Search

A new strategy to access macrocyclic enynes was developed. To block undesired ene-yne cyclization pathways, alkynes were protected via bromination and the resultant acyclic vic-(E)-dibromotrienes participated in selective ene-ene ring closing metathesis reactions. Zinc-promoted deprotection of (E)-dibromodienes provided macrocyclic enynes in high yields.

A series of intramolecular hetero-Diels-Alder reactions of iminium and imine dienophiles is analyzed by using density functional theory. Protonation of the nitrogen has led to a highly asynchronous transition state and the iminium dienophiles have a 17 kcal/mol lower barrier than the corresponding imines, even in aqueous solution.

The use of a gold-catalyzed, microwave protocol to activate alcohols through an intermolecular, SN1-type reaction to directly form unsymmetrical ethers and SN-benzyloxy carbamate- (Cbz) protected amines is reported. Results have shown this reaction to be highly reproducible and tolerant of moisture, and moderate to high product yields (53-99%) were obtained. Significantly, the intermolecular catalytic amination of alcohols to directly afford Cbz-protected amines is heretofore unprecedented using gold catalysis. [ABSTRACT FROM AUTHOR]

The organocatalytic transfer hydrogenation reactions of 3-phenyl-2-cyclopentenone with imidazolidinone catalysts are evaluated using the hybrid density functional (B3LYP/6-31G(d)) theory. The origin of the preference for the (E) iminium transition state is determined, and the stereoselectivity of hydride transfer is investigated.

A single transition state may lead to multiple intermediates or products if there is a post-transition-state reaction pathway bifurcation. These bifurcations arise when there are sequential transition states with no intervening energy minimum. For such systems, the shape of the potential energy surface and dynamic effects, rather than transition-state energetics, control selectivity. This Minireview covers recent investigations of organic reactions exhibiting reaction pathway bifurcations. Such phenomena are surprisingly general and affect experimental observables such as kinetic isotope effects and product distributions.

[reaction: see text] Density functional theory (B3LYP/6-31+G) has revealed the origin of stereoselectivity in intramolecular Diels-Alder reactions of vinyl oxocarbenium ions. The cycloaddition has endo preference and occurs with remote stereocontrol syn to the substituent at the stereogenic center. Torsional steering, the preference for the staggered conformation about forming sigma-bonds, dictates the preferred transition structure.