Relative and Absolute Stereochemistry of Secondary/Secondary Diols: Low-Temperature 1 H NMR of Their bis-MPA Esters § (original) (raw)
Related papers
Journal of Organic Chemistry, 2005
The absolute configuration of 1,2-, 1,3-, 1,4-, and 1,5-diols formed by two secondary (chiral) hydroxy groups can be deduced by comparison of the NMR spectra of the corresponding bis-(R)-and bis-(S)-MPA esters. The correlation between the NMR spectra of the bis-ester derivatives and the absolute stereochemistry of the diol involves the comparison of the chemical shifts of the signals for substituents R 1 /R 2 and for the hydrogens attached to the two chiral centers [H R (R 1 ) and H R (R 2 )] in the bis-(R)-and the bis-(S)-ester and is expressed as ∆δ. RS Theoretical calculations [energy minimization by semiempirical (AM1), ab initio (HF), DFT (B3LYP), and Onsager methods, and aromatic shielding effect calculations] and experimental data (NMR and CD spectroscopy) indicate that in these bis-MPA esters, the experimental ∆δ RS values are the result of the contribution of the shielding/deshielding effects produced by the two MPA units that combine according to the actual stereochemistry of the diol. The reliability of these correlations is demonstrated with a wide range of diols of known absolute configuration derivatized with MPA and 9-AMA as auxiliary reagents. A simple graphical model that allows the simultaneous assignment of the two asymmetric carbons of a 1,n-diol by comparison of the NMR spectra (∆δ RS signs) of its bis-(R)-and bis-(S)-AMAA ester derivatives is presented.
Silicon-29 NMR access to stereochemistry of molecules. 2. (Trimethylsilyl)cyclopropyl compounds
Organometallics, 1984
2.3-2.6 (br m, 1.6, SiCH2), 0.33-0.46 (d, 2.6, FSiCH3). A higher boiling fraction contained XIXb (0.49 9). X-ray Structure Analysis of VI. A crystal of 2,8-dibromo-5,10-dimethyl-10-fluoro-10,1l-dihydro-5H-dibenz[ b,A- [1,4]azasilepine with well-developed faces was attached to a glaas fiber. Pertinent crystal and intensity data are given in Systematic absences of h01,l odd, and OkO, k odd, uniquely determine the space group ml/c. Thirteen reflections with 20 > 1 5 O were centered with a programmed centering routine; cell parameters were obtained by least-squares refinement of these angles. The intensities of three standard reflections were measured every 97 reflections and decreased in intensity approximately 22% during the data collection. A hear correction for loss of intensity was applied during the data reduction. The data were reduced to F and u ( p ) by procedures similar to those described previ-0us1y.~~ Standard deviations were assigned as follows: u ( n = [ U~, , , ,~, ( Z )~ + (0.05Z)2]*/2, where ucountar = (I + PB)l12, I = net intensity, B = total background count, and K = ratio of scan time to background time. The structure was solved by an iterative application of the Z2 relationship with 129 normalized structure factors of magnitude 1.3 or greater. An E map based on the set of phases for the solution with the largest consistency index (0.98) gave the positions of the non-hydrogen atoms of the structure. Least-squares refinement of the non-hydrogen atoms with isotropic thermal parameters gave a discrepancy factors R = 0.052 and R, = 0.066; all hydrogen atoms were located in an electron density difference map (R = O.Os0) and included in the f i refinement with thermal parameters fixed at 5.0 A2. Final positional parameters with estimated standard deviations are given in Table V. Atomic scattering factors and real and imaginary anomalous dispersion (27) The synthesis of PhSi(Me)CH2C6H$ has been reported but the boiling point waa given without the pressure.28 (28) Drozdov, V. A,; Kreshkov, A. P.; Romanova, A. D. Zh. Obsch.
The Stereochemistry of 1,2,3-Triols Revealed by 1 H NMR Spectroscopy: Principles and Applications
Chemistry-a European Journal, 2009
The conformational compositions of the tris(α-methoxy-α-phenylacetic acid) ester derivatives of 1,2,3-prim,sec,sec-triols are presented. These conformations have been determined by theoretical and experimental data (i.e., energy- and chemical-shift calculations, circular dichroism (CD) experiments, coupling-constant analysis, enantioselective deuteration experiments, and low-temperature NMR spectroscopic studies). A detailed analysis of the anisotropic effects due to the most significant conformers in the 1H NMR spectra supported the correlation between the 1H NMR spectra (ΔδRS value of H(3′) and |Δ(ΔδRS)| parameters) and the absolute configuration of the substrate. The study also allows the identification of the pro-R and pro-S methylene protons from their vicinal coupling constants and relative chemical shifts.
