Short, Enantioselective Total Synthesis of Aflatoxin B₂ Using an Asymmetric [3+2]-Cycloaddition Step

A highly enantioselective [3+2]-cycloaddition reaction of 2,3-dihydrofuran with 1,4-benzoquinones using a chiral oxazaborolidinium triflimidate as catalyst has been developed which allows rapid access to a variety of chiral phenolic tricycles (enantiomeric excesses ranging from 91 to 98%). The utility of this new methodology is demonstrated by a short synthesis of the important pentacyclic natural product, aflatoxin B2. This exploratory study indicates that an even broader application of these catalysts to enantioselective cycloadditions may be possible

Short, Enantioselective Total Synthesis of Aflatoxin B₂ Using an Asymmetric [3+2]-Cycloaddition Step

A highly enantioselective [3+2]-cycloaddition reaction of 2,3-dihydrofuran with 1,4-benzoquinones using a chiral oxazaborolidinium triflimidate as catalyst has been developed which allows rapid access to a variety of chiral phenolic tricycles (enantiomeric excesses ranging from 91 to 98%). The utility of this new methodology is demonstrated by a short synthesis of the important pentacyclic natural product, aflatoxin B2. This exploratory study indicates that an even broader application of these catalysts to enantioselective cycloadditions may be possible

Short, Enantioselective Total Synthesis of Aflatoxin B₂ Using an Asymmetric [3+2]-Cycloaddition Step

A highly enantioselective [3+2]-cycloaddition reaction of 2,3-dihydrofuran with 1,4-benzoquinones using a chiral oxazaborolidinium triflimidate as catalyst has been developed which allows rapid access to a variety of chiral phenolic tricycles (enantiomeric excesses ranging from 91 to 98%). The utility of this new methodology is demonstrated by a short synthesis of the important pentacyclic natural product, aflatoxin B2. This exploratory study indicates that an even broader application of these catalysts to enantioselective cycloadditions may be possible

Short, Enantioselective Total Synthesis of Aflatoxin B₂ Using an Asymmetric [3+2]-Cycloaddition Step

A highly enantioselective [3+2]-cycloaddition reaction of 2,3-dihydrofuran with 1,4-benzoquinones using a chiral oxazaborolidinium triflimidate as catalyst has been developed which allows rapid access to a variety of chiral phenolic tricycles (enantiomeric excesses ranging from 91 to 98%). The utility of this new methodology is demonstrated by a short synthesis of the important pentacyclic natural product, aflatoxin B2. This exploratory study indicates that an even broader application of these catalysts to enantioselective cycloadditions may be possible

Short, Enantioselective Total Synthesis of Aflatoxin B₂ Using an Asymmetric [3+2]-Cycloaddition Step

A highly enantioselective [3+2]-cycloaddition reaction of 2,3-dihydrofuran with 1,4-benzoquinones using a chiral oxazaborolidinium triflimidate as catalyst has been developed which allows rapid access to a variety of chiral phenolic tricycles (enantiomeric excesses ranging from 91 to 98%). The utility of this new methodology is demonstrated by a short synthesis of the important pentacyclic natural product, aflatoxin B2. This exploratory study indicates that an even broader application of these catalysts to enantioselective cycloadditions may be possible

Short, Enantioselective Total Synthesis of Aflatoxin B₂ Using an Asymmetric [3+2]-Cycloaddition Step

A highly enantioselective [3+2]-cycloaddition reaction of 2,3-dihydrofuran with 1,4-benzoquinones using a chiral oxazaborolidinium triflimidate as catalyst has been developed which allows rapid access to a variety of chiral phenolic tricycles (enantiomeric excesses ranging from 91 to 98%). The utility of this new methodology is demonstrated by a short synthesis of the important pentacyclic natural product, aflatoxin B2. This exploratory study indicates that an even broader application of these catalysts to enantioselective cycloadditions may be possible

Mesitylboron-Substituted Ladder-Type Pentaphenylenes: Charge-Transfer, Electronic Communication, and Sensing Properties

