Abstract:in solutions where úd+ is high enough to protonate most of the 8-MOP.Although we have made no real attempt to characterize the products of the slower reactions, both UV and NMR spectra show that substantial changes in the structure occur. We suspect that ring opening occurs to give either cisand/orZrans-6-hydroxy-5-benzofuranylacrylic acid, which would exist as carboxy-protonated conjugate acids or the related acylium ions in the strongly acidic media.The rates in sulfuric acid solutions were highest in 95% ac… Show more
“…Over the 20th century, several methods have been proposed to overcome this problem. Well-known examples are sector rules for the CD of specific chromophores, of which the carbonyl “octant rule” − is a subset, as well as the exciton chirality method (also for CD), to name a few. These methods are invaluable tools for linking a molecule's CD to its absolute configuration.…”
The molar rotation of a solution of a natural alpha amino acid is changed in the positive direction by addition of a strong acid. Three decades ago, an attempt to rationalize this old rule, named for Clough, Lutz, and Jirgensons (CLJ), was made by assigning circular dichroism octants for overlapping carbonyl n to pi* transitions. Modern quantum chemical methods allow us to take a new look at this phenomenon. Time-dependent density functional theory was used to model the electronic structure and transitions responsible for CLJ. We show that sector rules originally developed for circular dichroism (CD) can be applied to the optical rotation in this case, but with some restrictions, and with great caution, due to the change of the overall charge of the acids upon protonation and the distortion of the COO- chromophore in the zwitterions. We have prepared sector maps based on first-principles computations to study the correspondence between CD and optical rotation for zwitterionic and protonated l-amino acid chromophores. The CLJ effect is correctly obtained from the computations for all 12 amino acids studied in this work.
“…Over the 20th century, several methods have been proposed to overcome this problem. Well-known examples are sector rules for the CD of specific chromophores, of which the carbonyl “octant rule” − is a subset, as well as the exciton chirality method (also for CD), to name a few. These methods are invaluable tools for linking a molecule's CD to its absolute configuration.…”
The molar rotation of a solution of a natural alpha amino acid is changed in the positive direction by addition of a strong acid. Three decades ago, an attempt to rationalize this old rule, named for Clough, Lutz, and Jirgensons (CLJ), was made by assigning circular dichroism octants for overlapping carbonyl n to pi* transitions. Modern quantum chemical methods allow us to take a new look at this phenomenon. Time-dependent density functional theory was used to model the electronic structure and transitions responsible for CLJ. We show that sector rules originally developed for circular dichroism (CD) can be applied to the optical rotation in this case, but with some restrictions, and with great caution, due to the change of the overall charge of the acids upon protonation and the distortion of the COO- chromophore in the zwitterions. We have prepared sector maps based on first-principles computations to study the correspondence between CD and optical rotation for zwitterionic and protonated l-amino acid chromophores. The CLJ effect is correctly obtained from the computations for all 12 amino acids studied in this work.
“…Fortunately, diisobutylaluminum hydride (DIBAL-H) has proven to be a suitable reagent for this purpose. The reduction was carried out at low temperature (0 °C) in CH 2 Cl 2 , and the alkaline workup of the reaction mixture allows obtaining the desired (6 Z, 9 Z )-henicosa-6,9-diene ( 4a ), (3 Z, 6 Z, 9 Z )-henicosa-3,6,9-triene ( 4b ), and henicosane ( 4c ) with an overall yield of 62%, 61%, and 70%, respectively. 4 …”
Readily preparable morpholine amides hitch in good yields with organocerium reagents to produce ketones. Even in the presence of substrates and reagents with high steric hindrance, the organometallic compounds prepared from dry cerium(III) chloride and organomagnesium or organolithium compounds at -78 degrees C add cleanly to morpholine amides. The low cost of starting materials makes this new scheme of synthesis very interesting for the preparation of biologically important pheromones.
“…Among many possible choices, we have considered (1S,4S)-norbornenone as a suitable candidate for its large optical rotation and for a number of other interesting facts briefly summarized as follows. Owing to substantially similar solution phase optical rotations measured at the sodium D-line for different solvents, [71][72][73] it was assumed for a long time that the optical rotation of this molecule is very little affected by solvent effects. The presumed lack of solvent effect was somehow hidden also by the apparently good agreement found with the B3LYP/aug-cc-pVDZ estimate of the OR obtained for the free molecule.…”
The interaction of a molecule with optical fields is customarily interpreted by means of induced time-dependent electric polarizabilities, magnetizabilities and mixed electric-magnetic polarizabilities. In general, these properties can be rationalized...
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