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Selective version of the HCC relay experiment
JOURNAL
OF MAGNETIC
RESONANCE
92,602-604
( 199 1)
Selective Version of the HCC Relay Experiment IGOR GOLJER, * TIBOR LIPTAJ, AND NAD’A
PR~NAYOVA
Central Laboratory of Chemical Technic, Department of Physical Chemistry, Faculty qf Chemicul Technology, Slovak Technical University, Radlinskkho 9, 812 37 Bratislava, Czechoslovakia Received September 16, 1990; revised October 3, 1990
Direct and unambiguous determination of the structure of organic compounds is in some cases possible only on the basis of a 13C- 13C INADEQUATE ( 1) NMR experiment. The bottleneck of this experiment is its inherent low sensitivity, especially if the structure involves quaternary carbons C,. In the study of molecules with fragments C,-C-H, more favorable methods are the HCC relayed experiment of Kessler et al. (2) and the HICCUP experiment of Lee and Morris (3, 4). The 2D version of the Kessler experiment provides the correlation of the resonances of protons and carbons separated by two bonds. The 13C COSY developed by Lee and Morris (4) gives complete correlation between protonated carbons and their neighbors. With respect to the structural determination, the information available from 2D correlation experiments is in many cases redundant. Partial information obtainable from less time-consuming ID experiments may be sufficient. To retain unequivocal proton and carbon correlation, however, 1D versions must be semiselective; i.e., only one preselected proton can serve as a source of magnetization in the coherence-transfer process. The selective excitation of the proton directly bonded to 13C can be achieved in various ways. One method is the selective inversion of one-half of the proton multiplet. The RF frequency of the soft pulse can be estimated from the proton spectrum measured with a J high-pass filter (selective inversion via a BIRD pulse of the main proton resonances, followed by a T, filter (5)). The excitation profile of the pulse should account for ‘H- ‘H and two-bond ‘H- 13C couplings of the given proton. Alternatively, the whole proton multiplet can be selectively excited during carbon decoupling, or nonselective excitation of the proton resonances can be combined with a chemical-shift filter set to allow or to suppress the excitation in the given spectral region. The general pulse scheme of the proposed experiment is shown in Fig. 1. Except for the selective proton excitation, the pulse scheme is identical to that of Lee and Morris (3). If the selective excitation is done by the selective inversion of one-half of the proton multiple& the first polarization step is omitted. To illustrate the proposed method, we present the elucidation of the structure of the derivative of isoxasole (I) synthesized earlier (6) (Fig. 2). Due to the oxygen effect the chemical shift of H-6a ( 5.27 ppm) is higher than the chemical shift of H-3a (4.7 1 * To whom correspondence should be addressed. 0022-2364/9 1 $3.00 Copyright 0 1991 by Academic Press. Inc All rights of repmduction in any form reserved.
602
603
NOTES
02
03
1 SELECTIVE ‘t-i
90
DECOUPLE
PULSE II I/
I I
FIG. 1. The general pulse sequence used to record the selective HCC relay experiment. The delays as follows: A, = 1/2’J(CH), A, is between 1/6’J(CH) and 1/2’/(CH) according to the multiplicity, A3 = l/Z’J(CC). Phases @I. . . @, of individual pulses are given in Ref. (3)
are set and
ppm). If structure Ia is valid and the polarization transfer starts at proton H-6a, it ends at carbons C-6 ( 106.8 ppm) and C-3a (54.0 ppm); if the transfer starts at proton H-3a, the target nuclei are C-4 ( 169.8 ppm), C-3 ( 152.6 ppm), and C-6a (87.2 ppm). On the other hand, if structure Ib is correct, then the polarization originating at H6a ends at carbons C-6 and C-3a, and polarization from H-3a ends at C-4 (aliphatic) . The measured spectra confirmed structure Ia. The measured coupling constants are J( C-3a, C-4) = 49. 9 Hz, J(C-3a, C-3) = 35.4 Hz, J(C-3a, C-6a) = 35.4 Hz, and J(C-6, C-6a) = 45.2 Hz. In our particular case, where only one proton resonance should be suppressed, we used nonselective excitation with the simplest form of chemical-shift filter: 45”( ‘H)--7/2-180”( i3C)---7/2-45“( ‘H) ( 7, 8). The decoupler frequency is set on resonance for the selected proton. The delay T should be chosen to suppress the excitation of the other proton: 7 = 1/(2A), where A is the difference
OEt
OEt
Ph
0
lb
la FIG
Ph
2. Two
possible
diastereoisomers
of isoxazole
derivatives
I.
