Cyclopropylazetoindolines as precursors to C(3)-quaternary-substituted indolines

Concise Total Synthesis of (+)-Gliocladins B and C

The first total synthesis of (+)-gliocladin B is described. Our concise and enantioselective synthesis takes advantage of a new regioselective Friedel-Crafts-based strategy to provide an efficient multigram-scale access to the C3-(3'-indolyl)hexahydropyrroloindole substructure, a molecular foundation present in a significant subset of epipolythiodiketopiperazine natural alkaloids. Our first-generation solution to (+)-gliocladin B involved the stereoselective formation of (+)-12-deoxybionectin A, a plausible biosynthetic precursor. Our synthesis clarified the C15 stereochemistry of (+)-gliocladin B and allowed its full structure confirmation. Further studies of a versatile dihydroxylated diketopiperazine provided a concise and efficient synthesis of (+)-gliocladin B as well as access to (+)-gliocladin C.

A Copper-Catalyzed Arylation of Tryptamines for the Direct Synthesis of Aryl Pyrroloindolines

An operationally simple, copper-catalyzed arylation of N-tosyltryptamines provides direct access to C3-aryl pyrroloindolines. A range of electron-donating and electron-withdrawing substituents is tolerated on both the indole backbone and the aryl electrophile. These reactions occur under ambient temperatures and with equimolar quantities of the coupling partners.

Ethylene in organic synthesis: a new route to anticholenergic pyrrolidinoindolines, and other molecules with all carbon-quaternary centers via asymmetric hydrovinylation

The asymmetric hydrovinylation (1 mol % Ni-cat., 1 atm, ethylene, >98% ee) products from 1-methylenetetralines are readily converted into 3,3-disubstituted oxindoles and subsequently to pyrrolidinoindolines. These hydrovinylation products are also useful for the syntheses of enantiopure benzomorphans.

Concise total synthesis and stereochemical revision of (+)-naseseazines A and B: regioselective arylative dimerization of diketopiperazine alkaloids

Concise and enantioselective total syntheses of (+)-naseseazines A and B are described. Our regioselective and directed dimerization of diketopiperazines provides their critical C3-C(sp(2)) linkages, an assembly with plausible biogenetic relevance. We revise the absolute stereochemistry of (+)-naseseazines A and B.

Directed heterodimerization: stereocontrolled assembly via solvent-caged unsymmetrical diazene fragmentation

A general strategy for the directed and stereocontrolled assembly of carbon-carbon linked heterodimeric hexahydropyrroloindoles is described. The stepwise union of complex amines in the form of mixed diazenes followed by photoexpulsion of dinitrogen in a solvent cage provides completely guided assembly at challenging C(sp(3))-C(sp(3)) and C(sp(3))-C(sp(2)) connections.

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.

Enantioselective synthesis of pyrroloindolines by a formal [3 + 2] cycloaddition reaction

(R)-BINOL·SnCl(4) was found to catalyze a formal [3 + 2] cycloaddition reaction between C(3)-substituted indoles and 2-amidoacrylates to provide pyrroloindolines. A variety of pyrroloindolines were prepared with high enantioselectivity in one step from simple precursors. This methodology is expected to facilitate the total synthesis of pyrroloindoline alkaloids, an important class of biologically active natural products.