Enantioselective synthesis of the ABC ring motif of norzoanthamine based on asymmetric Robinson annulation reactions

Organocatalyst Design for the Stereoselective Annulation towards Bicyclic Diketones and Analogues

The Wieland–Miescher ketone, Hajos–Parrish–Eder–Sauer–Wiechert ketone, and their analogues are bicyclic diketones essential as building blocks for the synthesis of several natural and bioactive molecules. For this reason, since 1971, when Hajos and Parrish and Eder, Sauer, and Wiechert reported the stereoselective synthesis of these compounds promoted by L-proline, numerous methodologies and organocatalysts have been studied over the years with the aim of identifying increasingly efficient asymmetrical syntheses of these bicyclic ketones. This review will outline the methodological and stereochemical features of the organocatalytic stereoselective synthesis of these bicyclic scaffolds based on the different organocatalysts employed from 1971 until today. Particular emphasis will be given to the structural features of the catalysts and to the reaction conditions.

Asymmetric Total Synthesis of Norzoanthamine

We report herein the asymmetric total synthesis of norzoanthamine using radical reactions as key steps for rapid access to the congested carbocyclic core, which is the major synthetic challenge for most zoanthamine alkaloids. (1) The Ueno-Stork radical cyclization was applied to construct the adjacent quaternary centers at the C-9 and C-22 positions; (2) a Co-catalyzed HAT radical reaction was successfully applied to construct the quaternary center at C-12 via Csp³ -Csp² bond formation; (3) a Mn-catalyzed HAT radical reaction was used to stereospecifically reduce the tetra-substituted olefin (C13=C18) and install the contiguous stereocenters in proximity to the quaternary center. A one-pot bio-inspired cyclization step was finally applied to forge the unstable bis-amino acetal skeleton. Our approach can precisely control the stereochemistry of seven vicinal stereocenters and effectively construct the highly congested heptacyclic skeleton.

Enantioselective Access to Robinson Annulation Products and Michael Adducts as Precursors

The Robinson annulation is a reaction that has been useful for numerous syntheses since its discovery in 1935, especially in the field of steroid synthesis. The products are usually obtained after three consecutive steps: the formation of an enolate (or derivative), a conjugate addition, and an aldol reaction. Over the years, several methodological improvements have been made for each individual step or alternative routes have been devised to access the Robinson annulation products. The first part of this Review outlines the most relevant developments towards the formation of monocarbonyl-derived Robinson annulation products (MRA products, MRAPs) and activated monocarbonyl-derived Robinson annulation products (AMRA products, AMRAPs). The following sections are then devoted to the diastereoselective and enantioselective synthesis of these products, while the last section describes the enantiomeric resolution of racemic mixtures.

Palladium-catalyzed sequential C-N/C-O bond formations: synthesis of oxazole derivatives from amides and ketones

A highly efficient method for the synthesis of oxazole derivatives from simple amides and ketones has been established via a Pd(II)-catalyzed sp(2) C-H activation pathway in one step. The reaction is supposed to proceed through a C-N bond formation followed by a C-O bond formation closing the ring. Because of the simple and readily available starting materials, easy operation, and high bioactivity of oxazoles, this strategy can be broadly applied to medical chemistry.

Enantioselective synthesis of decalin structures with all-carbon quaternary centers via one-pot sequential Cope/Rauhut-Currier reaction

The first example of one-pot sequential Cope/Rauhut-Currier reactions are reported and used to make functionalized decalin structures with all-carbon quaternary stereocenters. The substrates for the new sequential reaction are generated through a six-step sequence including an enantioselective Birch reduction-allylation reaction which makes the overall process asymmetric.

Synthesis of the tetracyclic core of Illicium sesquiterpenes using an organocatalyzed asymmetric Robinson annulation

An enantioselective synthesis of the core framework of neurotrophic Illicium majucin-type sesquiterpenes is described here. This strategy is based on an organocatalyzed asymmetric Robinson annulation and provides an efficient approach for a diversity-oriented synthesis of Illicium natural products that holds remarkable therapeutic potential for neurodegenerative diseases.

Illicium sesquiterpenes: divergent synthetic strategy and neurotrophic activity studies

Majucin-type sesquiterpenes from Illicium sp., such as jiadifenolide (2), jiadifenin (3), and (1R,10S)-2-oxo-3,4-dehydroxyneomajucin (4, ODNM), possess a complex caged chemical architecture and remarkable neurotrophic activities. As such, they represent attractive small-molecule leads against various neurodegenerative diseases. We present an efficient, enantioselective, and unified synthesis of 2, 3, and 4 and designed analogues that diverge from tetracyclic key intermediate 7. The synthesis of 7 is highlighted by the use of an enantioselective Robinson annulation reaction (construction of the AB rings), a Pd-mediated carbomethoxylation reaction (construction of the C ring), and a one-pot oxidative reaction cascade (construction of the D ring). Evaluation of the neurotrophic activity of these compounds led to the identification of several highly potent small molecules that significantly enhanced the activity of nerve growth factor (NGF) in PC-12 cells. Moreover, efforts to define the common pharmacophoric motif suggest that substitution at the C-10 center significantly affects bioactivity, while the hemiketal moiety of 2 and 3 and the C-1 substitution might not be critical to the neurotrophic activity.

Toward the synthesis of norzoanthamine: building carbocyclic core by a transannular Michael reaction cascade

A 12-step synthesis of the ABC carbocyclic core of norzoanthamine is described. It features an organocatalytic asymmetric intramolecular aldolization to set the stereochemistry of the entire molecule, a fragment coupling by selective alkylation of a bis-enolate, and a transannular Michael reaction cascade for rapid and stereoselective synthesis of the polycyclic core.

Intramolecular Cyclization Strategies Toward the Synthesis of Zoanthamine Alkaloids

Stabilized 2-amino-1,3-dienes can participate in intramolecular Diels-Alder (IMDA) reactions with pendant dienophiles. We found that these dienes can be readily prepared via standard palladium-mediated coupling reactions and have comparable reactivity to 2-oxodienes. Application of these substrates to the synthesis of tetracyclic model systems representing the ABCE motif of the zoanthamines is presented.

Enantioselective synthesis of (-)-jiadifenin, a potent neurotrophic modulator

The first enantioselective synthesis of (-)-jiadifenin (1), a potent neurite outgrowth promoter isolated from the Illicium species, is described. The synthetic strategy builds upon bicyclic motif 6, which represents the AB ring of the natural product and proceeds in 19 steps and 1.1% overall yield. Key to our approach is a Mn(III)-mediated oxidation reaction of A ring that, following a regio- and diastereoselective α-hydroxylation and methylation sequence, produces the desired functionalities of (-)-jiadifenin. The effect of synthetic 1 in NGF-mediated neurite outgrowth was also measured in PC-12 cells.