An aldol approach to the synthesis of the anti-tubercular agent erogorgiaene

Autoxidation of a C2-Olefinated Dihydroartemisinic Acid Analogue to Form an Aromatic Ring: Application to Serrulatene Biosynthesis

Dihydroartemisinic acid (DHAA) is a plant natural product that undergoes a spontaneous endoperoxide-forming cascade reaction to yield artemisinin in the presence of air. The endoperoxide functional group gives artemisinin its biological activity that kills Plasmodium falciparum, the parasite that causes malaria. To enhance our understanding of the mechanism of this cascade reaction, 2,3-didehydrodihydroartemisinic acid (2,3-didehydro-DHAA), a DHAA derivative with a double bond at the C2-position, was synthesized. When 2,3-didehydro-DHAA was exposed to air over time, instead of forming an endoperoxide, this compound predominantly underwent aromatization. This olefinated DHAA analogue reveals the requirement of a monoalkene functional group to initiate the endoperoxide-forming cascade reaction to yield artemisinin from DHAA. In addition, this aromatization process was exploited to illustrate the autoxidation process of a different plant natural product, dihydroserrulatene, to form the aromatic ring in serrulatene. This spontaneous aromatization process has applications in other natural products such as leubethanol and erogorgiaene. Due to their similarity in structure to antimicrobial natural products, the synthesized compounds in this study were tested for biological activity. A group of the tested compounds had minimum inhibitory concentration (MIC) values ranging from 12.5 to 25 μg/mL against the bacterial pathogen Staphylococcus aureus and the fungal pathogen Cryptococcus neoformans.

Developing a Methodology for Catalytic Asymmetric Crotylation of Aldehydes

Asymmetric crotylation has firmly earned a place among the set of valuable synthetic tools for stereoselective construction of carbon skeletons. For a long time the field was heavily dominated by reagents bearing stoichiometric chiral auxiliaries, but now catalytic methods are gradually taking center stage, and the area continues to develop rapidly. This account focuses primarily on preformed organometallic reagents based on silicon and, to some extent, boron. It narrates our endeavors to design new and efficient chiral Lewis base catalysts for the asymmetric addition of crotyl(trichloro)silanes to aldehydes. It also covers the development of a novel protocol for kinetic resolution of racemic secondary allylboronates to give enantio- and diastereomerically enriched linear homoallylic alcohols. As a separate topic, cross-crotylation of aldehydes by using enantiopure branched homoallylic alcohols as a source of crotyl groups is discussed. Finally, the synthetic credentials of the developed methodology are illustrated by total syntheses of marine natural products, in which crotylation plays a key role in setting up stereogenic centers.

1 Introduction

2 Pyridine N-Oxides as Lewis Base Catalysts

3 Bipyridine N,N′-Dioxides as Lewis Base Catalysts

4 Chiral Allylating Reagents

5 Synthetic Applications

6 Concluding Remarks

Total Synthesis of (+)-Erogorgiaene and the Pseudopterosin A-F Aglycone via Enantioselective Cobalt-Catalyzed Hydrovinylation

Due to their pronounced bioactivity and limited availability from natural resources, metabolites of the soft coral Pseudopterogorgia elisabethae, such as erogorgiaene and the pseudopterosines, represent important target molecules for chemical synthesis. We have now developed a particularly short and efficient route towards these marine diterpenes exploiting an operationally convenient enantioselective cobalt-catalyzed hydrovinylation as the chirogenic step. Other noteworthy C-C bond forming transformations include diastereoselective Lewis acid-mediated cyclizations, a Suzuki coupling and a carbonyl ene reaction. Starting from 4-methyl-styrene the anti-tubercular agent (+)-erogorgiaene (>98 % ee) was prepared in only 7 steps with 46 % overall yield. In addition, the synthesis of the pseudopterosin A aglycone was achieved in 12 steps with 30 % overall yield and, surprisingly, was found to exhibit a similar anti-inflammatory activity (inhibition of LPS-induced NF-κB activation) as a natural mixture of pseudopterosins A-D or iso-pseudopterosin A, prepared by β-D-xylosylation of the synthetic aglycone.

