Catalytic Asymmetric Reverse Prenylation of Indol-2-one Enabled a Synthesis of (-)-Debromoflustramine A

A catalytic asymmetric nucleophilic reverse prenylation of indol-2-ones in situ generated from 3-bromooxindoles with prenyltributylstannane promoted by Ni(II)/chiral N,N'-dioxide was developed. This reaction provides facile access to C3 reverse-prenylated oxindoles in good to excellent enantioselectivities, which enabled the asymmetric synthesis of debromoflustramine A in five steps.

Total Synthesis of the Proposed Structure of (-)-Novofumigatamide, Isomers Thereof, and Analogues. Part I

The total synthesis of the suggested structure of (−)-novofumigatamide, a natural product containing a C3-reverse prenylated N-acetyl-exo-hexahydropyrrolo[2,3-b]indole motif fused to a 10-membered ring lactam, was achieved using the macrolactam formation in advance of a diastereoselective bromocyclization and reverse prenylation steps. Since the NMR data of the synthetic sample did not match those of the natural product, the endo-bromo precursor of a N-Boc analogue and additional diastereomers derived from l-Trp were also synthesized. Five alternative synthetic routes, which differed in the order of final key steps used for the construction of the 10-membered ring lactam and the hexahydropyrrolo[2,3-b]indole framework within the polycyclic skeleton and also in the amide bond selected for the ring-closing of the macrolactam, were thoroughly explored. Much to our dismay, the lack of spectroscopic correlations between the proposed structure of natural (−)-novofumigatamide and the synthetic products suggested a different connectivity between the atoms. Additional synthetic efforts to assemble alternative structures of the natural product and isomers thereof (see accompanying paper; DOI: 10.1021/acs.joc.2c01228) further highlighted the frustrating endeavors toward the identification of a natural product.

Puzzling Out the Structure of Novofumigatamide: Total Synthesis of Constitutional Isomers. Part II

The total synthesis of several constitutional isomers showing a different connectivity of the macrolactam ring with the hexahydropyrrolo[2,3-b]indole core, as well as those arising from the positional exchange of the valine and the anthranilate units of the structure originally proposed for (−)-novofumigatamide, has been carried out. The constitutional isomers with 12-membered ring macrolactam connected with the pyrroloindoline framework through the indole nitrogen, and the acetyl group at the pyrrole nitrogen, of endo relative configuration, were prepared through the condensation between the tryptophan and valine edges derived from l- or d-tryptophan and l-valine amino acids. The corresponding exo products are highly unstable structures difficult to isolate and characterize. A second group of isomeric structures synthesized considered the positional exchange between the valine and the anthranilate residues within the macrolactam ring in the originally proposed macrocyclic structure. Comparison of the spectroscopic data allowed us to discard these alternative structures for the natural product.

Nickel-Catalyzed Reductive Vinylation of Chloro-hexahydropyrroloindoline Derivatives with Vinyl Triflates

This work emphasizes facile construction of C-3a vinyl substituted hexahydropyrrolidinoindolines based upon Ni-catalyzed reductive coupling of chloro-hexahydropyrroloindoline derivatives with a wide range of alkyl-decorated vinyl triflates. The remarkable compatibility of sterically hindered branched vinyl groups is highlighted.

Natural Products Originated from the Oxidative Coupling of Tyrosine and Tryptophan: Biosynthesis and Bioinspired Synthesis

The oxidative coupling of tyrosine and tryptophan units is a pivotal step in the total synthesis of some peptide-derived marine and terrestrial natural products, such as the diazonamides, azonazine and tryptorubin A. This Minireview details the biosynthesis and bioinspired synthesis of natural products with such structures. A special focus is put on the challenges of the synthesis of these natural products and the innovative solutions adopted by synthetic chemists.

Electrochemical synthesis of 3a-bromofuranoindolines and 3a-bromopyrroloindolines mediated by MgBr2

We report an efficient and environmentally friendly electrochemical approach to perform the bromo cyclization of tryptophol, tryptamine and tryptophan derivatives.

