Magnetic Resonance in Chemistry

doi: 10.1002/mrc.1260270801pmid: N/A

Kleinpeter, E.; Spitzner, R.; Schroth, W.

doi: 10.1002/mrc.1260270802pmid: N/A

The 13C NMR spectra and variable‐temperature 1H NMR spectra of some NR2‐substituted pyrylium and thiopyrylium salts were studied, and the rotational barriers thus obtained about the exocyclic partial CN double bonds are discussed according to their dependence on the counter anion and other structural variations. The counter anion has a negligible influence on the barrier, but substituents on the ring can either strengthen electronically or weaken sterically the mesomerism of the NR2‐heteroaromatic system. The competition between these two effects is readily estimated from the rotational barriers obtained. The pyrylium salts display higher rotational barriers than the identically substituted thiopyrylium salts.

Dlaz, Eduardo; Barrios, Héctor; Ortiz, Benjamin; Sánchez‐Obregón, Rubén; Yuste, Francisco; Reynolds, W. F.; Aguilera, Jose Luis; Caballero, Efrén

doi: 10.1002/mrc.1260270803pmid: N/A

The reaction of dibenzalcyclohexanone with hydroxylamine hydrochloride affords three compounds: the oxime of 2‐benzylidenecyclohexanone (2), 3‐phenyl‐7‐benzylidene‐3,3a,4,5,6,7‐hexahydro‐2,1‐benzisoxazole (3) and 3‐phenyl‐3,3a,4,5,6,7‐hexahydro‐2,1‐benzisoxazole‐7‐spiro‐2′‐(3′‐phenylaziridine) (4). Their structures were deduced by 2D NMR analysis. Single‐crystal x‐ray diffraction was used to confirm the structure and stereochemistry of 4. A possible mechanism for this reaction is proposed.

Neuvonen, Kari; Pohtola, Riitta; Pihlaja, Kalevi

doi: 10.1002/mrc.1260270804pmid: N/A

In addition to the parent compounds, nine methyl‐substituted 1,2‐dihydro‐4H‐3,1‐benzoxazines with and without N‐methyl substitution were prepared. The chain tautomer could only be detected in the case of 1,2‐dihydro‐2‐(p‐nitrophenyl)‐4H‐3,1‐benzoxazine in solution. The configurational and conformational assignments were based on 1H and 13C NMR data. The calculations of the methyl substituent effects on the heterocyclic ring (half‐chair structure) carbons gave a good fit between the observed and calculated shifts and made in possible to estimate that 1,2‐dihydro‐4‐methyl‐4H‐3,1‐benzoxazine is a 75:25 and its N‐methyl derivative a 69:31 mixture of the 4eq′‐ and 4ax′‐ forms. 1,2‐Dihydro‐trans‐1,2,4‐trimethyl‐4H‐3,1‐benzoxazine is also not conformationally homogeneous, but is a 43:57 mixture of the 2eq,4ax′‐ and 2ax,4eq′‐ forms. A comparison of the 13C chemical shift correlations with those of 3,4‐dihydro‐2H‐1,3‐benzoxazines also improves the structural knowledge about the latter series. Whereas the N‐methyl groups in 3,4‐dihydro‐2H‐1,3‐benzoxazines are axially orientated, model calculations reveal that the N‐methyl substitutions in the title compounds can be orientationally heterogeneous.

Wenzel, Thomas J.; Cameron, Kirk

doi: 10.1002/mrc.1260270805pmid: N/A

Lanthanide tetrakis(β‐diketonate) anions are effective NMR shift reagents for organic salts. The shifts in the 1H NMR spectra were analyzed and explained without invoking a contact shift mechanism. The equilibria of the shift reagent‐substrate complex was examined. No evidence was found for a 1:2 shift reagent‐substrate complex. Bound shifts and association constants were determined for 1‐methyltetrahydrothiophenium iodide (1) and 1‐ethylquinolinium (3) iodide. The observed shifts for these salts were fitted to calculated shifts using the simplified dipolar shift equation. A single lanthanide location was identified with 3. A time average of at least two configurations of shift reagent‐substrate complex was required with 1.

