Naturally Occurring Cyclotryptophans and Cyclotryptamines

Lewis Acid Catalyzed Displacement of Trichloroacetimidates in the Synthesis of Functionalized Pyrroloindolines

The pyrroloindoline core is found in many natural products. These structures often differ at the C3a position, which may be substituted with an oxygen, nitrogen, or sp(3)- or sp(2)-hybridized carbon. Utilizing a trichloroacetimidate leaving group, a diversity-oriented approach to these structures has been developed. The trichloroacetimidate intermediate allows for the rapid incorporation of anilines, alcohols, thiols, and carbon nucleophiles. This method was applied in the synthesis of arundinine and a formal synthesis of psychotriasine.

Total Synthesis of Dimeric HPI Alkaloids

In this paper, we report a full account of the synthesis of dimeric hexahydropyrroloindole alkaloids and its analogues. The key feature of our new strategy is the novel catalytic copper (10 %) mediated intramolecular arylations of o-haloanilides followed by intermolecular oxidative dimerization of the resulting oxindoles in one pot. This sequential reaction leads to the key intermediates for the synthesis of (+)-chimonanthine, (+)-folicanthine, (-)-calycanthine and (-)-ditryptophenaline. In the presence of catalytic amount of cuprous iodide (10 %), an intramolecular arylation of o-haloanilides followed by an intermolecular oxidative dimerization of the resulting oxindoles leads to a common intermediate for the synthesis of (+)-chimonanthine, (+)-folicanthine and (-)-calycanthine. Based on this cascade sequence, we also developed a flexible strategy towards the asymmetric syntheses of dimeric HPI alkaloids (-)-ditryptophenaline and its analogues.

Synthetic Strategies toward Natural Products Containing Contiguous Stereogenic Quaternary Carbon Atoms

Strategies for the total synthesis of complex natural products that contain two or more contiguous stereogenic quaternary carbon atoms in their intricate structures are reviewed with 12 representative examples. Emphasis has been put on methods to create quaternary carbon stereocenters, including syntheses of the same natural product by different groups, thereby showcasing the diversity of thought and individual creativity. A compendium of selected natural products containing two or more contiguous stereogenic quaternary carbon atoms and key reactions in their total or partial syntheses is provided in the Supporting Information.

Diastereodivergent Reverse Prenylation of Indole and Tryptophan Derivatives: Total Synthesis of Amauromine, Novoamauromine, and epi-Amauromine

A regio- and stereoselective reverse prenylation of indole and tryptophan derivatives is presented. All four possible stereoisomers are accessible through this iridium-catalyzed reaction. The stereoselectivity is controlled by a chiral phosphoramidite ligand in combination with an achiral borane additive and can be switched by changing the nature of the borane. One enantiomer of the ligand is thus sufficient to prepare all possible isomers. The synthetic potential of this method was demonstrated by a short total synthesis of amauromine and its two natural diastereomers.

Palladium-catalyzed highly atom-economical allylation of oxindoles with vinyl cyclopropanes

A highly atom-economical allylation of oxindoles with vinyl cyclopropanes catalyzed by Pd(PPh3)4 has been developed, setting up a practical venue for the installation of an all-carbon quaternary center at the 3-position in oxindoles under mild reaction conditions. Importantly, the reaction proceeded well without any additives and no waste was produced, affording linear products with high regioselectivities (>20 : 1). Enantioselective allylic alkylation has also been realized to provide the desired products in good yields and 35% ee.