604
NOTES
between the resonance frequencies of protons H-6a and H-3a. The 180” ( ‘C) pulse serves to effectively decouple the carbons. An attempt to prove structure Ia with the help of methods exploiting the J( CH ) long-range coupling constants failed. Using 2D J INEPT ( 9) we found that values of long-range coupling of protons H-3a, H-6, and H-6a to C-4 are very similar and cannot be used to discriminate between structures Ia and Ib. In unfavorable cases, strong carbon signals can obscure the observation of partner signals in the HCC relay spectrum. The signals of protonated carbons can be effectively decreased by inserting the defocusing delay without decoupling prior to data acquisition, as in the original experiment of Kessler et al. (2). Artifacts in the spectrum can arise from long-range J( CH). Usually they do not pose a problem, since the artifact signal is located in the middle of a well-separated i3C-13C antiphase doublet. They can be effectively suppressed by the cancellation scheme described by Lee and Morris (3). The selective HCC relay experiment allows unambiguous identification of the onebond vicinity of a CH group from which the polarization transfer is started. It can be considered as a supplementary method to the more sensitive methods based on the exploitation of the long-range proton-carbon couplings. REFERENCES 1. 2. 3. 4. 5.
A. H. K. K. J.
6. 7. 8. 9.
L’. G. D. D.
BAX, R. FREEMAN, AND S. P. KEMPSELL, J. Am. Chem. Sot. 102,4849 ( 1980). KESSLER, W. BERMEL, AND C. GRIESINGER, J. Mugn. Reson. 62,573 ( 1985). S. LEE AND G. A. MORRIS, J. Magn. Reson. IO, 332 ( 1986). S. LEE AND G. A. MORRIS, Magn. Reson. Chem. 25, 176 ( 1987). R. GARBOW, D. P. WEITEKAMP, AND A. PINES, Chem. Phys. Lett. 93,504 ( 1982); M. F. SUMMERS. L. G. MARZILLI, AND A. BAX, J. Am. Chem. Sot. 108,4285 ( 1986). FISERA AND P. ORAVEC, Collect Czech. Chem. Commun. 52, 146 ( 1987). M. CLORE, B. J. KIMBER, AND A. M. GRONENBORN, J. Magn. Reson. 54, 170 ( 1983). L. TURNER, J. Magn. Reson. 54, 146 ( 1983 ) UHR~N AND T. LIPTAJ, J. Mugn. Reson., 81, 82 ( 1989).
OF MAGNETIC
RESONANCE
92,602-604
( 199 1)
Selective Version of the HCC Relay Experiment IGOR GOLJER, * TIBOR LIPTAJ, AND NAD’A
PR~NAYOVA
Central Laboratory of Chemical Technic, Department of Physical Chemistry, Faculty qf Chemicul Technology, Slovak Technical University, Radlinskkho 9, 812 37 Bratislava, Czechoslovakia Received September 16, 1990; revised October 3, 1990
Direct and unambiguous determination of the structure of organic compounds is in some cases possible only on the basis of a 13C- 13C INADEQUATE ( 1) NMR experiment. The bottleneck of this experiment is its inherent low sensitivity, especially if the structure involves quaternary carbons C,. In the study of molecules with fragments C,-C-H, more favorable methods are the HCC relayed experiment of Kessler et al. (2) and the HICCUP experiment of Lee and Morris (3, 4). The 2D version of the Kessler experiment provides the correlation of the resonances of protons and carbons separated by two bonds. The 13C COSY developed by Lee and Morris (4) gives complete correlation between protonated carbons and their neighbors. With respect to the structural determination, the information available from 2D correlation experiments is in many cases redundant. Partial information obtainable from less time-consuming ID experiments may be sufficient. To retain unequivocal proton and carbon correlation, however, 1D versions must be semiselective; i.e., only one preselected proton can serve as a source of magnetization in the coherence-transfer process. The selective excitation of the proton directly bonded to 13C can be achieved in various ways. One method is the selective inversion of one-half of the proton multiplet. The RF frequency of the soft pulse can be estimated from the proton spectrum measured with a J high-pass filter (selective inversion via a BIRD pulse of the main proton resonances, followed by a T, filter (5)). The excitation profile of the pulse should account for ‘H- ‘H and two-bond ‘H- 13C couplings of the given proton. Alternatively, the whole proton multiplet can be selectively excited during carbon decoupling, or nonselective excitation of the proton resonances can be combined with a chemical-shift filter set to allow or to suppress the excitation in the given spectral region. The general pulse scheme of the proposed experiment is shown in Fig. 1. Except for the selective proton excitation, the pulse scheme is identical to that of Lee and Morris (3). If the selective excitation is done by the selective inversion of one-half of the proton multiple& the first polarization step is omitted. To illustrate the proposed method, we present the elucidation of the structure of the derivative of isoxasole (I) synthesized earlier (6) (Fig. 2). Due to the oxygen effect the chemical shift of H-6a ( 5.27 ppm) is higher than the chemical shift of H-3a (4.7 1 * To whom correspondence should be addressed. 0022-2364/9 1 $3.00 Copyright 0 1991 by Academic Press. Inc All rights of repmduction in any form reserved.
