Carbon-13 NMR studies of naturally occurring maytansinoids

The ¹³C NMR spectra of the antileukemic ansa macrolide maytansine and eight naturally occurring homologues were recorded. Signals were assigned to specific carbon atoms of the ansa macrolide ring and C-3 ester side chain using data from off-resonance decoupling, single frequency decoupling, and 180°–τ–907° double pulse experiments. The assignment of each carbon is discussed for maytansine and tabulated for the other eight maytansinoids.     

Carbon-13 NMR Studies: 82. Carbon-13 spectra of several 10-methyl-cis-decalins. Conformational preferences of some cis-decalin derivatives

The ¹³C spectra of a series of 21 cis-decalins, 17 of which bear 10-methyl substituents, have been recorded to examine geometrical and conformational effects on the ¹³C shieldings. Although the data for a few of the simpler examples have been reported, these compounds were examined to obtain results for a common medium and temperature. All of the derivatives are conformationally mobile and their spectra were examined as a function of temperature to obtain shieldings representative of the slow exchange limit. The examples include compounds which populate both available chair-chair conformations equally, some which exhibit a preference for one of these conformations and some which exist essentially in a single conformation. The data for the latter group combined with the observed temperature dependencies and results for other decalin systems led to specific assignments for each carbon. The internal consistency of the entire body of data offers strong support for these assignments. The conformational preferences observed for a number of decalones and Δ³-10-methyldecalin derivatives are discussed.     

Carbon-13 NMR studies of flavonoids—III: Naturally occurring flavonoid glycosides and their acylated derivatives

¹³C NMR spectra for a variety of flavonoid glycosidcs are presented and analysed. Evidence is presented which demonstrates that ¹³C NMR spectroscopy is a valuable technique for distinguishing the sites of methylation, glycosylation and acylation in flavonoid glycoiides, and in some cases the nature and sites of specific sugars and acyl groups. Shifts observed in the spectrum on derivization of the 5-OH group are unusual. The ring size and C-1 configuration in glycosidic sugars are also evident from the spectra. Structural assignments are made for several glycoides.     

Carbon-13 NMR characterisation of carboxyl derivatives of 1-ureidopyrroles

1-Ureido-5-methyl-2,3-dicarbethoxypyrrole under acidic conditions afforded different partially hydrolyzed or decarboxylated products. It has been possible to characterize two isomeric monoethyl esters of dicarboxylic acids of 1-ureidopyrroles by proton gated decoupled carbon-13 nmr spectra on the basis of multiplicities of the carbonyl carbons resonances.     

Carbon-13 NMR studies of benzoquinones

The assignment of all ring carbons of p-benzoquinones derived from perezone and from thymoquinone was completed using gated decoupled spectra. The long range proton-carbon couplings are discussed in terms of the degree of substitution of the quinone ring. The tautomeric interconversion of the two energetically equivalent forms of 2,5-dihydroxy-1,4-benzoquinones has been studied in various solvents and at several temperatures.     

Carbon-13 NMR studies of monohydroxylated and monochlorinated derivatives of Z- and E-p-menthanes

J(¹³C¹H) coupling constants for some methyl- and aminopyrimidines have been determined by ¹³C NMR. Both the one-bond and long-bond and long-range coupling constants follow general trends which can be summarized in a few simple rules. In particular, the ³J(C-i,H) coupling constants between a ring carbon C-i and the ring protons are larger than the ²J(C-i,H) coupling constants. The opposite is observed for the couplings between the ring carbons and the methyl protons: ³J(C,Me). These general rules are very useful for the assignment of resonances in complex ¹³C spectra of pyrimidines and seem to be valid for other 6-membered aromatic nitrogen heterocycles. Furthermore, the additivity of substituent effects on ¹J (CH) for monosubstituted pyrimidines allows the estimation of ¹J (CH) for polysubstituted pyrimidines with a very good accuracy.     

Carbon-13 NMR spectra of several dicyclopentadiene derivatives

The ¹³C shielding data for 69 derivatives of dicyclopentadiene are reported. These derivatives include compounds from both the exo and endo series having both saturated and unsaturated trimethylene bridges.     

