Total synthesis of the strychnos alkaloid (+)-minfiensine: tandem enantioselective intramolecular Heck-iminium ion cyclization

Molecular iodine induced/1,3-dipolar cycloaddition/oxidative aromatization sequence: an efficient strategy to construct 2-substituted benzo[f]isoindole-1,3-dicarboxylates

A useful method for a molecular iodine induced 1,3-dipolar cycloaddition/oxidative aromatization sequence to construct 2-substituted-benzo[f]isoindole-1,3-dicarboxylates is reported.

One-pot enantioselective construction of indoloquinolizidine derivatives bearing five contiguous stereocenters using aliphatic aldehydes, nitroethylenes, and tryptamine

An organocatalytic cascade reaction was established for the construction of indoloquinolizidine derivatives bearing five contiguous stereocenters from readily available starting materials.

Synthesis of indole-based propellane derivatives via Weiss-Cook condensation, Fischer indole cyclization, and ring-closing metathesis as key steps

A variety of highly functionalized indole-based [n.3.3]propellane derivatives is described. The synthesis of the propellane derivatives involves a Weiss–Cook condensation, a Fischer indole cyclization, and a ring-closing metathesis as key steps.

Metal-free α-amination of secondary amines: computational and experimental evidence for azaquinone methide and azomethine ylide intermediates

We have performed a combined computational and experimental study to elucidate the mechanism of a metal-free α-amination of secondary amines. Calculations predicted azaquinone methides and azomethine ylides as the reactive intermediates and showed that iminium ions are unlikely to participate in these transformations. These results were confirmed by experimental deuterium-labeling studies and the successful trapping of the postulated azomethine ylide and azaquinone methide intermediates. In addition, computed barrier heights for the rate-limiting step correlate qualitatively with experimental findings.

Palladium- and nickel-catalyzed alkenylation of enolates

Transition-metal-catalyzed alkenylation of enolates provides a direct method to synthesize broadly useful β,γ-unsaturated carbonyl compounds from the corresponding carbonyl compound and alkenyl halides. Despite being reported in the early seventies, this reaction class saw little development for many years. In the past decade, however, efforts to develop this reaction further have increased considerably, and many research groups have reported efficient coupling protocols, including enantioselective versions. These reactions most commonly employ palladium catalysts, but there are also some important reports using nickel. There are many examples of this powerful transformation being used in the synthesis of complex natural products.

Developments in direct C-H arylation of (hetero)arenes under microwave irradiation

This minireview describes the progress made in the development of direct C-H arylation approaches through the application of focused microwave irradiation. The synergistic combination of microwave-assisted techniques with the rapidly evolving domain of C-H arylation has opened new vistas in the efficient synthesis of a diverse array of biologically important (hetero)arenes.

Dynamic multi-component covalent assembly for the reversible binding of secondary alcohols and chirality sensing

Reversible covalent bonding is often employed for the creation of novel supramolecular structures, multi-component assemblies, and sensing ensembles. In spite of remarkable success of dynamic covalent systems, the reversible binding of a mono-alcohol with high strength is challenging. Here we show that a strategy of carbonyl activation and hemiaminal ether stabilization can be embodied in a four-component reversible assembly that creates a tetradentate ligand and incorporates secondary alcohols with exceptionally high affinity. Evidence is presented that the intermediate leading to binding and exchange of alcohols is an iminium ion. Further, to demonstrate the use of this assembly process we explored chirality sensing and enantiomeric excess determinations. An induced twist in the ligand by a chiral mono-ol results in large Cotton effects in the circular dichroism spectra indicative of the alcohol’s handedness. The strategy revealed in this study should prove broadly applicable for the incorporation of alcohols into supramolecular architecture construction.

Total synthesis of indoline alkaloids: A cyclopropanation strategy

Indoline alkaloids constitute a large class of natural products; their diverse and complex structures contribute to potent biological activities in a range of molecules. Designing an appropriate strategy for the total synthesis of indoline alkaloids is a difficult task that depends on being able to efficiently assemble the core architectures. The best strategies allow access to a variety of different indoline alkaloid structures in a minimum of steps. The cyclopropanation of simple olefins and the subsequent synthetic transformation of the resulting cyclopropyl intermediates has been intensively studied in recent decades. In contrast, the cyclopropanation of enamines, especially for the construction of complex nitrogen-containing ring systems, remained relatively unexplored. Previous success with the cyclopropanation of simple indoles to form stable indolylcyclopropanocarboxylates encouraged us to explore the assembly of indoline alkaloid skeletons with cyclopropanation as a key reaction. Theoretically, indolylcyclopropanocarboxylates are doubly activated by a vicinally substituted amino group and carboxyl group; that is, they are typical donor-acceptor cyclopropanes. Accordingly, they tend to yield an active iminium intermediate, which can undergo inter- and intramolecular nucleophilic reactions to form the core structure of indoline alkaloids with an expanded ring system. In this Account, we summarize our efforts to develop a cascade or stepwise reaction of cyclopropanation/ring-opening/iminium cyclization (the CRI reaction) on tryptamine derivatives for assembling indoline alkaloid skeletons. With the CRI approach, three types of indoline alkaloid skeletons have been efficiently constructed: (i) hexahydropyrrolo[2,3-b]indoline (type I), (ii) tetrahydro-9a,4a-iminoethano-9H-carbazole (type II), and (iii) tetrahydroquinolino[2,3-b]indoline (type III). The effects of substituents on tryptamine derivatives were carefully investigated for inter- and intramolecular CRI reactions during construction of type I and type II skeletons. These results provided a basis for the further design and synthesis of complex natural products containing nitrogen. The usefulness of the CRI reaction is well demonstrated by our total synthesis of structurally intriguing indoline alkaloids such as N-acetylardeemin, minfiensine, vincorine, and communesin F. In addition, we highlight advances by other groups in construction of the three types of skeletons as well as their total syntheses of these indoline alkaloids. Discussion of the total syntheses of these indoline alkaloids focuses on comparing the individual synthetic strategies for forming the ring systems embedded in the final products. We also describe the total synthesis of perophoramidine, which has the same type III skeleton as communesin F. The observation of a retro Diels-Alder reaction during our synthesis of communesin F inspired the hetero Diels-Alder reaction on which our total synthesis of perophoramidine was based.

Exploration of the interrupted Fischer indolization reaction

A convergent method to access the fused indoline ring system present in a multitude of bioactive molecules has been developed. The strategy involves the condensation of hydrazines with latent aldehydes to ultimately deliver indoline-containing products by way of an interrupted Fischer indolization sequence. The method is convergent, mild, operationally simple, broad in scope, and can be used to access enantioenriched products. In addition, our approach is amenable to the synthesis of furoindoline and pyrrolidinoindoline natural products as demonstrated by the concise formal total syntheses of physovenine and debromoflustramine B. The strategy will likely enable the synthesis of more complex targets such as the communesin alkaloids.

Nine-step enantioselective total synthesis of (+)-minfiensine

An enantioselective total synthesis of the Strychnos alkaloid (+)-minfiensine has been accomplished. Prominent features of this synthesis include (i) a new enantioselective organocatalytic Diels-Alder/amine cyclization sequence to build the central tetracyclic pyrroloindoline framework in four steps from commercial materials and (ii) a 6-exo-dig radical cyclization to forge the final piperidinyl ring system. This total synthesis of (+)-minfiensine was completed in nine chemical steps and 21% overall yield.