602
603
NOTES
02
03
1 SELECTIVE ‘t-i
90
DECOUPLE
PULSE II I/
I I
FIG. 1. The general pulse sequence used to record the selective HCC relay experiment. The delays as follows: A, = 1/2’J(CH), A, is between 1/6’J(CH) and 1/2’/(CH) according to the multiplicity, A3 = l/Z’J(CC). Phases @I. . . @, of individual pulses are given in Ref. (3)
are set and
ppm). If structure Ia is valid and the polarization transfer starts at proton H-6a, it ends at carbons C-6 ( 106.8 ppm) and C-3a (54.0 ppm); if the transfer starts at proton H-3a, the target nuclei are C-4 ( 169.8 ppm), C-3 ( 152.6 ppm), and C-6a (87.2 ppm). On the other hand, if structure Ib is correct, then the polarization originating at H6a ends at carbons C-6 and C-3a, and polarization from H-3a ends at C-4 (aliphatic) . The measured spectra confirmed structure Ia. The measured coupling constants are J( C-3a, C-4) = 49. 9 Hz, J(C-3a, C-3) = 35.4 Hz, J(C-3a, C-6a) = 35.4 Hz, and J(C-6, C-6a) = 45.2 Hz. In our particular case, where only one proton resonance should be suppressed, we used nonselective excitation with the simplest form of chemical-shift filter: 45”( ‘H)--7/2-180”( i3C)---7/2-45“( ‘H) ( 7, 8). The decoupler frequency is set on resonance for the selected proton. The delay T should be chosen to suppress the excitation of the other proton: 7 = 1/(2A), where A is the difference
OEt
OEt
Ph
0
lb
la FIG
Ph
2. Two
possible
diastereoisomers
of isoxazole
derivatives
I.
604
NOTES
between the resonance frequencies of protons H-6a and H-3a. The 180” ( ‘C) pulse serves to effectively decouple the carbons. An attempt to prove structure Ia with the help of methods exploiting the J( CH ) long-range coupling constants failed. Using 2D J INEPT ( 9) we found that values of long-range coupling of protons H-3a, H-6, and H-6a to C-4 are very similar and cannot be used to discriminate between structures Ia and Ib. In unfavorable cases, strong carbon signals can obscure the observation of partner signals in the HCC relay spectrum. The signals of protonated carbons can be effectively decreased by inserting the defocusing delay without decoupling prior to data acquisition, as in the original experiment of Kessler et al. (2). Artifacts in the spectrum can arise from long-range J( CH). Usually they do not pose a problem, since the artifact signal is located in the middle of a well-separated i3C-13C antiphase doublet. They can be effectively suppressed by the cancellation scheme described by Lee and Morris (3). The selective HCC relay experiment allows unambiguous identification of the onebond vicinity of a CH group from which the polarization transfer is started. It can be considered as a supplementary method to the more sensitive methods based on the exploitation of the long-range proton-carbon couplings. REFERENCES 1. 2. 3. 4. 5.
A. H. K. K. J.
6. 7. 8. 9.
L’. G. D. D.
BAX, R. FREEMAN, AND S. P. KEMPSELL, J. Am. Chem. Sot. 102,4849 ( 1980). KESSLER, W. BERMEL, AND C. GRIESINGER, J. Mugn. Reson. 62,573 ( 1985). S. LEE AND G. A. MORRIS, J. Magn. Reson. IO, 332 ( 1986). S. LEE AND G. A. MORRIS, Magn. Reson. Chem. 25, 176 ( 1987). R. GARBOW, D. P. WEITEKAMP, AND A. PINES, Chem. Phys. Lett. 93,504 ( 1982); M. F. SUMMERS. L. G. MARZILLI, AND A. BAX, J. Am. Chem. Sot. 108,4285 ( 1986). FISERA AND P. ORAVEC, Collect Czech. Chem. Commun. 52, 146 ( 1987). M. CLORE, B. J. KIMBER, AND A. M. GRONENBORN, J. Magn. Reson. 54, 170 ( 1983). L. TURNER, J. Magn. Reson. 54, 146 ( 1983 ) UHR~N AND T. LIPTAJ, J. Mugn. Reson., 81, 82 ( 1989).