Total Synthesis of the Monoterpenoid Indole Alkaloid (±)-Aspidophylline A

Total Synthesis of (+)-Alsmaphorazine C and Formal Synthesis of (+)-Strictamine: A Photo-Fries Approach

A bioinspired photo-Fries/imine capture cascade reaction was developed in continuous-flow mode, which facilitated the rapid construction of a series of diversely functionalized 2,7-heterocycle-fused tetrahydrocarbazoles, the ubiquitous core structures embedded in strychnos and akuammiline-type monoterpene indole alkaloids. The synthetic utility of this novel method has been preliminarily explored by the first total synthesis of (+)-alsmaphorazine C and formal synthesis of (+)-strictamine in a concise and efficient manner.

Pd/Xiang-Phos-catalyzed enantioselective intermolecular carboheterofunctionalization under mild conditions

A mild and practical Pd/Xiang-Phos-catalyzed enantioselective intermolecular carboheterofunctionalization reaction of 2,3-dihydrofurans is developed, leading to various optically active fused furoindolines and tetrahydrofurobenzofurans. The key to this transformation is employing two newly modified N-Me-Xiang-Phos ligands ((S, R S)- N-Me-X4/X5) as chiral ligands under mild conditions. Moreover, this synthetic methodology can be efficiently applied to a variety of complex polysubstituted heterocycles with high chemo-, regio-, and enantio-selectivities via introducing diverse substituents on furan rings, which were hard to access by other routes.

Towards the Sarpagine-Ajmaline-Macroline Family of Indole Alkaloids: Enantioselective Synthesis of an N-Demethyl Alstolactone Diastereomer

the strategy involving the use of functionalized tetrahydro-6H-cycloocta[b]indol-6-one is reported as a key intermediate for synthesis of members of the sarpagine-ajmaline-macroline family of monoterpene indole alkaloids. The desired tricycle was synthesized through the following key steps: 1) Evans' syn-selective aldolization; 2) Liebeskind-Srogl cross-coupling using the phenylthiol ester of 3-chloropropanoic acid as a surrogate of acrylic thioester for the synthesis of 2,3-disubstituted indoles; and 3) ring-closing metathesis (RCM) for the formation of the eight-membered ring. An N-allylation followed by intramolecular 1,4-addition was planned for synthesis of the vobasine class of natural products. However, attempted cyclizations under a diverse set of conditions involving anionic, radical, and organopalladium/organonickel species failed to produce the bridged ring system. On the other hand, esterification of the pendant primary alcohol function with acetoacetic acid, followed by intramolecular Michael addition, afforded the desired tetracycle with excellent diastereoselectivity. Subsequent functional group manipulation and transannular cyclization of the amino alcohol afforded the N(1)-demethyl-3,5-diepi-alstolactone. We believe that the same synthetic route would afford the alstolactone should the amino alcohol with appropriate stereochemistry be used as the starting material.

Mechanism and Selectivity of the Oxidative Ring Contraction of Cyclic α-Formyl Ketones

The oxidative contraction of α-formal ketone to form continuous all carbon chiral centers promoted by H₂ O₂ is widely used in natural product total synthesis. Typically, using this transformation, chiral cyclic ketones are obtained as the major products and ring-opening products as the minor products. Herein, DFT calculations have been used to investigate the detailed reaction mechanism and chemoselectivity. In addition, with the widely accepted mechanism of H₂ O₂ -promoted transformation, our systematic investigation with various explicit-solvent-model calculations for the first time shows that H₂ O and H₂ O₂ are comparable at catalyzing the rate-determining step of this reaction, which emphasis the importance of solvent effect in such transformations. It is found that both the less ring-constrain and a later transition state in an exothermic reaction account for the origin why the reaction favors ring-contraction pathway rather than ring-opening one. By a comprehensive analysis for the substituted groups, it has been disclosed that the steric effects of the substituted groups on R² and R³ contribute to the selectivity with larger steric hindrance favoring the chiral cyclic products. Moreover, the electronic effects on R¹ but not R³ affect the selectivity with electron-donating groups leading to the cyclic products. Based on our calculations, some predictions for higher selectivity have been made.