Synthesis of Javanicunines A and B, 9-Deoxy-PF1233s A and B, and Absolute Configuration Establishment of Javanicunine B

Javanicunines A-B and 9-deoxy-PF1233s A-B belong to a family of natural diketomorpholines with a unique isopropenyl group at C-10b or C-5a and a hydroxyl group at C-11a or C-10b. We herein reported the first total synthesis of javanicunines A-B and 9-deoxy-PF1233s A-B. Pivotal features of the synthesis included a nucleophilic substitution reaction, followed by a Davis' oxaziridine oxidation to assemble javanicunines A-B, and a chemoselective and stereoselective oxidation with Murray's reagent to install the requisite C-10b hydroxyl group in 9-deoxy-PF1233s A-B. The present synthesis also established the absolute configuration of javanicunine B.

Enantioselective Synthesis of Pyrroloindolines via Noncovalent Stabilization of Indole Radical Cations and Applications to the Synthesis of Alkaloid Natural Products

While interest in the synthetic chemistry of radical cations continues to grow, controlling enantioselectivity in the reactions of these intermediates remains a challenge. Based on recent insights into the oxidation of tryptophan in enzymatic systems, we report a photocatalytic method for the generation of indole radical cations as hydrogen-bonded adducts with chiral phosphate anions. These noncovalent open-shell complexes can be intercepted by the stable nitroxyl radical TEMPO· to form alkoxyamine-substituted pyrroloindolines with high levels of enantioselectivity. Further elaboration of these optically enriched adducts can be achieved via a catalytic single-electron oxidation/mesolytic cleavage sequence to furnish transient carbocation intermediates that may be intercepted by a wide range of nucleophiles. Taken together, this two-step sequence provides a simple catalytic method to access a wide range of substituted pyrroloindolines in enantioenriched form via a standard experimental protocol from a common synthetic intermediate. The design, development, mechanistic study, and scope of this process are presented, as are applications of this method to the synthesis of several dimeric pyrroloindoline natural products.

Selenium-Iodine Cooperative Catalyst for Chlorocyclization of Tryptamine Derivatives

Chlorocyclization of tryptamine derivatives has been developed with the use of a diphenyl diselenide-iodine cooperative catalyst. Various tryptamine derivatives can be smoothly converted to the corresponding C3a-chlorohexahydropyrrolo[2,3-b]indoles. Additionally, we demonstrate the formal total syntheses of (-)-psychotriasine and (-)-acetylardeemin by introducing nucleophiles to the C3a position of the products.

Formal total synthesis of the akuammiline alkaloid (+)-strictamine

An asymmetric formal total synthesis of the akuammiline alkaloid (+)-strictamine is reported.

Total synthesis of (-)-depyranoversicolamide B

Starting from easily prepared (R)-C3-isoprenylated pyrroloindoline, the C3-isoprenylated indolyl diketopiperazine is prepared by an efficient reductive opening of the pyrrolo ring, and undergoes biomimetic Diels-Alder reaction to generate an anti-adduct as a sole stereoisomer. Oxidation of the indoline moiety to oxindole completes the synthesis of (-)-depyranoversicolamide B.

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.

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.

Chiral Phosphine Oxide-Sc(OTf)3 Complex Catalyzed Enantioselective Bromoaminocyclization of 2-Benzofuranylmethyl N-Tosylcarbamates. Approach to a Novel Class of Optically Active Spiro Compounds

An efficient enantioselective bromoaminocyclization of 2-benzofuranylmethyl N-tosylcarbamates catalyzed by a chiral phosphine oxide-Sc(OTf)(3) complex is described. A wide variety of optically active spiro benzofuran oxazolidinones can be obtained with high enantioselectivities.

Synthesis and ABCG2 inhibitory evaluation of 5-N-acetylardeemin derivatives

An efficient and versatile synthesis of 5-N-acetylardeemin (1a) and sixteen 2-, 3- and 13-substituted derivatives 1b-q was achieved through Ugi three-component reaction of 3,3a,8,8a-tetrahydropyrrolo[2,3-b]indole and cyclization/epimerization. Their inhibitory activity on the drug efflux of breast cancer resistance protein (ABCG2) was evaluated by flow cytometric analysis of accumulation of Hoechst 33342 stain in Flp-In-293/ABCG2 cells. Most of the derivatives exhibited a stronger ABCG2 inhibitory effect compared with natural product 1a. The derivative 1m with a 4-tolyl substituent at the C-13 position exhibited the most potent ABCG2 inhibition. This preliminary structure-activity relationship study indicates that an electron-rich aryl moiety as the 13-substituent is key to increasing the inhibitory activity.

