A novel and highly stereoselective approach to aza-spirocycles. A short total synthesis of 2-epi-(+/-)-perhydrohistrionicotoxin and an unprecedented decarboxylation of 2-pyrones

The 6π-azaelectrocyclization of azatrienes. Synthetic applications in natural products, bioactive heterocycles, and related fields

Covering: 2006 to 2018 The application of the 6π-azaelectrocyclization of azatrienes as a key strategy for the synthesis of natural products, their analogs and related bioactive or biomedically-relevant compounds (from 2006 to date) is comprehensively reviewed. Details about reaction optimization studies, relevant reaction mechanisms and conditions are also discussed.

Highly torquoselective electrocyclizations and competing 1,7-hydrogen shifts of 1-azatrienes with silyl substitution at the allylic carbon

Highly torquoselective electrocyclizations of chiral 1-azatrienes are described. These 1-azatrienes contain an allylic stereocenter that is substituted with a silyl group and are derived in situ from condensation of γ-silyl-substituted enals with vinylogous amides. The ensuing stereoselective ring closures are part of a tandem sequence that constitutes an aza-[3 + 3] annulation method for constructing 1,2-dihydropyridines. Several mechanisms for the formal 1,7-hydrogen shift of these 1-azatrienes were evaluated computationally.

Studies on the formal [3 + 2] cycloaddition of aziridines with alkenes for the synthesis of 1-azaspiroalkanes

The Lewis acid-mediated [3 + 2] cycloaddition of N-sulfonyl- and N-sulfamoylaziridines with alkenes provides a rapid and efficient access to 1-azaspiro[4.n]alkanes. Experimental studies have been combined with DFT calculations to explore the mechanism of the reaction. They demonstrate that the nature of the electron-withdrawing nitrogen protecting group has a very limited influence on the course of the reaction and, particularly, on the initial formation of the 1,3-zwitterionic species through C-N bond cleavage, which has been found to be the rate-determining step. Compared to N-sulfonylaziridines, N-sulfamoylaziridines have proved to be more synthetically useful synthons that afford crystalline polycyclic structures in good yields. A short sequence of catalytic C(sp(3))-H amination-cyclization-[3 + 2] cycloaddition has then been successfully designed to afford the homologue 1-azaspiro[5.n]alkanes, thereby illustrating the higher versatility of sulfamates in these cycloadditions.

Establishing the concept of aza-[3 + 3] annulations using enones as a key expansion of this unified strategy in alkaloid synthesis

A successful enone version of an intramolecular aza-[3 + 3] annulation reaction is described here. Use of piperidinium trifluoroacetate salt as the catalyst and toluene as the solvent appears to be critical for a successful annulation. We also demonstrated for the first time that microwave irradiation can accelerate aza-[3 + 3] annulation reactions. An attempt to expand the scope of the enone aza-[3 + 3] annulation was made in the form of propyleine synthesis as a proof of concept. While synthesis of the enone annulation precursor was successfully accomplished, the annulation proved to be challenging and was only modestly successful.

Asymmetric electrocyclic reactions

This critical review offers an overview of asymmetric electrocyclic processes, where diastereo- or enantioselectivity is a consequence of the influence of a chiral component (be it substrate or catalyst) on the electrocyclic bond-forming process (195 references).

A Stereodivergent Approach for Accessing Both C2,8a-Syn and C2,8a-Anti Relative Stereochemical Manifolds in the Lepadin Family via a TiCL(4)-Promoted Aza-[3 + 3] Annulation

Details in developing a stereodivergent approach to the lepadin family and establishing an entry to both C2,8a-syn and C2,8a-anti relative stereochemical manifolds through a common intermediate are described here. This works paves a foundation for constructing all members of the lepadin family, which consists of three subsets based on an array of interesting relative configurations. These efforts underline the prominence of aza-[3 + 3] annulation as a unified strategy in alkaloid synthesis.

Total synthesis of (+)-lepadin F

An enantioselective total synthesis of (+)-lepadin F is described. The synthetic sequence features an intermolecular aza-[3 + 3] annulation, homologation of a vinylogous amide via Eschenmoser's episulfide contraction, and a highly stereoselective hydrogenation essential for achieving the 1,3-anti relative stereochemistry at C2 and C8a.

Aza- and Carbo-[3 + 3] Annulations of Exo-Cyclic Vinylogous Amides and Urethanes. Synthesis of Tetrahydroindolizidines and An Unexpected Formation of Hexahydroquinolines

[3 + 3] Annulations of exo-cyclic vinylogous amides and urethanes with vinyl iminium salts are described here. We observed an intriguing dichotomy in their reaction pathways. For pyrrolidine- and azepane-based vinylogous amides or urethanes, aza-[3 + 3] annulation would dominate to give tetrahydroindolizidines, whereas, unexpectedly, for piperidine-based vinylogous amides or urethanes, carbo-[3 + 3] annulation was the pathway, leading to hexahydroquinolines. The origin for such a contrast is likely associated with a switch in the initial reaction pathway between C-1,2-addition and C-1,4-addition.