Barber, Jill; Lian, Luyun; Morris, Gareth A.; Tehrani, M. Hassan

doi: 10.1002/mrc.1260270806pmid: N/A

The 1H and 13C nuclear magnetic resonance spectra of sodium fusidate in methanol‐d4 and in D2O have been assigned using a range of one‐ and two‐dimensional NMR techniques.

Barber, Jill; Derome, Andrew E.; Howard, Tina D.; Lian, Luyun; Tebb, Graham

doi: 10.1002/mrc.1260270807pmid: N/A

The 1H nuclear magnetic resonance spectra of kirromycin in acetone‐d6 and in buffered D2O have been assigned using a range of one‐ and two‐dimensional NMR techniques. The 13C NMR spectrum of kirromycin in acetone‐d6 has also been assigned, and compared with the 13C NMR spectrum of its N‐methyl analogue, aurodox.

Pieters, Luc A. C.; Van Zoelen, Agnes M.; Vrieling, Klaas; Vlietinck, Arnold J.

doi: 10.1002/mrc.1260270808pmid: N/A

A method for the determination of the pyrrolizidine alkaloids from Senecio jacobaea (Compositae) by 1H and 13C NMR is presented. 1H NMR is a reliable method for the determination of the total alkaloid level. 13C NMR appears to be a suitable method for the determination of the individual alkaloids. The same techniques can also be applied to other plants containing pyrrolizidine alkaloids.

Sohár, P.; Kövesdi, I.; Szabó, J.; Katócs, Á.; Fodor, L.; Szücs, E.; Bernáth, G.; Tamás, J.

doi: 10.1002/mrc.1260270809pmid: N/A

N‐(3,4‐Dialkoxyphenylthiomethyl)aroylamides (la, b) reacted with phosphoryl chloride to give not only the expected 4H‐ and 2H‐1,3‐benzothiazine derivatives (4a, b and 5a, b), but also dibenzodithiocins of new (dibenzo[d,g][1,3]dithiocins 2a, b) and known (dibenzo[b,f][1,5]dithiocins 3a, b) types. The analogous reaction of the 4‐methylaroylamide 8a furnished the 4H‐1,3‐benzothiazine 9a, the dibenzo[b, f][1,5]dithiocin derivative 10a and benzonitrile. In contrast, 8b (the chloro analogue of 8a) furnished only benzonitrile and bis(4‐chlorophenylmercapto)methane (11). The structures of the new compounds were confirmed by IR, 1H and 13C NMR, and (in part) by mass spectrometry. Temperature‐dependent 1H NMR studies were used for the conformational analysis of 2a and its disulphone 6a; the nature and free enthalpies of activation of the two different conformational motions occurring at higher temperatures were determined.

Duus, Fritz

doi: 10.1002/mrc.1260270810pmid: N/A

2‐Ethoxycarbonylthiolane‐3‐thione (1) and its 5‐methyl, 5‐phenyl, 5,5‐dimethyl and 4,5‐dimethyl derivatives (2–5) were studied in solution by 13C and 1H NMR spectroscopy, using solvents of various polarity. Compounds 1–4 exist entirely in the tautomeric (Z)‐enethiol form, i.e. they are in fact 2‐ethoxycarbonyl‐3‐mercapto‐4,5‐dihydrothiophenes (1C‐4C), apparently irrespective of their molecular surroundings. Compound 5 exists predominantly as the pair of enantiomeric (and hence NMR‐indistinguishable) 2‐ethoxycarbonyl‐3‐mercapto‐4,5‐dimethyl‐4,5‐dihydrothiophenes having a trans location of their ring‐methyl groups, i.e. 5C(4R, 5R) and 5C(4S, 5S). The diastereomeric counterparts 5C(4R, 5S)/5C(4S, 5R) are observable as minor constituents, co‐existing with the former in a double two‐step equilibrium system, involving tautomeric 2‐ethoxycarbonyl‐3‐mercapto‐4,5‐dimethyl‐2,5‐dihydrothiophenes as intermediates.