The Orthoester Johnson-Claisen Rearrangement in the Synthesis of Bioactive Molecules, Natural Products, and Synthetic Intermediates - Recent Advances: Orthoester Johnson-Claisen Rearrangements

Recent advances in the total synthesis of galantamine, a natural medicine for Alzheimer's disease

Covering: 2006 to 2023(-)-Galantamine is a natural product with distinctive structural features and potent inhibitory activity against acetylcholine esterase (AChE). It is clinically approved for the treatment of Alzheimer's disease. The clinical significance and scarcity of this natural product have prompted extensive and ongoing efforts towards the chemical synthesis of this challenging tetracyclic structure. The objective of this review is to summarize and discuss recent progress in the total synthesis of galantamine from 2006 to 2023. The contents are organized according to the synthetic strategies for the construction of the quaternary center. Key features of each synthesis have been highlighted, followed by a summary and outlook at the end.

Asymmetric total synthesis of humulane sesquiterpenoids alashanoids B, C, E, and F and 2,9-humuladien-6-ol-8-one

Naturally occurring sesquiterpenes having humulane frameworks are structurally intriguing and possess significant biological profiles. Asymmetric synthesis of the alashanoids B, C, E, and F and 2,9-humuladien-6-ol-8-one is achieved for the first time through a linear synthetic strategy. Intramolecular late-stage Nozaki-Hiyama-Kishi (NHK) coupling is employed to access the eleven-membered macrocyclic core present in the target molecules. The NHK precursors are accessed using the Evans and non-Evans syn and anti-aldol reaction as a key transformation. X-ray and ECD analysis reconfirmed the synthesized compounds' structures and chirotopical properties.

Synthetic Approaches to α-, β-, γ-, and δ-lycoranes

Lycorane is a pentacyclic core presented in alkaloids isolated from the Amaryllidaceae family of herbaceous flowering plants. Members of this class of natural products have shown to display important biological properties including analgesic, antiviral, and antiproliferative activities. This review presents the known synthetic routes toward α-, β-, γ-, and δ-lycoranes. α-(19 routes), β-(10 routes), γ-(38 routes), and δ-(6 routes).

Reshuffle Bonds by Ball Milling: A Mechanochemical Protocol for Charge-Accelerated Aza-Claisen Rearrangements

This study presents the development of a mechanochemical protocol for a charge-accelerated aza-Claisen rearrangement. The protocol waives the use of commonly applied transition metals, ligands, or pyrophoric Lewis acids, e.g., AlMe₃. Based on (heterocyclic) tertiary allylamines and acyl chlorides, the desired tertiary amides were prepared in yields ranging from 17% to 84%. Moreover, the same protocol was applied for a Belluš-Claisen-type rearrangement resulting in the synthesis of a 9-membered lactam without further optimization.

Formal Total Synthesis of Salvinorin A

The generation of the quaternary stereocenter at the C9 position of salvinorin A precursors by the Claisen rearrangement was investigated. The required allyl alcohol was prepared from a Wieland-Miescher ketone using a known γ-hydroxylation, reduction of the enone double bond, cyanohydrin formation, and elimination, yielding an unsaturated nitrile. A two-step reduction led to the required allyl alcohol. The subsequent Johnson-Claisen rearrangement provided a mixture of two diastereomeric 1,4-unsaturated esters in a ratio of around 2.6 : 1. The major isomer could be converted to a key intermediate of the Hagiwara synthesis of salvinorin A.

Asymmetric Total Syntheses of (-)-Lycoramine, (-)-Lycoraminone, (-)-Narwedine, and (-)-Galanthamine

A concise asymmetric total synthesis of naturally occurring Amaryllidaceae alkaloids sharing dihydrobenzofuran scaffolds, (-)-galanthamine (1a), (-)-lycoramine (1b), (-)-narwedine (2a), and (-)-lycoraminone (2b), is reported. Orthoester Johnson-Claisen rearrangement of allyl alcohol (+)-9 (98% ee) in diisopropylethylamine furnished enantioenriched cyclohexene (+)-8 (97.4% ee) with a quaternary stereogenic center.