A series of dimesitylboron (B)- or ditolylamino (N)-substituted ladder-type pentaphenylenes (PP) has been designed and synthesized. The UV−vis absorption spectra of compounds BPPN, BPPB, and NPPN reveal an identical maximum wavelength at 432 nm, which indicates that the B and N centers have very similar contributions to the extended conjugation. A rather weak solvatochromism in the UV−vis absorption spectra is observed for compound BPPN, while a remarkable solvatochromic emission is achieved even though the distance between the B and the N centers is as huge as 22 Å. The photoluminescence of BPPN shows a bathochromic shift of 108 nm when the solvent polarity is increased from cyclohexane (453 nm) to acetone (561 nm). Compound BPPN acts as a colorimetric and fluorescent chemosensor with high sensitivity (10−5 M) and selectivity for F− over other halogen ions. By inhibiting the charge transfer (CT) from the N center to the B center, the intense green CT emission of compound BPPN rapidly switches into the sky-blue emission of PP when F− is bound to the B center. Furthermore, a CT emission can be switched “on” and “off” when compound BPPB is used as F− sensory material. Such an intramolecular CT emission between the two B centers has so far never been reported. Corresponding studies by cyclic voltammetry and differential pulse voltammetry reveal a two-step reduction of the two bridged B centers in compound BPPB, which might suggest that the charge delocalizes through the whole molecule and that the terminal redox centers communicate through the pentaphenylene bridge

Single Molecule Conductance, Thermopower, and Transition Voltage

We have measured the thermopower as well as other important charge transport quantities, including conductance, current–voltage characteristics, and transition voltage of single molecules. The thermopower has little correlation with the conductance, but it decreases with the transition voltage, which is consistent with a theory based on Landauer’s formula. Since the transition voltage reflects the molecular energy level alignment, our finding also shows that the thermopower provides valuable information about the relative alignment between the molecular energy levels and the electrodes’ Fermi energy level

Arylamine-Substituted Oligo(ladder-type pentaphenylene)s: Electronic Communication between Bridged Redox Centers

Novel bis(arylamine-substituted) oligo(ladder-type pentaphenylene)s 1−3, with bridge lengths estimated to be 2.2, 4.2, and 6.3 nm, respectively, have been developed, and the model compound 4 with a mono-arylamine substituent was also synthesized. Their absorption spectra in different solvents are almost identical, while distinct bathochromic shifts of the photoluminescence (PL) spectra were observed with increasing solvent polarity due to the polarized excited states. The cyclic voltammetry (CV) and differential pulse voltammetry (DPV) spectra display a two-step oxidation of the bridged diamines in compound 1, which suggests that the electron and charge delocalize in mixed-valence (MV) cation 1+• and that both redox centers can communicate through the pentaphenylene bridge. Only unresolved curves in CV and DPV spectra were observed in the first two oxidation processes of diamines 2 and 3, indicating that the bridges are too long for efficient delocalization over the entire molecules and the radical cations localize at each arylamine center. This finding was further supported by chemical oxidation with SbCl5 and studies of the corresponding UV−vis−NIR absorption spectra of compounds 1−4. A significant intervalence charge-transfer (IVCT) band around 5283 cm-1 (1893 nm) was observed in 1+•. This is the first report of such a highly intense IVCT band in the NIR region with intensity similar to that of the visible band of the radicals, enabling further analysis of the CT process and the coupling matrix element V, classifying 1+• as a class II derivative (V = 1.6 kcal/mol). This study may offer an effective way to improve the understanding of charge transfer and charge-carrier transport in various conjugated oligomers or polymers and facilitate their ongoing exploration in optoelectronic applications

Nonparallelism between Reaction Rate and Dienophile−Catalyst Affinity in Catalytic Enantioselective Diels−Alder Reactions

The above reaction is much faster with Y = CF3CH2O than with Y = CH3O. However, the methyl ester is a strong inhibitor of the Diels−Alder reaction of the trifluoroethyl ester, since it has a higher affinity for the catalyst 1