Total Syntheses of Echitamine, Akuammiline, Rhazicine, and Pseudoakuammigine

Echitamine (1) and akuammiline (2) are representative members of a fascinating class of monoterpenoid indole alkaloids. We report the syntheses of 2 and its congener deacetylakuammiline (3). The azabicyclo[3.3.1]nonane motif was assembled through silver-catalyzed internal alkyne cyclization, and one-pot C-O bond cleavage/C-N bond formation furnished the pentacyclic scaffold. Compound 3 then served as a common intermediate for preparing a series of structurally diverse and synthetically challenging congeners including 1. A position-selective Polonovski-Potier reaction followed by formal N-4 migration built the core of N-demethylechitamine (4) and 1. An alternative route featuring Meisenheimer rearrangement gave 4 as well. Oxidation of the alcohol within 3 gave rhazimal (5), which underwent tandem indolenine hydrolysis, hemiaminalization, and hemiketalization to form rhazicine (6). A sequence of N,O-ketalization and reductive amination secured the chemoselectivity of N-methylation, leading to pseudoakuammigine (7).

Bioinspired Oxidative Cyclization of the Geissoschizine Skeleton for the Total Synthesis of (-)-17-nor-Excelsinidine

We report the first total synthesis of (-)-17-nor-excelsinidine, a zwitterionic monoterpene indole alkaloid that displays an unusual N4-C16 connection. Inspired by the postulated biosynthesis, we explored an oxidative coupling approach from the geissoschizine framework to forge the key ammonium-acetate connection. Two strategies allowed us to achieve this goal, namely an intramolecular nucleophilic substitution on a 16-chlorolactam with the N4 nitrogen atom or a direct I₂ -mediated N4-C16 oxidative coupling from the enolate of geissoschizine.

Nickel-Catalyzed Intramolecular C-O Bond Formation: Synthesis of Cyclic Enol Ethers

An efficient and exceptionally mild intramolecular nickel-catalyzed carbon-oxygen bond-forming reaction between vinyl halides and primary, secondary, and tertiary alcohols has been achieved. Zinc powder was found to be an essential additive for obtaining high catalyst turnover and yields. This operationally simple method allows direct access to cyclic vinyl ethers in high yields in a single step.

Total Synthesis of Calophyline A

Reported herein is the total synthesis of calophyline A, an indoline natural product possessing distinct ring connectivity which has not been synthesized previously. The synthetic route features several key transformations, including an aza-pinacol rearrangement to construct the nitrogen-containing bridged [3.2.2] bicycle, a Heck cyclization to assemble the fused 6/5/6/5 ring system, and a challenging late-stage aldol reaction to generate both a neopentyl quaternary stereogenic center and an oxygen-containing bridged [3.2.1] bicycle.

Total Synthesis of (±)-Strictamine

The total synthesis of strictamine has been achieved in nine steps from a known enol triflate. Characteristic features of our approach included: a) creation of a C7 all-carbon quaternary stereocenter at an early synthetic stage; b) use of an N,N-dimethyl tertiary amine as a surrogate of the primary amine for the rapid build-up of a functionalized 2-azabicyclo[3,3,1]nonan-9-one skeleton (achieved by using a reaction sequence of α-bromination of the ketone, followed by a stereoconvergent intramolecular nucleophilic substitution reaction); and c) a late-stage construction of the indolenine unit.

Cascade reactions: a driving force in akuammiline alkaloid total synthesis

The akuammiline alkaloids are a family of intricate natural products which have received considerable attention from scientists worldwide. Despite the fact that many members of this alkaloid class were discovered over 50 years ago, synthetic chemistry has been unable to address their architectures until recently. This minireview provides a brief overview of the rich history of the akuammiline alkaloids, including their isolation, structural features, biological activity, and proposed biosyntheses. Furthermore, several recently completed total syntheses are discussed in detail. These examples not only serve to highlight modern achievements in alkaloid total synthesis, but also demonstrate how the molecular scaffolds of the akuammilines have provided inspiration for the discovery and implementation of innovative cascade reactions for the rapid assembly of complex structures.

Total synthesis of dixiamycin B by electrochemical oxidation

N–N-linked dimeric indole alkaloids represent an unexplored class of natural products for which chemical synthesis has no practical solution. To meet this challenge, an electrochemical oxidative dimerization method was developed, which was applied as the pivotal step of the first total synthesis of dixiamycin B. This method is also general for N–N dimerization of substituted carbazoles and β-carbolines, providing entry into seldom explored chemical space.