New Insight into Chiroptical Properties of 1,2-Diols Cyclic Sulfites.

The present work examines the relationship between the structure and chiroptical properties of cyclic sulfites utilizing the electronic circular dichroism (CD) and time-dependent density functional theory (TD-DFT). For some of the model compounds the study was additionally supported by the X-ray diffraction analysis. A comparison of the experimental and simulated CD spectra gave a reasonable interpretation of the Cotton effects observed in the 200−220 nm spectral range. The study revealed a high sensitivity of the CD spectra with regard to the configuration at the sulfur atom as well as the conformation of the ring bearing the sulfite chromophore. The results demonstrated that such a combined treatment enabled the determination of absolute configuration with a high degree of confidence

New Insight into Chiroptical Properties of 1,2-Diols Cyclic Sulfites.

The present work examines the relationship between the structure and chiroptical properties of cyclic sulfites utilizing the electronic circular dichroism (CD) and time-dependent density functional theory (TD-DFT). For some of the model compounds the study was additionally supported by the X-ray diffraction analysis. A comparison of the experimental and simulated CD spectra gave a reasonable interpretation of the Cotton effects observed in the 200−220 nm spectral range. The study revealed a high sensitivity of the CD spectra with regard to the configuration at the sulfur atom as well as the conformation of the ring bearing the sulfite chromophore. The results demonstrated that such a combined treatment enabled the determination of absolute configuration with a high degree of confidence

Configurational Assignment of 5-Substituted Pyrazolidin-3-ones Using Circular Dichroism Spectroscopy

The chiroptical properties of the title compounds bearing various substituents at the N(1) and N(2) nitrogen atoms are discussed. It was found that the sign of the n−π* Cotton effect centered at about the 230−250 nm region can be correlated with the absolute configuration of the stereogenic center at C(5). It was also found that the sign of this Cotton effect is predictable by Weigang's lactam sector rule. MMX calculations, supported by X-ray measurements, showed that substituents at the nitrogen atoms significantly affect the conformation of the five-membered ring. It was additionally concluded that the conformation of the five-membered ring in 5-substituted pyrazolidin-3-ones is the sign-determining factor for an n−π* transition

Chiral Base-Catalyzed Enantioselective Synthesis of 4-Aryloxyazetidinones and 3,4-Benzo-5-oxacephams

Readily available 4-formyloxyazetidinone was enantioselectively transformed into 3,4-benzo-2-hydroxy-5-oxacephams and 4-phenyloxyazetidinones upon treatment with 0.1 equiv of the cinchona alkaloid in toluene via intermolecular nucleophilic trapping of N-acyliminium intermediate by the hydroxyl moiety of phenols or o-hydroxybenzaldehydes. Additionally, the absolute configuration of title compounds was established by CD spectroscopy

Photoinduced Isomerization of 23-Oxosapogenins: Conformational Analysis and Spectroscopic Characterization of 22-Isosapogenins

The first synthesis of 22-isospirostane derivatives is described. They were obtained by photochemical isomerization of 23-oxosapogenins. The structure of 23-oxo-22-isotigogenin acetate (12) was proved by a single crystal X-ray diffraction, while structures of 23-oxo-22-isodiosgenin acetate (13) and 23-oxo-22-isosarsasapogenin acetate (14) were elucidated by spectroscopic methods. 22-Isodiosgenin acetate (17) was obtained by NaBH4 reduction of the 23-oxo derivative 13 followed by the two-step Barton–McCombie deoxygenation procedure. Conformational analysis of 22-iso compounds was carried out with CD and NMR, as well as DFT calculations

Dimolybdenum Tetracarboxylates as Auxiliary Chromophores in Chiroptical Studies of <i>vic</i>-Diols

The aim of the present work was to check the suitability of dimolybdenum carboxylates, other than commonly used [Mo₂(OAc)₄], as auxiliary chromophores for determining the absolute configuration of optically active <i>vic</i>-diols by means of electronic circular dichroism (ECD). To this end, a set of dimolybdenum tetracarboxylates was synthesized, and subsequently, the two most promising compounds were selected, namely dimolybdenum tetrakis­(μ-pivalate) and tetrakis­(μ-isovalerate). The selection was based on their solubility in commonly used solvents, their stability in solution, their tolerance to air exposure, as well as their utility for dichroic studies. The stability of the obtained <i>in situ</i> chiral complexes was verified by measuring the dependence of ECD, UV–vis, and NMR spectra on time, temperature, and concentration. We have shown that the ECD spectra of diverse <i>vic</i>-diols with these complexes are suitable for configurational assignment based on the correlation between signs of Cotton effects (CEs) arising in the spectra and the stereostructure of the ligand. Furthermore, to aid in the interpretation of experimental results, a separate set of DFT calculations has been incorporated to provide additional insight into the structure of the chiral complexes involved. In contrast to the earlier assumptions, experiments showed that the chelating mode of ligation is preferred for the studied complexes

Structure−Chiroptical Properties Relationship of Carbapenams by Experiment and Theory

The present work examines the relationship between the molecular structure and chiroptical properties of carbapenams through use of electronic circular dichroism spectroscopy (ECD). The applicability of the helicity rule that correlates the molecular structures of various β-lactam analogues and their ECD spectra is examined against a set of differently substituted carbapenams. It is demonstrated that the studied compounds conform to the rule. The rule can be also applied to the carbapenams with an additional chromophoric unit interfering with the amide chromophore. For the representative carbapenams, the experimental curves are compared to the ECD spectra computed using time-dependent density functional theory (TDDFT) in order to validate the experimental data. The study reveals a high effectiveness of the ECD spectroscopy for the configurational assignment at the bridgehead carbon atom and demonstrates a strong dependence of the molecular conformation on substitution of the five-membered ring and side-chain flexibility of investigated carbapenams

Photoinduced Isomerization of 23-Oxosapogenins: Conformational Analysis and Spectroscopic Characterization of 22-Isosapogenins

The first synthesis of 22-isospirostane derivatives is described. They were obtained by photochemical isomerization of 23-oxosapogenins. The structure of 23-oxo-22-isotigogenin acetate (12) was proved by a single crystal X-ray diffraction, while structures of 23-oxo-22-isodiosgenin acetate (13) and 23-oxo-22-isosarsasapogenin acetate (14) were elucidated by spectroscopic methods. 22-Isodiosgenin acetate (17) was obtained by NaBH4 reduction of the 23-oxo derivative 13 followed by the two-step Barton–McCombie deoxygenation procedure. Conformational analysis of 22-iso compounds was carried out with CD and NMR, as well as DFT calculations

Photoinduced Isomerization of 23-Oxosapogenins: Conformational Analysis and Spectroscopic Characterization of 22-Isosapogenins

The first synthesis of 22-isospirostane derivatives is described. They were obtained by photochemical isomerization of 23-oxosapogenins. The structure of 23-oxo-22-isotigogenin acetate (<b>12</b>) was proved by a single crystal X-ray diffraction, while structures of 23-oxo-22-isodiosgenin acetate (<b>13</b>) and 23-oxo-22-isosarsasapogenin acetate (<b>14</b>) were elucidated by spectroscopic methods. 22-Isodiosgenin acetate (<b>17</b>) was obtained by NaBH₄ reduction of the 23-oxo derivative <b>13</b> followed by the two-step Barton–McCombie deoxygenation procedure. Conformational analysis of 22-iso compounds was carried out with CD and NMR, as well as DFT calculations