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Recent trends in conformational analysis

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An outlook of the various research fields of organic chemistry where conformational analysis plays a fundamental role is presented. The section Methodologies for Conformational Analysis is focused on the information that can be obtained by various spectroscopic techniques such as nuclear magnetic resonance and optical methods such as electronic circular dichroism and vibrational circular dichroism. A comprehensive screening of the more recent ab initio and density functional theory theoretical approaches to conformational analysis is presented in the section Theoretical Conformational Analysis. The section Application of Conformational Analysis to the Determination of the AC of Organic Molecules shows how the synergic use of experimental and theoretical methods can solve challenging conformational tasks arising from modern organic chemistry. © 2011 John Wiley & Sons, Ltd.

Figure 1.

Left, comparisons of the measured VCD and IR spectra of baccatin III with the predicted spectra. Right, the optimized geometries of the three conformations [B3LYP/6‐31G(d)]. (Reprinted with permission from Ref 22. Copyright 2008 American Chemical Society.)

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Figure 2.

ROA spectra of (R)‐[2H1, 2H2, 2H3]‐neopentane. The bottom color traces show the computed individual ROA spectra for the nine rotamers (R1–R9). (Reprinted with permission from Ref 24. Copyright 2007 Nature Publishing Group.)

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Figure 3.

Top, calculated structures for the three conceivable conformations of [3.3]meta(heterocyclo) paracyclophanes. Bottom, 19F‐NMR spectra (564.6 MHz in CDFCl2/CHF2Cl). The signals belonging to the C1 conformation are indicated with the arrows in the spectrum at −128°C. The line shape simulations with the corresponding rate constants are also reported. (Note: the major signals of Cs conformer GS‐1 get closer on lowering the temperature, and eventually overlap at −128°C). (Reprinted with permission from Ref 37. Copyright 2010 John Wiley & Sons.)

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Figure 4.

VT‐NMR spectra in CDFCl2/CBrF3 2:1 (v/v). Left, 19F‐NMR (564.6 MHz). Right, 1H‐NMR (600 MHz) with line shape simulations and rate constants. The arrows indicate the minor conformation with Cs symmetry. (Reprinted with permission from Ref 37. Copyright 2010 John Wiley & Sons.)

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Figure 5.

Dynamic behavior of 9,10‐diferrocenyltriptycene. Top, 1H spectrum; bottom, EXSY spectra. (Reprinted with permission from Ref 38. Copyright 2010 American Chemical Society.)

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Figure 6.

NOE experiments (600 MHz at −80°C in CD2Cl2) carried out by excitation of the CH multiplet of the ortho‐isopropyl group of the minor (ap) and major (sc) atropisomer (traces b and c, respectively). The control spectrum in the region 1.6–4.6 ppm (trace a) is also reported. (Reprinted with permission from Ref 59. Copyright 2005 American Chemical Society.)

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Figure 7.

DPFGSE–NOE spectra obtained (600 MHz at −10°C in CD3CN). Trace (a), control spectrum. Traces (b–e), NOE spectra obtained on saturation of H‐A, H‐2, H‐6, and Me(3). Observed NOEs are indicated as arrows in the DFT‐optimized structure. (Reprinted with permission from Ref 62. Copyright 2009 Wiley‐VCH.)

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Figure 8.

MM‐computed energy surface with the pathway of a conformational interconversion. (Reprinted with permission from Ref 73. Copyright 2007 American Chemical Society.)

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Figure 9.

Intermediate and transition structures (AM1 Hamiltonian) involved in flipping processes as viewed along the fivefold or perpendicular to the twofold symmetry axes in C5Ph5 (top, D5h symmetry) and C5Ph4H (bottom, C2v), respectively. Relative energies are in kcal/mol. (Reprinted with permission from Ref 82. Copyright 2002 American Chemical Society.)

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Figure 10.

The five atropisomers of tetratolylbenzene. In square brackets the experimental population obtained by low‐temperature NMR is reported. In brackets are reported the calculated energies and the red numbers represent the 1H chemical shifts of the methyls. (Reprinted with permission from Ref 114. Copyright 2005 American Chemical Society.)

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Figure 11.

Top, the two conformational isomers of 1,8‐di‐(m‐tolyl)‐anthracene. Bottom, computed structures of the three possible conformers of 1,8‐di‐(m‐tolyl)‐biphenylene, having dihedral angles of 44° between the planes of toluene and biphenylene. Relative energies are in kcal/mol. (Reprinted with permission from Refs 116 and 117. Copyright 2007 American Chemical Society.)

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Figure 12.

Top boxes, calculated ECD spectra for the four conformations of the reported compound (S absolute configuration) using the geometries obtained by gas‐phase calculations. TD‐DFT calculations were performed at the CAM‐B3LYP/6‐311++G(2d,p) and BH&HLYP/6‐311++G(2d,p) level. Bottom, superimposition between the experimental trace (black) and the two simulated spectra. (Reprinted with permission from Ref 177. Copyright 2011 John Wiley & Sons.)

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Figure 13.

COSMO‐BLYP‐D/def‐TZVP structures of the seven conformations (traces a–g) of the reported β‐lactam and their computed ECD spectra at the TD‐PBE0/def2‐TZVPP level of theory. (h) Boltzmann‐averaged spectrum compared with experiment. The symbol ε denotes the molar decadic absorption coefficient, λ is the excitation wavelength, and R is the computed rotatory strength. (Reprinted with permission from Ref 181. Copyright 2011 American Chemical Society.)

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Figure 14.

Experimental (solid trace) and DFT computed (S absolute configuration, dashed trace) ECD spectrum of (S)‐2‐naphthyl tert‐butylsulfoxide (R = t‐Bu). The E/Z ratio derived from dynamic NMR was used to calculate the theoretical spectrum. (Reprinted with permission from Ref 192. Copyright 2009 Wiley‐Liss.)

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Figure 15.

Anomalous dispersion X‐ray structure and best calculated structures. The ECD simulation is also shown [B3LYP/6‐31+G(d,p) level]. (Reprinted with permission from Ref 198. Copyright 2009 Wiley‐VCH.)

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Figure 16.

Plots of VCD intensities (1148, 1117, and 1113 cm−1) versus the number of alkyl carbon atoms in (S)‐CH3CHOHCnH2n+1 (n = 2–8). (Reprinted with permission from Ref 201. Copyright 2004 American Chemical Society.)

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Figure 17.

(a) IUPAC conformational code. (b) Updated conformational code. (Reprinted with permission from Ref 203. Copyright 2009 American Chemical Society.)

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