Conformational analysis of ketoximes by the application of carbon-13 nuclear magnetic resonance spectroscopy

Abstract

DPparte~?lerrt cle Chimie, Urriversite' d e Sherbrooke, Sherbrooke (Que'.) C a~l a t f a JIK 2RI Received July 25, 19772 PATRICK GENESTE, ROBERT DURAND, JEAN-MARC KAMENKA, HELMUT BEIERBECK, ROBERT MARTINO, and JOHN K. SAUNDERS. Can. J. Chern. 56, 1940Chern. 56, (1978.The carbon-13 chemical shifts for a number of relatively rigid ketoximes are presented. It is shown that the chemical shift difference, A6 (sj~rr-onri), for the carbons a to the oxime carbon depends on the dihedral angle between the C=N and C. -H bonds. This stereochernical dependence is then used to determine the preferred conforn~ation of substituted cyclohexanone oximes.PATRICK GENESTE, ROBERT DURAND, JEAN-MARC KAMENKA, HELMUT BEIERBECK, ROBERT MARTINO et JOHN K. SAUNDERS. Can. J. Chen~. 56, 1940Chen~. 56, (1978.O n rapporte les diplacements chiniiques du carbone-13 d'un certain nombre de cetoximes relativement rigides. O n dinlontre que la difference des deplacements chimiques, A6(sj~rr-orrli), pour les carbones en a du carbone de I'oxirne depend de I'angle diedre entre les liens C=N et C,-H.O n utilise ensuite cette correlation sterCochimique pour determiner la conformation privilegie des oxinles de cyclohexanone substitues.[Traduit par 1e journal]Currently, carbon-13 nuclear magnetic resonance spectroscopy (13Cmr) is an infornlative and important technique for conformational analysis. The shielding parameters have been shown to be very sensitive to environment (I) such that relatively small conformational changes lead to significant chemical shift variations. During a study of the reactivity of ketones and ketoximes, it was necessary to have increased knowledge of their confor~liations i l l solution. From earlier reports (2, 3), it appeared that 13Cmr may furnish such information since AS(syna l~t i ) values for the carbons a to the oxime carbon are significant and hence we undertook a study of the 13Cmr chemical shifts of a number of relatively rigid ketoximes. In this paper we present the results of this study and their subsequent use in the conformational analysis of substituted cyclohexanone oximes.signments for the remaining conlpounds were made with the aid of single frequency off-resona~lce decoupled (sford) spectra, from the chemical shifts f o r the corresponding ketones (4), a n d the observation (2,3) that for carbons a to the C=N bond, the carbon sy17 to the OH group is shielded relative to the a l~t i position. Also, for oximes whose corresponding ketones do not have two-fold symmetry, disting~~ishable isomers exist, which were generally present in unequal amounts. For compound 2, carbons I, 2,4, 6, 7 (the quaternary carbon), and 8 (the CH, carbons) were readily assigned from the sford spectrunl. T h e protons on carbon 2 have a chemical shift difference of 1 ppm (5) and thus this carbon appeared in t h e sford spectrum a s the X part of an ABX system, whereas the other methylenes appeared as the normal triplets. The remaining resonances were assigned t o carbons 3 and 5 o n the basis of the different shifts in- Resultsduc...

13C shieldings in syn and anti aldimines and ketimines

“…As already noted for the acyclic compounds, the data in Table 3 show much larger As, values for ketimines than for ketoximes (4) cyclopentanone derivatives are 8.6 vs. 3.5 and for the cyclohexanones 11.4 vs. 6.5. It is also interesting to note a parallel behavior for the shift of the methine carbon attached to nitrogen in that it shows a pronounced high field shift in 25 and 26 relative to 24.…”

Oximoaspergillimide, a Fungal Derivative from a Marine Isolate of Aspergillus sp.

“…Oximoaspergillimide (1) formally derives from leucine linked to isovaleramide, and it was isolated as a single geometrical isomer. From earlier reports, [21][22][23] it appears that NMR chemical shifts of oximes may give information about their geometry. The oximation of ketones results in considerable changes in the carbon chemical shifts, and the carbon of the methylene group in the position neighboring the C=N bond resonates at a higher field in E isomers than in Z isomers by about 5-6 ppm.…”

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

“…Most of the oximes in this paper have been the subject of earlier I3C studies (19)(20)(21)(22). One of the signal assignments, specifically that made for the a carbon syrl to the oxime function in 8, (19,20) must be altered.…”

“…One of the signal assignments, specifically that made for the a carbon syrl to the oxime function in 8, (19,20) must be altered. The signal appearing at 6 24.4, previously assigned to C-4, is the peak whose LIS in the presence of Eu(dpm), is zero and thus represents C-2 (syn to the oxime oxygen).…”

“…For 6, present as a minor component with 5 in a mixture of stereoisomers, the configurations were assigned utilizing the general rule that the upfield methyl signal belongs to the E isomer (19,20,23). In 11 and 12, it was first necessary to establish the methyl assignments by measuring the fully coupled spectrum of each isomer.…”