Intramolecular [4 + 2] cycloaddition reactions of substituted 1,2-cyclohexadienes with pendent furans enables the synthesis of complex tetracyclic scaffolds in a single step under mild conditions. All Diels–Alder cycloadducts were obtained as single diastereomers, assigned as the endo isomer. Substrates were easily assembled via Stork–Danheiser alkylation of 3-ethoxy-2-bromocyclohex-2-enone to accommodate a range of tethers and furan traps. Cleavage of enol acetate moieties resulted in room-temperature Diels–Alder cycloreversion to tethered furyl cyclohexenones.

1,2-Cyclohexadienes are transient intermediates that undergo rapid dimerization and intermolecular trapping with activated olefins and heteroatomic nucleophiles. Fluoride-mediated desilylative elimination of readily accessible 6-silylcyclohexene-1-triflates allows the mild, chemoselective, and functional-group tolerant generation of cyclic allene intermediates, which undergo efficient trapping reactions with stable 1,3-dipoles. The reactions proceed with high levels of both regio- and diastereoselectivity. The reaction of cyclic allenes with azides is accompanied by the facile loss of dinitrogen, resulting in the formation of tetrahydroindoles or polycylic aziridines depending on the azide employed.

Alkyl-substituted η5-pentadienyl complexes of cobalt have been reported to undergo [5 + 2] cycloaddition reactions with alkynes to form substituted η2,η3- and η5-cycloheptadienyl products, providing a new route to the synthesis of substituted cycloheptadienes. A series of cyclopentadienyl and pentamethylcyclopentadienyl cobalt(III) η5-pentadienyl complexes have been prepared, incorporating alkyl and aryl substituents at various positions on the pentadienyl ligand. The crystalline complexes have been completely characterized spectroscopically and, in the solid state, by X-ray crystallography. The alkyl-substituted pentadienyl complexes can be prepared by a range of methodologies, most generally by acid-promoted dehydration of in situ-derived η2- or η4-dienol complexes. Two variations on this classic strategy have been developed, starting from either conjugated (1,3-) or nonconjugated (1,4-) dienyl alcohols. For both cyclopentadienyl and pentamethylcyclopentadienyl ancillary ligands, the substituted η5-pentadienyl complexes are obtained in reasonable to good isolated yields, limited by the extent of substitution on the starting allylic alcohol. The cationic cobalt(III) η5-pentadienyl complexes are indefinitely stable to air and moisture; isolation and purification is accomplished by chromatography on the bench. The substitutional lability of the pentadienyl ligand has been investigated using both neutral and anionic donor ligands (CO, isonitrile, acetonitrile, and halide salts). The results reveal that η5-pentadienyl complexes react by equilibrium dissociation of the most substituted end of the pentadienyl moiety, providing the corresponding η3-coordinated pentadienyl adducts. A comparative study of 11 X-ray crystal structures covering a range of pentadienyl substitution patterns is also reported, revealing long Co−C1 and Co−C5 bonds and, consistently, short Co−C3 bond lengths. The internal bond angles of substituent-bearing carbon atoms are smaller than those observed at unsubstituted carbons, with the compression as much as 10° at the substituted center. As expected, alkyl substituents invariably deviate out of the pentadienyl plane, toward the metal. In unbiased cases, greater deviations are noted for C1 substituents than for C2 substituents, although the deviations at C2 increase significantly for 1,2-disubstituted ligands. In some cases, the carbon−carbon bond distances display distortions toward η2,η3- or η1,η4-hapticity, although no particular correlation between structural distortion and solution reactivity is evident.

The one-pot, three-component, coupling reaction of indoles/pyrroles, dimethyl malonate, and acetic acid was performed using Mn(III) acetate as an oxidant. In the presence of Mn(OAc)3, indole-2, and indole-3-carbonyl compounds were alkylated at the 3- and 2- positions, respectively, with subsequent oxidation and nucleophilic capture occurring at the newly formed benzylic carbon. In contrast, oxidation of 2- and 3-indole carboxylic acids afforded the corresponding 2-oxindol-3-ylidenes and 3-oxindol-2-ylidenes. The reaction conditions, scope, and mechanism are discussed herein.

Abstract1,2‐Cyclohexadienes generated under mild fluoride‐mediated desilylative conditions undergo efficient intramolecular [2+2] trapping, providing tricyclic alkylidene cyclobutanes with complete diastereoselectivity for the cis‐fused products. Pendent styrenes or electron‐deficient olefins can trap simple 1,2‐cyclohexadienes or their oxygenated counterparts, with 14 substrates being disclosed. Reactions proceed at ambient temperature using just cesium fluoride in up to 91 % yield, and the necessary precursors are easily accessed from substituted 2‐bromocyclohexenones. Multiple synthetic routes have been developed to install the appropriate functional groups required for [2+2] trapping.