Aza-Quaternary Scaffolds from Selective Bond Cleavage of Bridgehead-Substituted 7-Azabicyclo[2.2.1]heptane: Total Synthesis of (+)-Cylindricines C-E and (−)-Lepadiformine A

Enantioselective total synthesis of (+)-cylindricine B

This article describes the first enantioselective synthesis of the Tasmanian marine alkaloid (+)-cylindricine B. The concise construction of the compound hinged on dearomative retrosynthetic logic combined with a tactical advance in the generation of congested, cyclic, alpha-tertiary amine centers. The scope of this key coupling reaction was explored in addition to providing a synthetic application for Cu-catalyzed enantioselective dearomatization of N-acyl-pyridiniums. The synthesis proceeds in five or six steps from commercially available starting materials.

Synthesis of the Marine Alkaloid Cylindricine C and Serendipitous Synthesis of Its 2,13-Di-epi Stereoisomer

A new approach to the marine alkaloid cylindricine C afforded its previously unreported (±)-2,13-di-epi stereoisomer as the major product along with a minor amount of the racemic parent alkaloid. Key steps included a stereoselective dianion alkylation of a monoester of 1,2-cyclohexanedicarboxylic acid and an annulation based on the tandem conjugate addition of a primary amine to an acetylenic sulfone, followed by intramolecular acylation of the resulting sulfone-stabilized carbanion. The cis-azadecalin moiety thus formed, comprising the cyclohexane A-ring and enaminone B-ring of the products, was further elaborated by the selenenyl chloride-induced cyclofunctionalization of a pendant butenyl substituent with the enaminone moiety, followed by a seleno-Pummerer reaction. Desulfonylation and enaminone reduction afforded the final products. Molecular modeling and X-ray crystallography provided further insight into these processes.

Collective Asymmetric Total Synthesis of Cylindricines

Collective asymmetric total synthesis of marine tricyclic alkaloids, cylindricines A-H, and the proposed structures of cylindricines I and J was achieved in a concise manner from a single common spirocyclic pyrrolidine intermediate. A tandem chemoselective oxidation/intramolecular aza-Michael addition/epimerization was exploited to complete the tricyclic skeleton. This work provides a versatile synthetic entry to the cylindricine family of marine tricyclic alkaloids.

Access to Diverse Seleno-spirocyclohexadienones via Ag(II)-Catalyzed Selenylative ipso-Annulation with Se and Boronic Acids

An efficient and straightforward synthesis of diversified seleno-azaspiro-2,5-cyclohexadienones from N-(4-methoxy aryl)propiolamides using elemental selenium and boronic acids has been demonstrated. The reaction proceeds through silver-catalyzed oxidative dearomatization in the presence of potassium persulfate (K₂S₂O₈) as the oxidant. Further, this approach was extended to N-(4-methoxy aryl)propiolates and biaryl ynones to access the corresponding selenylated oxospiro-2,5-cyclohexadienones and spiro[5,5]trienones, respectively. The present three-component method offers the diverse substitutions on selenium involving two C-Se and one C-C bond formations.

Total Synthesis of (-)-Cylindricine H

Starting from (R)-phenylglycinol-derived tricyclic lactam 1, the enantioselective synthesis of (-)-cylindricine H is reported. From the stereochemical standpoint, the key steps are the stereoselective generation of the quaternary C₁₀ stereocenter, the stereoselective introduction of the C₄ acetoxy and C₂ butyl substituents taking advantage of the lactam carbonyl functionality, and the assembly of the pyrrolidine ring with the required functionalized one-carbon chain at C₁₃ by intramolecular opening of an epoxide.

Enantioselective total syntheses of marine natural products (+)-cylindricines C, D, E and their diastereomers

On the basis of the amide strategy, we have achieved the concise enantioselective total synthesis of (+)-cylindricines D, C, and E, and their 2-epimers in seven, eight, and nine steps, respectively.