Concise Total Synthesis of (+)-Asperazine, (+)-Pestalazine A, and (+)-iso-Pestalazine A. Structure Revision of (+)-Pestalazine A

The concise, enantioselective total syntheses of (+)-asperazine (1), (+)-iso-pestalazine A (2), and (+)-pestalazine A (3) have been achieved by the development of a late-stage C3-C8' Friedel-Crafts union of polycyclic diketopiperazines. Our modular strategy enables the union of complex polycyclic diketopiperazines in virtually their final forms, thus providing rapid and highly convergent assembly at the challenging quaternary stereocenter of these dimeric alkaloids. The significance of this carbon-carbon bond formation can be gauged by the manifold constraints that were efficiently overcome, namely the substantial steric crowding at both reactive sites, the nucleophilic addition of C8' over N1' to the C3 carbocation, and the multitude of reactivity posed by the use of complex diketopiperazine fragments in the coupling event. The success of the indoline π-nucleophile that evolved through our studies is notable given the paucity of competing reaction pathways observed in the presence of the highly reactive C3 carbocation generated. This first total synthesis of (+)-pestalazine A also allowed us to revise the molecular structure for this natural alkaloid.

Access to the Pyrroloindoline Core via [3 + 2] Annulation as well as the Application in the Synthetic Approach to (±)-Minfiensine

A [3 + 2] formal cycloaddition reaction using aza-oxyallyl cation as a synthetic synthon was developed to construct the pyrroloindololine core. With this novel method, a variety of C3-substituted indoles were readily converted into the corresponding pyrroloindoline analogues at room temperature in the mixed solvents. To further demonstrate the utility of this method, a synthetic approach to the total synthesis of (±)-minfiensine was developed in quite concise fashion.

DMSO/Tf2O-mediated cross-coupling of tryptamine with substituted aniline to access C3a-N1'-linked pyrroloindoline alkaloids

The cross-coupling of tryptamine with substituted aniline to access C3a-nitrogen-linked pyrroloindolines has been developed via the consecutive cyclization of tryptamine with DMSO/Tf2O and the substitution of 3a-pyrroloindolylthionium intermediate with aniline. The use of 2,3-dihydrotryptamine instead of aniline enabled easy access to 3a-(1-indolyl)pyrroloindoline and the concise synthesis of C3a-N1'-linked pyrroloindoline alkaloid (±)-psychotriasine was accomplished.

Enantioselective Synthesis of 3a-Amino-Pyrroloindolines by Copper-Catalyzed Direct Asymmetric Dearomative Amination of Tryptamines

A direct asymmetric dearomative amination of tryptamines with O-(2,4-dinitrophenyl)hydroxylamine (DPH) was achieved using CuBr-bisoxazoline complex as a catalyst, affording 3a-amino-pyrroloindolines in good to excellent enantioselectivity under mild reaction conditions. Furthermore, the synthetic value of this method was demonstrated in the total synthesis of (-)-psychotriasine in a highly concise manner.

Concise Total Synthesis of (+)-Luteoalbusins A and B

The first total synthesis of (+)-luteoalbusins A and B is described. Highly regio- and diastereoselective chemical transformations in our syntheses include a Friedel-Crafts C3-indole addition to a cyclotryptophan-derived diketopiperazine, a late-stage diketopiperazine dihydroxylation, and a C11-sulfidation sequence, in addition to congener-specific polysulfane synthesis and cyclization to the corresponding epipolythiodiketopiperazine. We also report the cytoxicity of both alkaloids, and closely related derivatives, against A549, HeLa, HCT116, and MCF7 human cancer cell lines.

[Total synthesis of biologically active alkaloids using bio-inspired indole oxidation]

Many tryptophan-based dimeric diketopiperazine (DKP) alkaloids including WIN 64821 and ditryptophenaline, which exhibit fascinating biological activities, have been isolated from fungi. These alkaloids possess a unique architecture; therefore several total syntheses of these compounds have been accomplished via bio-inspired reactions. Despite these elegant strategies, we were convinced that a more direct bio-inspired solution for the preparation of tryptophan-based DKP alkaloids was possible because in a true biosynthesis, direct dimerization of tryptophan occurs in aqueous media without incorporation of a protecting group on the substrates. Thus we developed direct bio-inspired dimerization reactions in aqueous, acidic media, along with a novel biomimetic pathway, to provide C2-symmetric and non-symmetric dimeric compounds from commercially available amine-free tryptophan derivatives using Mn(OAc)3, VOF3, and V2O5 as one-electron oxidants. In addition, concise two-pot or three-step syntheses of the naturally occurring dimeric DKP alkaloids (+)-WIN 64821, (-)-ditryptophenaline, and (+)-naseseazine B were accomplished with total yields of 20%, 13%, and 20%, respectively. The present synthesis has several noteworthy features: 1) the tryptophan-based C2-symmetric and non-symmetric dimeric key intermediates can be prepared on a multigram scale in one step; 2) the developed oxidation reaction was carried out in aqueous, acidic solution without deactivation of the metal oxidants; 3) protection of the primary amine can be avoided by salt formation in aqueous acid; 4) for the total two-pot operation, the reaction media are environmentally friendly water and ethanol; 5) satisfactory total yields are obtained compared with previously reported syntheses.