A General Approach to the Quinolizidine Alkaloids via an Intramolecular Aza-[3 + 3] Annulation. Synthesis of (±)-2-Deoxy-Lasubine II

The first success in constructing a member of quinolizidine family of alkaloids employing an intramolecular aza-[3 + 3] annulation strategy is described here. The key feature is the usage of vinylogous urethane tethered to a vinyl iminium intermediate with trifluoroacetate serving as the counter anion. The proof-of-concept is illustrated with the synthesis of 2-deoxy-lasubine II.

An Intramolecular aza-[3 + 3] Annulation Approach to Azaphenalene Alkaloids. Total Synthesis of Myrrhine

A detailed account on the stereoselective total syntheses of azaphenalene alkaloids via an intramolecular aza-[3+3] annulation strategy is described here. All five members of the Coccinellidae family of defensive alkaloids were prepared from the same common intermediate, which was derived from a stereoselective aza-[3+3] annulation reaction.

Thirty-five years of synthetic studies directed towards the histrionicotoxin family of alkaloids

This article brings together for the first time reviews of all the synthetic attempts towards the spirocyclic histrionicotoxin alkaloids published since the discovery of the group in 1971. This covers 5 total syntheses of the fully unsaturated parent alkaloid HTX-283A, 7 total syntheses of perhydrohistrionicotoxin, 15 total syntheses of other members of this alkaloid family, 25 formal syntheses, and 19 partial syntheses involving the successful formation of the core azaspirocyclic structure but lacking advancement towards the target structure.

S-(3-Chloro-2-oxo-propyl)-O-ethyl xanthate: a linchpin radical coupling agent for the synthesis of heterocyclic and polycyclic compounds

The preparation and use of S-(3-chloro-2-oxo-propyl)-O-ethyl xanthate to directly introduce an alpha-chloroketone motif is described, along with applications for the synthesis of heterocyclic and polycyclic structures.

Total syntheses of enantiomerically enriched R-(+)- and S-(–)-deplancheine

Total syntheses of indoloquinolizidine alkaloid (+/-)-, R-(+)-, and S-(-)-deplancheine are described here. The synthesis features an enantioselective intramolecular formal aza-[3 + 3] cycloaddition for the construction of the quinolizidine CD-ring. This application serves to introduce a new synthetic strategy for the synthesis of indoloquinolizidine alkaloids.

[3 + 3] Cycloadditions and related strategies in alkaloid natural product synthesis

In this article we describe a strategically unusual approach to piperidines via formal [3 + 3] cycloaddition reactions. The scope and limitations of these processes are outlined and their employment in the synthesis of alkaloid natural products is delineated.

Development of Highly Stereoselective Asymmetric 6π-Azaelectrocyclization of Conformationally Flexible Linear 1-Azatrienes. From Determination of Multifunctional Chiral Amines, 7-Alkyl cis-1-Amino-2-indanols, to Application as a New Synthetic Strategy: Formal Synthesis of 20-Epiuleine

The highly stereoselective asymmetric 6pi-azaelectrocyclization was achieved as a general synthetic method based on the reaction between the (E)-3-carbonyl-2,4,6-trienal compounds and the (-)-7-alkyl-cis-1-amino-2-indanol derivatives which are effective chiral amines. The 7-alkyl-substituted 2-indanol moiety of the cyclized products was efficiently removed by the novel manganese dioxide oxidation under remarkably mild conditions, and the method was successfully applied to the formal synthesis of optically active 20-epiuleine.

Intramolecular Formal Aza-[3 + 3] Cycloaddition Approach to Indoloquinolizidine Alkaloids. A Stereoselective Total Synthesis of (±)-Tangutorine

[reaction: see text] A 19-step stereoselective total synthesis of (+/-)-tangutorine is described here. The total synthesis features an intramolecular aza-[3 + 3] formal cycloaddition strategy and also a Heck coupling for constructing the C2-C3 bond. This work provides a novel approach toward the indoloquinolizidine family of alkaloids.

Stereoselective formal [3 + 3] cycloaddition approach to cis-1-azadecalins and synthesis of (-)-4a,8a-diepi-pumiliotoxin C. evidence for the first highly stereoselective 6pi-electron electrocyclic ring closures of 1-azatrienes

Evidence is described here to support that a highly stereoselective 6pi-electron electrocyclic ring closure of 1-azatrienes is a key step in formal [3 + 3] cycloaddition or annulation reactions of chiral vinylogous amides with alpha,beta-unsaturated iminium salts. This would represent the first highly stereoselective 6pi-electron electrocyclic ring closure of 1-azatrienes. We have also unambiguously demonstrated that these specific ring closures are reversible, leading to the major diastereomer that is also thermodynamically more stable, and that a rotation preference likely also plays a role. A synthetic application is illustrated here to stereoselectively transform the resulting dihydropyridines to cis-1-azadecalins with unique anti relative stereochemistry at C2 and C2a, leading to synthesis of epi isomers of (-)-pumiliotoxin C.