Highly stereoselective synthesis of the indolo[2,3-a]quinolizine ring system and application to indole natural product synthesis

Acid-catalyzed three-component addition of carbonyl compounds with 1,2,3-triazoles and indoles

A facile and efficient acid-catalyzed three-component reaction of indoles, 1-tosyl-1,2,3-triazoles and carbonyl compounds has been developed. The use of TsOH with a small amount of water significantly promoted the reaction yield. This method provided a general and one-pot approach for the synthesis of structurally diverse C3-alkylated indole derivatives. The alkylation exclusively occurred at the N² position of triazoles. Various functional groups were tolerated under the optimized simple reaction conditions.

Studies on the Enantioselective Synthesis of E-Ethylidene-bearing Spiro[indolizidine-1,3'-oxindole] Alkaloids

A synthetic route for the enantioselective construction of the tetracyclic spiro[indolizidine-1,3′-oxindole] framework present in a large number of oxindole alkaloids, with a cis H-3/H-15 stereochemistry, a functionalized two-carbon substituent at C-15, and an E-ethylidene substituent at C-20, is reported. The key steps of the synthesis are the generation of the tetracyclic spirooxindole ring system by stereoselective spirocyclization from a tryptophanol-derived oxazolopiperidone lactam, the removal of the hydroxymethyl group, and the stereoselective introduction of the E-ethylidene substituent by acetylation at the α-position of the lactam carbonyl, followed by hydride reduction and elimination. Following this route, the 21-oxo derivative of the enantiomer of the alkaloid 7(S)-geissoschizol oxindole has been prepared.

Dissecting the Gold(I)-Catalyzed Carboaminations of N-Allyl Tetrahydro-β-carbolines to Allenes

N-Allyl tetrahydro-β-carbolines undergo gold-catalyzed cyclizations that lead to tetracyclic compounds, resulting from both ring closure and the transfer of the allylic group from the nitrogen to the carbon backbone. The final skeleton obtained depends on the nature of both the R² group of the allene and the R³ group of the allylic residue. Mechanistic studies and DFT calculations allowed the determination of all the mechanistic pathways involved in these processes, stemming from a common intermediate that evolves differently according to the substituents nature.

A general strategy for the synthesis of indoloquinolizine alkaloids via a cyanide-catalyzed imino-Stetter reaction

A general synthetic strategy applicable to indoloquinolizine alkaloids has been developed using a cyanide-catalyzed imino-Stetter reaction as a key step.

Stereoselective Synthesis of Chiral Polycyclic Indolic Architectures through Pd(0) -Catalyzed Tandem Deprotection/Cyclization of Tetrahydro-β-carbolines on Allenes

Enantioenriched N-allyl tetrahydro-β-carbolines were prepared by chiral phosphoric acid-catalyzed Pictet-Spengler reactions. The compounds undergo Pd(0) -catalyzed cyclizations through a tandem deprotection/cyclization process. The regioselectivity of the attack is controlled by the chain length and by the substitution pattern of the allene function. Products resulting from 5-exo- or 6-exo-attack were obtained with diastereoisomeric ratio up to 95:5. Azepinopyrrido[3,4-b]indoles were obtained by 7-endo-cyclizations.

N-Heterocyclic carbene-catalyzed annulation of cyclic β-enamino esters with enals: access to functionalized indolo[2,3-a]quinolizidines

A novel synthetic approach to functionalized indolo[2,3-a]quinolizidines is developed via an N-heterocyclic carbene-catalyzed annulation of cyclic β-enamino esters with enals.

Enantioselective formal synthesis of ent-rhynchophylline and ent-isorhynchophylline

Starting from (S)-tryptophanol, a formal synthesis of ent-rhynchophylline and ent-isorhynchophylline, involving stereoselective cyclocondensation, spirocyclization, and alkylation reactions, and the final adjustment of the oxidation level at the oxindole and piperidine moieties, is reported.

Asymmetric N-Acyliminium Cyclization as an Approach to Heterocyclic Natural Product Synthesis

This paper describes the development of a highly stereoselective N-acyliminium cyclization protocol for the construction of a range of non-racemic heterocyclic templates. Due to the nature of this review, we have focused primarily on developments made within our own research group, but have included relevant and noteworthy contributions in the same area from others, most notably the research groups of Amat, Bosch and Lete. As a result, we apologize in advance for the omission of much excellent work carried out by many other talented researchers in this field.

Et2Zn-catalyzed intramolecular hydroamination of alkynyl sulfonamides and the related tandem cyclization/addition reaction

Intramolecular hydroamination of alkynyl amides was effected by a catalytic amount of Et2Zn (20 mol %) to form indole derivatives, and a tandem cyclization/nucleophilic addition procedure involving reaction of the indole zinc salt intermediate with acid chlorides or halides was developed to provide an efficient approach to C3-substituted indole derivatives when an excess of Et2Zn (120 mol %) was used.

Chiral Oxazolopiperidone Lactams: Versatile Intermediates for the Enantioselective Synthesis of Piperidine-Containing Natural Products

Phenylglycinol-derived oxazolopiperidone lactams are exceptionally versatile building blocks for the enantioselective construction of structurally diverse piperidine-containing natural products and bioactive compounds. These lactams are readily available in both enantiomeric series by cyclocondensation of the chiral amino alcohol with a delta-oxo acid derivative and allow the substituents to be introduced at the different ring positions in a regio- and stereocontrolled manner, providing access to enantiopure polysubstituted piperidines bearing virtually any type of substitution pattern, and also quinolizidines, indolizidines, perhydroquinolines, hydroisoquinolines, as well as complex indole alkaloids. Of particular interest are cyclocondensation reactions with racemic or prochiral delta-oxo (di)acid derivatives in processes involving dynamic kinetic resolution and/or differentiation of enantiotopic or diastereotopic ester groups, as they directly lead to lactams that already incorporate the carbon substituents on the heterocyclic ring. The use of (S)-3,4-dimethoxyphenylalaninol or (S)-tryptophanol in the above cyclocondensation reactions expands the potential and the scope of the methodology, providing a straightforward route to enantiopure benzo[a]- and indolo[2,3-a]quinolizidines.