Reversal of multidrug resistance in vitro and in vivo by 5-N-formylardeemin, a new ardeemin derivative

Because multidrug resistance (MDR) is a serious impediment to the use of chemotherapy in treating cancer patients, great efforts have been made to search for effective MDR-reversing agents. We have developed a brand new synthetic ardeemin derivative, 5-N-formylardeemin, and investigated the activity of which in reversing MDR in MDR cancer cell lines derived from human breast cancer (MCF-7-R) or lung cancer (A549-R). 5-N-formylardeemin strongly enhanced the anti-cancer efficacy of doxorubicin, vincristine through potentiation of apoptosis in both MCF-7-R and A549-R at relatively noncytotoxic concentrations in vitro. Mechanistic studies showed that 5-N-formylardeemin inhibited the expression of MDR-1 (P-gp) and increased the intracellular accumulation of cytotoxic drugs in the MDR cells, suggesting that 5-N-formylardeemin reverses MDR activities through inhibiting MDR-1 expression. Interestingly, 5-N-formylardeemin also sensitized the parent wild-type cancer cells toward these chemotherapeutic agents to various extents. Importantly, in vivo studies demonstrated that 5-N-formylardeemin significantly improved the therapeutic effects of doxorubicin in nude mice bearing A549-R xenografts, which was associated with reduced expression of MDR-1 protein level and increased apoptosis in tumor tissues. These results underscore 5-N-formylardeemin as a potential sensitizer for chemotherapy against multidrug resistant cancers.

Evolution of a unified, stereodivergent approach to the synthesis of communesin F and perophoramidine

Expedient synthetic approaches to the highly functionalized polycyclic alkaloids communesin F and perophoramidine are described using a unified approach featuring a key decarboxylative allylic alkylation to access a crucial and highly congested 3,3-disubstituted oxindole. Described are two distinct, stereoselective alkylations that produce structures in divergent diastereomeric series possessing the critical vicinal all-carbon quaternary centers needed for each synthesis. Synthetic studies toward these challenging core structures have revealed a number of unanticipated modes of reactivity inherent to these complex alkaloid scaffolds. Additionally, several novel and interesting intermediates en route to the target natural products, such as an intriguing propellane hexacyclic oxindole encountered in the communesin F sequence, are disclosed. Indeed, such unanticipated structures may prove to be convenient strategic intermediates in future syntheses.

Recent developments in the chemistry of quinazolinone alkaloids

This review focuses on the recent advances in the chemistry of quinazolinone alkaloids which covers the newly isolated quinazolinone alkaloids with their biological activities and the recently reported total syntheses of quinazolinone alkaloids from 2006 to 2015.

Simple indole alkaloids and those with a nonrearranged monoterpenoid unit

This review summarizes the isolation, structure determination, total syntheses and biological activities of simple indole alkaloids and those with a nonrearranged monoterpenoid unit, with literature coverage from 2012 to 2013.

Ir-catalyzed reverse prenylation of 3-substituted indoles: total synthesis of (+)-aszonalenin and (-)-brevicompanine B

The selective reverse prenylation of 3-substituted-1H-indoles at C3 is described. The iridium-catalyzed reaction proceeds with high branched to linear selectivity (>20:1) for a variety of indoles. In addition, a diastereoselective reverse prenylation of tryptophan methyl ester is disclosed, and its synthetic utility is demonstrated in the synthesis of (+)-aszonalenin and (-)-brevicompanine B.

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.

A diastereodivergent synthetic strategy for the syntheses of communesin F and perophoramidine

An efficient, unified, and stereodivergent approach toward communesin F and perophoramidine was examined. The C(3) all-carbon quaternary center of an oxindole was smoothly constructed by base-promoted indolone-malonate alkylation chemistry. The complementary relative stereochemistry of the crucial vicinal quaternary centers found in communesin F and perophoramidine was selectively installed by substrate-controlled decarboxylative allylic alkylations.

Regioselective hydroarylation reactions of C3 electrophilic N-acetylindoles activated by FeCl3: an entry to 3-(hetero)arylindolines

A method for the direct and rare umpolung of the 3 position of indoles is reported. The activation of N-acetylindole with iron(III) chloride allows the C-H addition of aromatic and heteroaromatic substrates to the C2=C3 double bond of the indole nucleus to generate a quaternary center at C3 and leads regioselectively to 3-arylindolines. Optimization, scope (50 examples), practicability (gram scale, air atmosphere, room temperature), and mechanistic insights of this process are presented. Synthetic transformations of the indoline products into drug-like compounds are also described.