Design, Synthesis, Evaluation and Structure of Allenic 1α,25-Dihydroxyvitamin D3 Analogs with Locked Mobility at C-17

Vitamin D receptor ligands have potential for the treatment of hyperproliferative diseases and disorders related to the immune system. However, hypercalcemic effects limit their therapeutical uses and call for the development of tissue-selective new analogs. We have designed and synthesized the first examples of 1α,25-dihydroxyvitamin D3 analogs bearing an allenic unit attached to the D ring to restrict the side-chain conformational mobility. The triene system was constructed by a Pd0 -mediated cyclization/Suzuki-Miyaura cross-coupling process in the presence of an allenic side chain. The allenic moiety was built through an orthoester-Claisen rearrangement of a propargylic alcohol. The biological activity and structure of (22S)-1α,25-dihydroxy-17,20-dien-24-homo-21-nor-vitamin D3 bound to binding domain of the vitamin D receptor, provide information concerning side-chain conformational requirements for biological activity.

Palladium-Catalyzed Oxidative Coupling of the Allenic C-H Bond with α-Diazo Esters: Synthesis of [3]Dendralenes

A palladium-catalyzed highly regio- and stereoselective allenic C-H oxidative coupling with α-diazo esters is developed. The reaction pathway involves allylic palladium carbene as the key intermediate, which is followed by a carbene migratory insertion process. The reaction proceeds efficiently under mild conditions without external base, providing substituted [3]dendralenes bearing various functional groups.

Asymmetric Total Synthesis of (+)-21-epi-Eburnamonine Via a Photocatalytic Radical Cascade Reaction

An asymmetric total synthesis of (+)-21-epi-eburnamonine has been achieved. Key features of the synthesis include a visible-light photocatalytic intra-/intramolecular radical cascade reaction to assemble the tetracyclic ABCD ring system, and a highly diastereoselective Johnson-Claisen rearrangement to establish the C20 all-carbon quaternary stereocenter.

Catalytic Asymmetric Total Synthesis of Exiguolide

The catalytic asymmetric total synthesis of (-)-exiguolide, a complex 20-membered macrolide embedded with a bis(tetrahydropyran) motif, is reported. The convergent synthesis involves the construction of the C1-C11 tetrahydropyran segment via catalytic asymmetric allylation and Prins cyclization, and the formation of the C12-C21 phosphonate segment via catalytic asymmetric cyclocondensation reaction and Johnson-Claisen rearrangement. The synthesis of 15-epi-exiguolide is also described.

Applications of alkyl orthoesters as valuable substrates in organic transformations, focusing on reaction media

In this review we focus on applications of alkyl orthoesters as valuable and efficient substrates to perform various classes of two-component and multi-component organic reactions. The article has classified them according to two aspects, which are: (i) a focus on the reaction medium (solvent-free conditions, aqueous media, and organic solvents); and (ii) an examination of product structures. Reaction accomplishment under solvent-free conditions is an eco-friendly process with the absence of volatile toxic solvents, which puts it in line with green chemistry goals. Water is an interesting choice in organic transformations due to its inexpensiveness and safety. The authors hope their assessment will help chemists to attain new approaches for utilizing alkyl orthoesters in various organic synthetic methods. The review covers the corresponding literature up to the beginning of 2020.

Recent progress in the synthesis of limonoids and limonoid-like natural products

Recent progress in syntheses of limonoids and limonoid-like natural products is reviewed. The current “state-of-art” advance on novel synthetic strategy are summarized and future outlook will be presented.

Practical and Scalable Organic Reactions with Flow Microwave Apparatus

Microwave irradiation has been used for accelerating organic reactions as a heating method and has been proven to be useful in laboratory scale organic synthesis. The major drawback of microwave chemistry is the difficulty in scaling up, mainly because of the low penetration depth of microwaves. The combination of microwave chemistry and flow chemistry is considered to overcome the problem in scaling up of microwave-assisted organic reactions, and some flow microwave systems have been developed in both academic and industrial communities. In this context, we have demonstrated the scale-up of fundamental organic reactions using a novel flow microwave system developed by the academic-industrial alliance between the University of Shizuoka, Advanced Industrial Science and Technology, and SAIDA FDS. In this Personal Account, we summarize the recent progress of our scalable microwave-assisted continuous synthesis using the SAIDA flow microwave apparatus.

