors.

NMR study of four-membered Rings. IX. Conformation of thietane-1,1-dioxide and thietane-1-oxide from their NMR spectra in a nematic solvent

Abstract The molecular structures of thietane-1-oxide and thietane-1,1-dioxide have been studied by their proton magnetic resonance spectra in the nematic phase. The monooxide derivative exists preferentially in one strongly puckered conformation with the oxygen in equatorial position and with an angle of puckering of approximately 38°. The dioxide has a planar or a slightly distorted average conformation with a low barrier to ring planarity. These results are discussed and related to those obtained for other four membered ring compounds, whose potential function for ring puckering is known.

Sign determination of carbon-proton and proton-proton coupling constants in thiete-1,1-dioxide

Abstract All CH and HH coupling constants of thiete-1, 1-dioxide are measured from 13 C , 1 H , and 13 C satellite spectra, and sign determinations are performed by tickling and triple resonance experiments. The values obtained are related to the known data and discussed.