Asymmetric Total Syntheses of Di- and Sesquiterpenoids by Catalytic C-C Activation of Cyclopentanones

To show the synthetic utility of the catalytic C-C activation of less strained substrates, described here are the collective and concise syntheses of the natural products (-)-microthecaline A, (-)-leubehanol, (+)-pseudopteroxazole, (+)-seco-pseudopteroxazole, pseudopterosin A-F and G-J aglycones, and (+)-heritonin. The key step in these syntheses involve a Rh-catalyzed C-C/C-H activation cascade of 3-arylcyclopentanones, which provides a rapid and enantioselective route to access the polysubstituted tetrahydronaphthalene cores presented in these natural products. Other important features include 1) the direct C-H amination of the tetralone substrate in the synthesis of (-)-microthecaline A, 2) the use of phosphoric acid to enhance efficiency and regioselectivity for problematic cyclopentanone substrates in the C-C activation reactions, and 3) the direct conversion of serrulatane into amphilectane diterpenes by an allylic cyclodehydrogenation coupling.

Applications of Friedel-Crafts reactions in total synthesis of natural products

Over the years, Friedel-Crafts (FC) reactions have been acknowledged as the most useful and powerful synthetic tools for the construction of a special kind of carbon-carbon bond involving an aromatic moiety. Its stoichiometric and, more recently, its catalytic procedures have extensively been studied. This reaction in recent years has frequently been used as a key step (steps) in the total synthesis of natural products and targeted complex bioactive molecules. In this review, we try to underscore the applications of intermolecular and intramolecular FC reactions in the total syntheses of natural products and complex molecules, exhibiting diverse biological properties.

Stereoselective Total Syntheses of Acutifolone A, Bisacutifolone A and B, Pinguisenol, and Isonaviculol

Stereoselective total syntheses of pinguisane type of sesquiterpenoids are described following a unified approach using a chiral building block derived from (R)-pulegone. The functionality embodied in the key intermediate enables their facile elaboration into more complex structures of biological relevance such as acutifolone A (9 steps, 22% overall yield) and bisacutifolone A and B (6 and 8%, respectively) following Furukawa's modified Simmons-Smith cyclopropanation, Luche reduction, Saegusa-Ito oxidation, pyridinium chlorochromate-mediated 1,3-oxidative transposition, and Diels-Alder dimerization as key steps.

Applications of oxazolidinones as chiral auxiliaries in the asymmetric alkylation reaction applied to total synthesis

In this review, a number of applications of chiral oxazolidinones in the asymmetric alkylation reaction applied to total synthesis are described.

The daphniphyllum alkaloids: total synthesis of (-)-calyciphylline N

Presented here is a full account on the development of a strategy culminating in the first total synthesis of the architecturally complex daphniphyllum alkaloid, (-)-calyciphylline N. Highlights of the approach include a highly diastereoselective, intramolecular Diels-Alder reaction of a silicon-tethered acrylate; an efficient Stille carbonylation of a sterically encumbered vinyl triflate; a one-pot Nazarov cyclization/proto-desilylation sequence; and the chemoselective hydrogenation of a fully substituted diene ester.

Formation of substituted tetrahydropyrans through oxetane ring opening: application to the synthesis of C1-C17 fragment of salinomycin

The stereoselective synthesis of C1-C17 fragment of salinomycin is achieved. The strategy employs a desymmetrization approach and utilizes an intramolecular oxetane opening reaction with O-nucleophile to result in the tetrahydropyran skeleton as the key step.

Total synthesis of (-)-calyciphylline N

The total synthesis of the architecturally complex Daphniphyllum alkaloid (-)-calyciphylline N has been achieved. Highlights of the synthesis include a Et2AlCl-promoted, highly stereoselective, susbtrate-controlled intramolecular Diels-Alder reaction, a transannular enolate alkylation, an effective Stille carbonylation/Nazarov cyclization sequence, and a high-risk diastereoselective hydrogenation of a fully substituted conjugated diene ester.