Positive and negative intramolecular nuclear Overhauser effects in an all-proton system

Abstract

The Overhauser effects recorded for the alkaloid ochotensin~ine (1) show positive effects (area increases) for adjacent protons but negative effects (area decreases) for two protons separated by an intervening proton. A qualitative explanation based on the dominance of a dipole-dipole relaxation mechanism is advanced. The Overhauser effect between geminal protons on an sp2-carbon has been measured.Canadian Journal of Chemistry, 46, 3421 (1968) In an investigation of the Overhauser effect in F~rmaria and CorydaIis alkaloids, we have exainined the alkaloid ochote~isimine (1) (1). We report here the results obtained for this alkaloid since it has furnished the first definite example of positive and negativeL ii~tramolecular nuclear Overhauser effects in an all-proton system and the first measurement of the Overhauser effect between geminal protons.area changes recorded are expressed as a percentage of the total area of each proton. The measurements were obtained on a 0.17 M solution of 1 in carefully degassed CDCI,.When the proton HA-15 is saturated, the area of the signal for the proton H,-15 increases by 40% (positive effect) but the area for t h e proton H-13 decreases by 8 % (negative effect). In addition, there is a small, but nonetheless reproducible, area increase of 3 % for the proton H-12.Similarly, when the proton H,-15 is saturated, the area for the adjacent proton H-13 increases by 24% and the area for the proton H-12 decreases by 7 %. The observed area decreases were not the result of spurious i n s t r~~m e n t a l~ effects nor overloading of the audio phase-sensitive detector, since any arbitrary displacement of the saturating field from the trailsitions of HA-15 of H,-15 resulted in a return to normal areas for all ~r o t o n s .The observed negative effects are thus an Qualitatively, the occurrence of a negative lines, sufficiently separated from other transitions effect can be accounted for by a colisideration to be accurately integrated. For personal use only.

Recent trends in conformational analysis

“…Indeed, when r XY refers to a known internal distance (a ‘control’ enhancement), the NOE ratio smoothly provides the unknown internuclear distance r IS up to a distance of 4.5–5.0 Å, where the NOE enhancements are close to the detection limit. NOE effect is therefore an important tool that provides a reliable and highly accurate51,52 description of the space around the selected nuclei.…”

Assignments of configuration by nuclear Overhauser effects in some dibenzthiepin derivatives

“…The negative sign for the n.0.e. of H, on irradiation of H, is not unusual and its origin has been discussed (7). The fact that the n.0.e.…”

Nuclear Magnetic Resonance Study of Several Derivatives of 1,3,5,7-Tetramethyltricyclo[5.1.0.03,5]octane. Stereochemical Assignments by Nuclear Overhauser Enhancements

“…It is interesting that the geminal proton enhancements, i.e. H*{H,) etc,, are somewhat smaller than those found for a variety of methylene groups in other systems (9); the values for the C-6 methylene protons in 16 were found to be 19 and 24% for the upfield and downfield protons, respectively. These results, with those in Table 2, indicate that other pathways also make substantial contributions to the overall relaxation for these protons.…”

“…Similarly, a distinction between the cyclopropyl methylene protons should be possible since the proton cis to the methyl groups should be affected more than its geminal partner. It has been shown (9), however, that the n.0.e. for a methylene proton caused by saturation of vicinal or further separated protons is small because the major relaxation occurs by interaction with its geminal neighbor.…”

“…measurements to determine the relative orientations of protons in a wide variety of molecules is becoming increasingly popular (2,(8)(9)(10). It has been amply demonstrated that closely-neighboring protons provide a major mechanism for spin-lattice relaxation of each proton such that saturation of one of the neighbors causes an increase in the integrated intensity of the other's absorption.…”