Synthesis of Pyrrolidine- and γ-Lactam-Containing Natural Products and Related Compounds from Pyrrole Scaffolds

Polycyclic alkaloid natural products featuring pyrrolidine and pyrrolidinone motifs remain enduring targets of total synthesis endeavors. Pyrrole and its derivatives have been exploited to access many such frameworks, including alkaloids belonging to the Aspidosperma, Stemona, and batzelladine families. In this article, a selection of exemplars that highlight the utility of pyrrole-based approaches to facilitate total syntheses of pyrrolidine- and pyrrolidinone-containing alkaloids and related molecules are showcased.

Expeditious Access to Spiro-Fused 2,5-Cyclohexadienones via Thio(seleno)cyanative ipso-Cyclization

A facile oxidative dearomatization of N-(p-methoxyaryl)propiolamides has been established for the synthesis of spiro-fused 2,5-cyclohexadienone frameworks via thio(seleno)cyanative ipso-cyclization in the presence of ceric ammonium nitrate (CAN) as the oxidant. The present method, involving the formation of C-S and C-C bonds, was also extended to (p-methoxyaryl)propiolates for thiocyanative ipso-cyclization. Furthermore, the obtained chalcogeno-spirocyclohexadienones were transformed into uniquely functionalized spirocyclohexadienone derivatives.

Application of The Dess-Martin Oxidation in Total Synthesis of Natural Products

Dess-Martin periodinane (DMP), a commercially available chemical, is frequently utilized as a mild oxidative agent for the selective oxidation of primary and secondary alcohols to their corresponding aldehydes and ketones, respectively. DMP shows several merits over other common oxidative agents such as chromiumand DMSO-based oxidants; thus, it is habitually employed in the total synthesis of natural products. In this review, we try to underscore the applications of DMP as an effective oxidant in an appropriate step (steps) in the multi-step total synthesis of natural products.

Approach to Fully Substituted Cyclic Nitrones from N-Hydroxylactam Derivatives: Development and Application to the Total Synthesis of Cylindricine C

An approach to cyclic nitrones from N-hydroxylactam derivatives is documented. The nucleophilic addition of an organolithium reagent to an N-OSEM [SEM=2-(trimethylsilyl)ethoxymethyl] lactam forms a five-membered chelated intermediate, which undergoes both elimination and deprotection to give a fully substituted nitrone in a one-pot process. When combined with the N-oxidation of easily available chiral lactams, this method becomes especially useful for the quick synthesis of chiral nitrones in enantio-pure form, enabling the concise total synthesis of cylindricine C.

Formation of Aminocyclopentadienes from Silyldihydropyridines: Ring Contractions Driven by Anion Stabilization

Highly functionalized aminocyclopentadienes can be formed through the rearrangement of anions generated from readily prepared 6-silyl-1,2-dihydropyridines by desilylation with fluoride. The scope and generality of the reaction was defined, and diverse transformations were performed on the highly functionalized products. A mechanism and driving force for the rearrangement were identified from experiments and DFT calculations.

Enantioselective synthesis of (−)-phaseic acid

A short, enantioselective synthesis of a newly identified ABA receptor agonist (−)-phaseic acid is described.

Synthesis of Fasicularin

The synthesis of a tricyclic marine alkaloid, fasicularin, was accomplished. Stereoselective synthesis of the aza-spirocyclic BC-ring precursor and ensuing construction of the A-ring with stereocontrolled installation of the C2 hexyl group feature prominently in the synthesis.

Synthetic approaches towards alkaloids bearing α-tertiary amines

Alkaloids account for some of the most beautiful and biologically active natural products. Although they are usually classified along biosynthetic criteria, they can also be categorized according to certain structural motifs. Amongst these, the α-tertiary amine (ATA), i.e. a tetrasubstituted carbon atom surrounded by three carbons and one nitrogen, is particularly interesting. A limited number of methods have been described to access this functional group and fewer still are commonly used in synthesis. Herein, we review some approaches to asymmetrically access ATAs and provide an overview of alkaloid total syntheses where those have been employed.

Synthesis of Lepadiformine Using a Hydroamination Transform

Dissection of lepadiformine by a double hydroamination transform affords a simple achiral amino diene. This reaction is accomplished in the forward sense by amine-directed hydroboration and an oxidative alkyl shift to nitrogen, both of which occur with high stereoselectivity to generate three stereogenic centers and the lepadiformine skeleton.