Carbon-13 NMR spectra of fusicoccin and its derivatives

The ¹³C NMR spectra of fusicoccin, some of its cometabolites and derivatives were studied. Using standard Fourier transform techniques, T₁ relaxation time measurements and lanthanide shift reagents, the resonances of individual carbon-13 nuclei were assigned.     

Carbon-13 NMR study of some isomeric n-nitroindazole derivatives

The ¹³C NMR spectra of 1,5- and 2,x-dinitroindazoles “x=4-7” and 3-bromo-1,x-dinitroindazoles are recorded and discussed. The data on he 2,x-dinitroindazoles confirm our previous structurea assignment based on ¹H NMR spectroscopy. The ¹³C NMR spectra established that in the novel 3-bromo-1,x-dinitroindazoles the N-nitro group is attached at N-1.     

Carbon-13 NMR spectra of some o-carbonyl benzeneselenenyl derivatives

The ¹³C NMR signals have been assigned for some ortho-carbonyl benzene-selenenyl derivatives, COR.C₆H₄. Sex, where R = H, CH₃, OCH₃ and OC₂H₅ and X = Cl, Br, CN, SCN, SeCN and CH₃. The effects of the nature of R and X have been discussed.     

Carbon-13 NMR studies of twenty-four methyl-substituted morpholines

Carbon-13 NMR spectra of all the isomers of monomethyl-, 2,3-, 2,5-, 2,6-, 3,5-dimethyl-, 2,3,5-, 2,3,6-trimethyl- and 2,3,5,6-tetramethylmorpholine have been obtained at both ambient (25 °C) and low temperature (～ ?100 to ?120 °C). The ring carbon shifts appear to be additive with respect to the position of the methyl groups. A good correlation between predicted and experimental shift values was obtained (r = 0.998₉). The values were used in an attempt to assign, conformationally, the ‘all cis’ isomer 2,3,5,6-tetramethylmorpholine, which from ¹HNMR spin–spin coupling studies has been unsuccessful. Methyl carbon shifts to high field were found for axially oriented carbons. The extracted ‘steric shift’ values for such carbons were compared to their corresponding proton shift data.     

Carbon-13 and proton NMR studies of 1,4-benzodiazepines

One bond and long range ¹H-¹H, ¹H-¹⁹F, ¹³C-¹H and ¹³C-¹⁹F coupling constants for six 1,4-benzodiazepines are described. An unambiguous assignment of their carbon-13 resonances is carried out based on chemical shift theory and on the analysis of the fine splittings caused by one bond and long range couplings. Chemical shift and coupling constant values are reported for diazepam, chlordiazepoxide, oxazepam, nitrazepam, chlonazepam and flunitrazepam.     

Carbon-13 spectral analysis,tautomeric structures and conformations of four two-ring methylene-bridged phloroglucinol derivatives

The ¹³C NMR spectra of phloropyron BB, desaspidin BB, albaspidin BB and margaspidin BB were recorded and the structures of the compounds investigated with the aid of chemical shifts and CH coupling constants. The filicinic acid ring of the first three compounds appeared to have a monoketonic structure with the carbonyl group in position 2 (acyl group in position 3). The pyronone ring of phloropyron BB also has a monoketonic structure, with the carbonyl function adjacent to the ring oxygen. The two rings of the first three compounds attain a conformation where a stabilizing hydrogen bond(s) is (are) formed between the two rings, as shown by the observed CH couplings to some of the hydroxyl protons. The spectrum of margaspidin BB, whichconsists of two aromatic acyl phloroglucinol rings, indicates less inter-ring hydrogen bonding interactions.     

13C NMR spectroscopy of naturally occurring iridoid glucosides and their acylated derivatives

The ¹³C NMR spectra of twenty one iridoid glucosides and fourteen acyl iridoid glucosides of various cyclopentane oxidation states have been analysed and their carbon shifts assigned. Evidence is presented which demonstrates that ¹³C NMR spectroscopy is a valuable and reliable technique for distinguishing the sites of acylation in iridoid glucosides and confirming the predictions of the configuration at C-6 and C-8. A cis configuration of vicinal substituents is generally associated with a substantial increase in shielding, as compared with the trans analog. The ring size and C-1 configuration in the glucose moiety are also evident from the spectra.