The use of shift reagents and ¹³C nuclear magnetic resonance for assignment of stereochemistry to oximes

Abstract

The effects of the shift reagents Eu(dpm), and Eu(fod), on the 'H and "C nmr spectra of twelve oximes of diverse structure were examined. The proton data show that the use of proton shifts to assign oxime stereochemistry, as has been done in the past, is unreliable. In contrast, Eu(dpm), causes LIS values for the "C signals of a carbons which are strongly dependent on stereochemistry. Signals for all a carbons anti to the oxime oxygen experience large downfield shifts while all syrl carbons are either unaffected or shifted upfield. Thus the effects of Eu(dpm), on these I7C signals provides an unambiguous method of configurational assignment to oximes. The effects of Eu(fod),, though similar to Eu(dpm),, were less consistent and thus less promising for making configurational assignments.ROBERT R. FRASER, RAJ CAPOOR, JOHN W. BOVENKAMP, BENOIT V. LACROIX et JACK PAGOTTO. Can. J. Chem. 61,2616Chem. 61, (1984.On a CtudiC les effets des rtactifs de contact Eu(dpm), et Eu(fod), sur les spectres de rmn du 'H et du I3C de douze oximes ayant diverses structures. Les donnCes de la rmn du proton rCvklent que I'utilisation des rkactifs de contact pour attribuer la stCrCochimie d'oximes n'est pas rigoureuse. Le Eu(dpm), au contraire provoque des dtplacements des signaux du "C des carbones en position a qui dtpendent beaucoup de la stCrCochimie. Les signaux de tous les carbones a en position anti par rapport 2 l'oxime-oxygkne subissent de forts dCplacements vers les champs faibles tandis que tous les carbones en position syn ou bien ne bougent pas ou alors se dtplacent vers les champs forts. Ainsi I'effet du Eu(dpm), sur ces signaux du "C foumit une mCthode non equivoque d'attribution de configuration des oximes. Les effets du Eu(fod),, quoiqu'identiques 2 ceux du E~( d p m )~, sont moins cohtrents et par constquent peu sDrs pour faire des attributions de configuration.[Traduit par le journal]The effect of the addition of the shift reagent, E u ( d~m ) ,~ on the proton spectra of oximes has been the subject of some controversy and of considerable misunderstanding. The first study by Wolkowski (1) described the observation of a dependence of the observed downfield shifts on the proton to oxime oxygen distance. In the same year, a paper by Berlin and Rengaraju (2) described a study of the effect of adding Eu(dpm), to a mixture of E and Z 2-butanone oximes. The conclusion of this paper, based on the observation of larger lanthanide induced shifts (LIS values) for all of the protons in the Z isomer, was that complexation occurred at the nitrogen atom. Concurrently, a third paper by Tronchet et al. (3) described a study of LIS values for pairs of isomeric aldoximes. For this class of oximes the Z isomers were also observed to give much larger downfield shifts, indicative of coordination at nitrogen. Subsequently, the proposed distance relation of Wolkowski was employed in assigning configuration to pairs of oximes. In one paper the assignment on this basis (4) turned out to be in error (5), while a second utilization of the depe...

Convenient Synthesis of Pyrazolo[4′,3′:5,6]pyrano[4,3-c][1,2]oxazoles via Intramolecular Nitrile Oxide Cycloaddition

“…Finally, a heteronuclear 2D J -resolved NMR experiment was used in order to determine 1 J CH coupling constants throughout the series of aldoximes. It is well established from previous studies that there is a large and constant difference between the magnitudes of 1 J CH coupling constants of the iminyl moiety in syn - anti isomers [ 39 ], which is larger by at least 10–15 Hz for the syn isomer. The measurements of compounds 4a – c showed that the relevant 1 J CH coupling constants of the iminyl moiety were around 175.0 Hz for the predominant syn isomer, while the minor anti isomer provided significantly lower coupling constant values by around 13.0 Hz.…”

Axial/equatorial populations in ?-hetero-substituted cyclohexanone Oximes andO-methyl oximes

“…The chemical shift differences between H-6eq and H-6ax are unusually large (¾1 ppm) and also, with the exception of the axial proton for the cis series, they are easily identified in the spectra, providing an excellent probe for the determination of these equilibria. Figure 6 presents the HETCOR spectrum of a mixture of the E and Z isomers of the 2-chlorocyclohexanone Omethyl oxime (10). It can be seen that the C-6 signal for the E isomer at 20.6 ppm couples with two different protons, showing that the protons cis and trans to the chlorine atom have different chemical shifts.…”

“…The deshielding effect of the oxime oxygen on the neighboring proton provides an easy method to determine E/Z populations, through integration of signals arising from the syn and anti protons. 9,10 The carbon syn to the OH or OMe groups is shielded and can be a diagnostic tool.…”

“…HECTOR 13 C-1 H spectrum of 2-chlorocyclohexanone O-methyl oxime(10). The arrow indicates the two protons which are coupled to C-6.…”

1H, 13C and 17O NMR spectroscopic study of four bicyclo [3.1.1]heptenes (derivatives of α‐pinene) and four bicyclo[3.1.1]heptanes (derivatives of β‐pinene)

“…According to Fraser et a1., 26 Eu(DPM), causes a large induced shift for the c~-'~C signal, and this is strongly dependent Proton chemical shift changes in the presence of Eu(DPM), have been found to be an unreliable indicator of oxime stereochemistry. 26 In this study Eu(DPM), was found to cause a large downfield shift in the C-1 resonance position and a negligible shift for that of C-3. This result confirms that the oxime hydroxyl is located anti to c-1.…”