59(P-37) 複雑な化合物の立体学研究に有用なNMRの新しい測定方法 : HR-HMBC法とBIRD-J-resolved HMBC法(ポスター発表の部)

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We present two new NMR techniques prepared by modification of HMBC, High Resolution HMBC (HR-HMBC) and BIRD-J- resolved HMBC. 1) High Resolution HMBC (HR-HMBC) In the conventional HMBC spectra, the cross peaks contain information on the long-range J_<CH> splittings and/or J_<HH> splittings. Due to the poor resolution in the F_2 or F_1 dimension, however, it is difficult to measure the long-range J_<CH> couplings directly from HMBC cross peaks. In order to solve this problem, we have incorporated the J-scaling method into HMBC and developed a new technique, HR-HMBC. The J-scaling pulse (--nt_1/2-180(H, C)-nt_1/2--) is introduced after the spin evolution period of HMBC. In this pulse sequence, the magnetizations of chemical shift, J_<HH> and J_<CH> coupling constants evolve with t_1, (n+1)t_1 and nt_1, respectively. As a result, the spin coupling constants are amplified by a factor of n for J_CH and n+1 for J_<HH> (n=20 to 30). This modification enables to determine easily long range J_<CH> couplings of complicated compounds and will be useful for conformational analysis of non-cyclic systems. Advantages of this method are simple operation, high sensitivity and general applicability. 2) BIRD-J-resolved HMBC For stereochemical studies of organic molecules by NMR, it is important to analyze proton-proton spin coupling constants. Natural products such as polyketide often contain the spin system which includes a secondary methyl group (-CH_A-CH_B(CH_3)-CH_C-). In such a spin system, proton H_B splits complicatedly. Thus, it becomes very difficult to obtain spin coupling constants from the multiplet signal of H_B. Analysis of spin coupling pattern of HB is sometimes essential for stereochemical studies. The new NMR techniques BIRD-J-resolved HMBC and BIRD-HR-HMBC are useful in order to overcome this problem. We have already reported J-resolved HMBC methods to observe long range J_<CH> couplings. By incorporation of the scaling pulse into HMBC, proton-proton spin splittings in the F_1 dimension are magnified by a factor of n (n=20 to 30) making measurement of J_<CH> very easy. In these pulse sequences, 180 pulse is put into BIRD pulse. The BIRD pulse modulates only protons directly connected to ^<12>C and not protons connected to ^<13>C. In other words, protons connected directly to ^<13>C are decoupled. For example, in a spin system (-CH_A-CH_B(^<13>CH_3)-CH_C-), proton H_B with its adjacent methyl protons being decoupled are coupled only to H_A and H_C in J-resolved HMBC and HR-HMBC spectra. Thus this technique enables easier analysis of the multiplet H_B with complicated splitting pattern. An application of this method to a model compound portmicin is explained.
2004-10-01

59(P-37) 複雑な化合物の立体学研究に有用なNMRの新しい測定方法 : HR-HMBC法とBIRD-J-resolved HMBC法(ポスター発表の部)

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