Organic Letters, 2006
The absolute configuration of 1,2,3-prim,sec,sec-triols can be assigned by comparison of the 1 H NMR spectra of the tris-(R)-and the tris-(S)-MPA ester derivatives. An experimental demonstration of this correlation with 24 triols of known absolute configuration and a protocol using two parameterss∆δ RS (H3) and the difference between ∆δ RS (H2) and ∆δ RS (H3) ) |∆(∆δ RS )|sfor its application to the determination of the absolute configuration of other triols are presented.
The Journal of Organic Chemistry, 1997
Molecular mechanics, semiempirical (AM1), and aromatic shielding effect calculations and DNMR experiments show that MTPA esters are constituted by three main conformers in close populations due to restricted rotation around the C R -CO and C R -Ph bonds. The small predominance of one conformer and the simultaneous operation of aromatic shielding and deshielding effects on the alcohol part, due to the orientation of the Ph ring, explains the small ¢‰ RS values observed. A graphical description of the aromatic magnetic field distribution in the conformers of MTPA and MPA esters and its use to correlate the average chemical shifts with the absolute stereochemistry is presented. Comparison of MTPA with MPA (two conformers) as reagents for determination by NMR of absolute stereochemistry indicates that MTPA esters are intrinsically limited by the greater complexity of their conformational composition, yield very small ¢‰ RS values, and are consequently less reliable for configurations assignment of chiral alcohols than MPA or other arylmethoxyacetic acid reagents such as (R)and (S)-R-methoxy-R-(9-anthryl)acetic acids. ¢‰ RS is the difference between the chemical shift in the (R)derivative minus that of the same proton in the (S)-derivative. (5) (a) Latyov, Sh. K.; Seco, J. M.; Quiñ oá , E.; Riguera, R. Mosher, H. S. J. Am. Chem. Soc. 1973, 95, 512-519 and references cited therein. (8) (a) Yu, J.; Hu, X.; Ho, D.; Bean, M.; Stephens, R.; Cassady, J.; Brinen, L.; Clardy, J. J. Org. Chem. 1994, 59, 1598-1599. (b) Rieser, M. J.; Hui, Y.; Rupprecht, J. K.; Kozlowski, J. F.; Wood, K. V.; Mclaughlin, J. L.; Hanson, P. R.; Zhuang, Z.; Hoye, T. E.
P A R T I FUNDAMENTALS AND TECHNIQUES PRINCIPLES IN NMR SPECTROSCOPY
More than any other analytical method the nuclear magnetic resonance (NMR) spectroscopy provides information about the chemical structure and the dynamics of organic 3 molecules. The interpretation of the NMR data depends on a minimum amount of basic information that will be provided in this chapter. For a better appliance, these basics of oneand two-dimensional NMR techniques are demonstrated by means of practical examples. 0 2 J ° = 8.5 Hz J ° = 11.5 Hz J ° = 14.5 Hz J 180 = 15.5 Hz J 180 = 12.5 Hz J 180 = 9.5 Hz 4
Chemistry-a European Journal, 2005
The absolute configuration of 1,2-diols formed by a primary and a secondary (chiral) hydroxyl group can be deduced by comparison of the 1H NMR spectra of the corresponding (R)- and bis-(S)-MPA esters (MPA=methoxyphenylacetic acid). This method involves the use of the chemical shifts of substituents L1/L2 attached to the secondary (chiral) carbon, and of the hydrogen atom linked to the chiral center (CαH) as diagnostic signals. Theoretical (AM1, HF, and B3 LYP calculations) and experimental data (dynamic and low-temperature NMR spectroscopy, studies on deuterated derivatives, constant coupling analysis, circular dichroism (CD) spectra, and NMR studies with a number of diols of known absolute configuration) prove that the signs of the ΔδRS obtained for those signals correlate with the absolute configuration of the diol. A graphical model for the reliable assignment of the absolute configuration of a 1,2-diol by comparison of the NMR spectra of its bis-(R)- and bis-(S)-MPA esters is presented.