Application of Claisen Rearrangement and Olefin Metathesis in Organic Synthesis

Several new synthetic methods to diverse polycycles and heterocycles on the basis of [3,3]-sigmatropic rearrangement and ring-closing metathesis as key steps are summarized. In this regard, Claisen, aza-Claisen, and Overman rearrangements are used in combination with other popular reactions such as ring-closing metathesis, Suzuki coupling, Diels-Alder reaction, and retro-Diels-Alder reaction. To this end, we prepared polycycles (e.g., cyclophanes, cage molecules), heterocycles (e.g., oxepins, azapins), carbocycles, and spirocycles with the use of the above protocol.

Unusual mechanisms in Claisen rearrangements: an ionic fragmentation leading to a meta-selective rearrangement

A mechanistic investigation of the acid-catalysed redox-neutral oxoarylation reaction of ynamides using electrospray ionisation mass-spectrometry (ESI-MS) and quantum chemical calculations (DFT and MP2) is presented. This study reveals the diversity of pathways and products available from an otherwise deceptively simple-looking, classical transformation: fragmentation, an unusual meta-arylation and competing α-carbonyl cation pathways are some of the alternatives unveiled by ESI-MS and mechanistic experiments. Detailed calculations explain the observed trends and rationalise the results.

Navigating the Chiral Pool in the Total Synthesis of Complex Terpene Natural Products

The pool of abundant chiral terpene building blocks (i.e., "chiral pool terpenes") has long served as a starting point for the chemical synthesis of complex natural products, including many terpenes themselves. As inexpensive and versatile starting materials, such compounds continue to influence modern synthetic chemistry. This review highlights 21st century terpene total syntheses which themselves use small, terpene-derived materials as building blocks. An outlook to the future of research in this area is highlighted as well.

Recent advances in the Overman rearrangement: synthesis of natural products and valuable compounds

This review documents the reports since 2005 on the Overman rearrangement, an important C–N bond forming reaction that has been profoundly used in the synthesis of natural products, synthetic intermediates, building blocks and valuable compounds.

Enantioselective Synthesis and Racemization of (-)-Sinoracutine

Sinoracutine is a recently isolated alkaloid with unusual stereochemical and biological properties. It features an unprecedented tetracyclic 6/6/5/5 skeleton that bears an N-methylpyrrolidine ring fused to a cyclopentenone. Interestingly, both enantiomers of sinoracutine have been independently isolated from the same plant, yet the molecule does not appear to occur as a racemate. Here, we present a short synthesis of (-)-sinoracutine that relies on a highly diastereoselective Pauson-Khand reaction and a Mandai-Claisen reaction to install the quaternary stereocenter. Our work establishes the absolute configuration of the levorotatory isomer and suggests that the optical purity of sinoracutine varies in nature due to its gradual racemization. Experimental evidence supports this proposal, and a plausible mechanism for the racemization is provided.

Synthesis of Angularly Substituted trans-Fused Decalins through a Metallacycle-Mediated Annulative Cross-Coupling Cascade

A convergent coupling reaction is described that enables the stereoselective construction of angularly substituted trans-fused decalins from acyclic precursors. The process builds on our alkoxide-directed titanium-mediated alkyne-alkyne coupling and employs a 1,7-enyne coupling partner. Overall, the reaction is thought to proceed through initial formation of a tetrasusbstituted metallacyclopentadiene, stereoselective intramolecular [4+2] cycloaddition, elimination, isomerization, and regio- and stereoselective protonation. Distinct from our early studies directed at the synthesis of trans-fused hydrindanes, the current annulative coupling reveals an important effect of TMSCl in controlling the final protonation-the event that establishes the stereochemistry of the ring fusion.

Modular synthesis of allyl vinyl ethers for the enantioselective construction of functionalized quaternary stereocenters

A flexible synthesis of allyl vinyl ethers, precursors of functionalized quaternary stereocenters, was developed by employing a regioselective Petasis olefination of haloallyl oxalates and subsequent cross-coupling with alkyl- and aryl boronic acids.