Benjamin W. Gung

Gold-Catalyzed Transannular [4+3] Cycloaddition Reactions

Macrocyclic propargyl acetates containing a furan ring were prepared by using a CrCl(2)-promoted reaction. In the presence of either a Au(I) or Au(III) catalyst, a tandem 3,3-rearrangement/transannular [4+3] cycloaddition reaction occurred to give propargyl acetates that are regio- and diastereospecific. The regiochemistry of the product is controlled by the position of the acetoxy group in the starting material and the stereochemistry of the reaction depends on the ring size.

Stacking Interactions between Nitrogen-Containing Six-Membered Heterocyclic Aromatic Rings and Substituted Benzene: Studies in Solution and in the Solid State

The stacking interactions between an aromatic ring and a pyridine or a pyrimidine ring are studied by using a series of triptycene-derived scaffolds. The indicative ratios of the syn and anti conformers were determined by variable-temperature NMR spectroscopy. The syn conformer aligns the attached aromatic ring and the heterocycle in a parallel-displaced orientation while the anti conformer sets the two rings apart from each other. Comparing to the corresponding control compounds where a benzene ring is in the position of the heterocycle, higher attractive interactions are observed as indicated by the higher syn/anti ratios. In general, the attractive interactions are much less sensitive to the substituent effects than the corresponding nonheterocycles. The greatest attractive interactions were observed between a pyrimidine ring and a N,N-dimethylaminobenzene, consistent with a predominant donor-acceptor interaction. The interactions between a pyridine ring and a substituted benzene ring show that the pyridine is comparable to that of a NO2- or a CN-substituted benzene ring except for the unpredictable substituent effects.

Quantification of CH···π interactions: implications on how substituent effects influence aromatic interactions.

Attractive interactions between a substituted benzene ring and an α-substituted acetate group were determined experimentally by using the triptycene model system. The attractive interaction correlates well with the Hammett constants σ(m) (R(2)=0.90), but correlates much better with the acidity of the α-protons (R(2)=0.98).

Remarkable increase in the diastereofacial selectivity of the addition of β-methyl α-(alkoxy)allylstannane to aldehydes: substituent effects on diastereofacial selectivity

Several new chiral allylstannanes were prepared and reacted with aldehydes. Excellent diastereofacial selectivity was observed for the reactions of allylstannane 1 with aldehydes in the presence of BF3·Et2O. This is in contrast to the results from other allylstannanes which do not bear a β-methyl group. These observations were rationalized based on a combination of steric and electronic effects.

An anomalous case of diastereofacial selectivity in the addition of chiral allylstannanes to benzaldehyde: is the “inside alkoxy” effect involved?

Abstract The “inside alkosy” effect has been found to be important in the addition of α-(alkoxy) allylstannanes to aldehydes in the presence of BF3·Et2O.

A concise synthesis of (+)- and (−)-adociacetylene B

Abstract The synthesis of (−)-adociacetylene B was completed in six steps in a yield of 9.3%, while the (+)-enantiomer was synthesized in seven steps and 7.5%. An enzymatic resolution of racemic 1 using the lipase from Pseudomonas sp. was employed to obtain (+) and (−)- 1 . This synthesis of ( S , S )- 1 represents the first total synthesis of a naturally occurring acetylenic alcohol that has two chiral centers and four acetylene units.

Gold(I)-catalyzed divergence in the preparation of bicyclic enol esters: from exclusively [3C+2C]-cycloaddition reactions to exclusive formation of vinylcyclopropanes.

With the use of benzonitrile-stabilized Au(I) catalyst [Au(IPr)(NCPh)]SbF(6) (Ic; IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), a spectrum of reactivity is observed for propargyl ester 4a with cyclic vinyl ethers, ranging from exclusively [3C+2C] cycloaddition reactions to exclusively cyclopropanation depending only on the structure of the substrate. Some initially formed cyclopropanation products rearrange into the corresponding formally [3C+2C] cycloaddition products after treatment with fresh Au(I) complex at 80 °C. Vinylcyclopropanes formed from dihydrofuran and dihydropyran resisted such rearrangement, even in the presence of fresh Au(I) catalyst at elevated temperature. This study addresses an important mechanistic question concerning whether the five-membered-ring products were produced by a direct [3C+2C] cycloaddition reaction or by a sequential cyclopropanation/ring-expansion reaction. A dual pathway is proposed for the Au(I)-catalyzed reactions between propargyl esters and cyclic vinyl ethers. The different behavior among vinyl cyclic ethers is attributed to the difference in the polarization of the π bond. Highly polarized bonds appear to undergo the cycloaddition reaction whereas less polar π-bonds produce cyclopropanes.

A SHORT SYNTHESIS OF AN ACETYLENIC ALCOHOL FROM THE SPONGE CRIBROCHALINA VASCULUM

ABSTRACT A short synthesis of the acetylenic alcohol from cribrochalina vasculum is described. No protecting groups are used in this synthesis. The key step is an enzymatic resolution of a racemic mixture of the title compound.

The difference in strength of BF3-complexes of aromatic and aliphatic aldehydes

Abstract ab initio MO computations have shown that the difference in energy between syn and anti complexation is much larger for benzaldehyde/BF3 than for acetaldehyde/BF3. The BO bond strength is much greater for the anti PhCHO·BF3 than for its syn isomer and for all the CH3CHO·BF3 complexes.

Enhanced intramolecular amide-amide hydrogen bonding through cooperativity

Abstract Model peptides 1–6 are prepared and studied by IR and variable temperature 1H NMR spectroscopies. The α-hydroxy diamides 3 and 4 form intramolecular amide-amide hydrogen bond through an eight-membered and a nine-membered ring, respectively. When compared to Gellmans simple diamides of same chain length, the enhanced hydrogen bond strength is considered as an indication of a cooperative effect.