Nuclear Overhauser enhancement proton decoupling

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

Nuclear Overhauser enhancements and spin-lattice relaxation times are individual for each carbon. As a result, signal intensities cannot be evaluated from PFT 13C NMR spectra obtained with continuous proton broadband decoupling. 

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

NMR analyses were done on an IBM Instruments NR-300 spectrometer and an Oxford 7 Tesla superconducting narrow-bore magnet. Silicon-29 (Si-29) NMR spectra were recorded at 59.6 MHz and hydrogen (also commonly called proton or H-l) NMR spectra at 300.13 MHz. Spectra were recorded using conventional single-pulse techniques with proton decoupling for Si-29 acquisitions. Si-29 experiments were structured so as to suppress nuclear Overhauser enhancement (NOE). For Si-29 acquisitions, spectral widths were 50 kHz and Fourier transform (FT) sizes were 4K points. For protons, spectral widths were 7.5 kHz and FT sizes were 16K points. Si-29 rf pulse widths were approximately 12 fits and proton rf pulse widths were 8 jj.s. 

With diastereomerically pure 32 now available, it was possible to determine its stereochemistry. Regiochemistry about the vinyl silane moiety in 32 was ascertained by nuclear Overhauser enhancement difference (DNOE) experiments. Decoupling experiments with either downfield methylene proton adjacent to the carboxylic acid 8 2.62 (dd, 1 H, J = 9.5,15.0 Hz) or 8 2.48 (dd, 1 H, J = 5.9, 15.0 Hz) identified the C-6 proton resonance 8 2.78 (m, 1 H). Similarly, the C-2 proton resonance was located [8 2.11 (m, 1H)]. Irradiation of the vinylsilane proton singlet at 8 5.38 led to an enhancement of 10% of the C-6 and none of the C-2 proton resonance, thus demonstrating a syn relationship between the vinyl proton and the C-6 proton. The fact that acid 32 was converted to the natural product confirmed that a p-oriented side chain had been produced at C-6 and clinched the stereochemistry of 32 as depicted. 

G NMR Spectra. G spectra were obtained with a Varian Associates FT-80 spectrometer at 20,000 MHz under several conditions. Samples from 10-, 35-, 60-, and 120-min reaction times were examined in THF-dg in the presence of O.IM fm-(acetylacetonate)-chromium(IH) (Gr(acac)3) using 90° pulses and 1.4-sec recycle times with gated proton decoupling for Nuclear Overhauser enhancement (NOE) suppression (6). These parameters gave quantitative results for all carbons of 2,2-dihydroxy diphenylmethane as well as pyrene, whose innermost carbons (10b and 10c) exhibit Tj values of greater than 200 sec (7), Ladner has reported a 14.0-sec delay and a 0.3-sec acquisition time with 0.04M Gr(acac)3 (8) using a 35°... 

Wide band proton decoupling results not only in the appearance of individual carbon resonances as sharp singlets but also in an additional increase in sensitivity to maximum of 1-99. The increased sensitivity is the consequence of the collapse of proton decoupled multiplets and the nuclear Overhauser enhancement (Noggle and Schirmer, 1971). 

Figure 1.2.13 (a) Partial 400 MHz IH-NMR spectrum of the methine protons of testosterone. A hopeless case for assignment, (b) Decoupling and nuclear Overhauser enhancement effects can be used to disect the multiline, one-dimensional spectrum into a variety of two-dimensional spectra. Assignment of signals to individual hydrogen atoms can thus be achieved. (From Sanders and Hunter, 1993.)... 

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