An organocatalytic Mannich/denitration reaction for the asymmetric synthesis of 3-ethylacetate-substitued 3-amino-2-oxindoles: formal synthesis of AG-041R

The highly enantioselective organocatalytic addition of ethyl nitroacetate to isatin-derived N-Boc ketimines (Boc = tert-butoxycarbonyl), followed by the removal of the nitro group, is described. The scalable reaction sequence leads to the title compounds as important intermediates of pyrroloindoline alkaloids and related drugs in excellent yields and enantioselectivities. The synthesis of the hexahydrofurano[2,3-b]indole skeleton, the spirocarbamate oxindole unit, and the formal synthesis of AG-041R have been carried out to demonstrate the synthetic utility of this protocol.

3-Pyrrolyl-oxindoles as efficient nucleophiles for organocatalytic asymmetric synthesis of structurally diverse 3,3′-disubstituted oxindole derivatives

An organocatalytic asymmetric reaction of 3-pyrrolyl-oxindoles to nitroalkenes was developed to afford a range of 3-pyrrolyl-3,3′-disubstituted oxindoles in excellent yields.

Synthesis of pyrroloindolines and furoindolines via cascade dearomatization of indole derivatives with carbenium ion

Intermolecular cascade dearomatization of substituted indoles with benzodithiolylium tetrafluoroborate has been developed, providing C3 methyl-substituted pyrroloindolines and furoindolines.

Synthesis of naked amino-pyrroloindoline via direct aminocyclization of tryptamine

Direct access to unprotected amino-pyrroloindolineviaaminocyclization of tryptamine and tryptophan catalyzed by Rh₂(esp)₂has been described usingO-(2,4-dinitrophenyl)hydroxylamine (DPH) as the nitrogen source.

Thionium-based one-pot construction of homo-/heterodimeric pyrroloindoline from tryptamine

We report a one-pot procedure for forming a dimeric pyrroloindoline framework with a thionium reagent. The cyclization of tryptamine with DMSO and Tf(2)O, followed by substitution with indole derivatives, produced racemic 3a-indolylpyrroloindolines. The method enables rapid access to heterodimeric pyrroloindolines as well as to homodimeric pyrroloindolines.

Construction of the N1-C3 linkage stereogenic centers by catalytic asymmetric amination reaction of 3-bromooxindoles with indolines

The catalytic asymmetric amination reaction of 3-bromooxindoles with indolines for the construction of the N1-C3 linkage stereogenic centers has been realized for the first time. Moreover, the racemic substrates (3-substituted indolines) were also applicable under the same chiral conditions. The newly developed method conveniently led to a formal synthesis of (+)-psychotrimine.

Recent developments in the catalytic, asymmetric construction of pyrroloindolines bearing all-carbon quaternary stereocenters

Pyrroloindoline alkaloids constitute a large family of natural products that has inspired the development of an impressive array of new reactions to prepare the key heterocyclic motif. This synopsis will address catalytic, asymmetric reactions developed to synthesize pyrroloindolines bearing C3a all-carbon quaternary stereocenters. The methods described herein include both transition-metal-catalyzed and organocatalyzed reactions that have been demonstrated to be suitable for the synthesis of the pyrroloindoline framework.