The Claisen rearrangement in the syntheses of bioactive natural products

Brønsted Acid-Catalyzed, Asymmetric Allenoate Claisen Reaction

An auxiliary-based protocol is described for an asymmetric allenoate Claisen rearrangement. Silicated tosic acid (10 mol %) was used as an inexpensive, user-friendly catalyst. Stereochemical analysis revealed a preferential attack at the si face of prostereogenic olefin. The amine auxiliary was readily hydrolyzed and can be isolated from the reaction mixture (85-87% recovery). The resulting unsaturated β-keto esters were isolated in ≤99% yield and 98% ee. Diastereoselective examples provided a 96:4 syn:anti ratio of the resulting vicinal stereocenters.

Domino Conjugate Addition-1,4-Aryl Migration for the Synthesis of α,β-Difunctionalized Amides

A domino difunctionalization of sulfonyl(acryl)imides to form β-substituted α-aryl amides is reported. This transformation involves a 1,4-addition followed by a polar Truce-Smiles rearrangement process, entropically driven by release of SO2. A wide range of carbon- and heteroatom-based nucleophiles and sulfonyl imides were employed, allowing rapid access to highly functionalized amides. In contrast to related reactions with a radical pathway, unbiased substrates could be employed. Despite the usual requirement of an electron-poor migrating moiety for the SNAr event, we herein report unique and unprecedented vinylogous migrations of electron-neutral arenes. Additionally, a one-pot process toward β-amido amides starting from acrylic acids has been developed.

Divergent and Nucleophile-Assisted Rearrangement in the Construction of Pyrrolo[2,1-b][3]benzazepine and Pyrido[2,1-a]isoquinoline Scaffolds

Under microwave (MW) irradiation at 150 °C in toluene and in the presence of nucleophiles (DMAP, triphenylphosphine and tetrahydrothiophene) 1-substituted 1-ethynyl-2-vinyldi- and tetrahydroisoquinolines undergo [3,3]-sigmatropic rearrangement providing pyrrolo[2,1-b][3]benzazepines in good yields. The replacement of toluene with acetonitrile directs the rearrangement towards the formation of 7,11b-dihydro-6H-pyrido[2,1-a]isoquinolines.

Cobalt(II)-Catalyzed Enantioselective Propargyl Claisen Rearrangement: Access to Allenyl-Substituted Quaternary β-Ketoesters

Highly enantioselective propargyl Claisen rearrangement of O-propargyl β-ketoesters was achieved under 2.5 mol % of the chiral cobalt complex as the catalyst under mild reaction conditions. With Co(OTf)2 as the Lewis acid and C1-symmetric imidazoline-pyrroloimidazolone pyridine as the ligand, diverse chiral allenyl-substituted all-carbon quaternary β-ketoesters were obtained in good yields (up to 97% yield) and high enantioselectivities (up to 98% ee).

The aromatic Claisen rearrangement of a 1,2-azaborine

The first aromatic Claisen rearrangement of a 1,2-azaborine is described along with a quantitative kinetic comparison of the reaction of the azaborine with its direct all-carbon analogue. The azaborine A rearranged in a clean, regioselective fashion and reacted faster than the all-carbon substrate B at all temperatures from 140-180 °C. Activation free energies were extracted from observed first-order rate constants (A: ΔG‡298K = 32.7 kcal mol-1; B: ΔG‡298K = 34.8 kcal mol-1) corresponding to a twenty fold faster rate for A at observed reaction temperatures. DFT calculations show that the rearrangement proceeds via a concerted six-membered transition state and that the electronic structure of the BN and CC rings is mostly responsible for the observed regioselectivity and relative reactivity.

Iodine-Catalyzed Claisen-Rearrangements of Allyl Aryl Ethers and Subsequent Iodocyclizations

Iodine can be considered as the simplest halogen-bond donor. Previous investigations have revealed its remarkable catalytic effect in various reactions. The catalytic activity of iodine can often even compete with that of traditional Lewis acids. So far, iodine was typically used to activate carbonyl derivatives like Michael acceptors. We now demonstrate that iodine can also be used to activate allyl aryl ethers in Claisen rearrangements. The formed ortho-allylic phenols rapidly undergo iodocyclizations to afford dihydrobenzofurans, which are important building blocks for medicinal applications. A comparison with different catalysts further highlights the potential of iodine catalysis for this reaction. Computational and mechanistic investigations provide deeper insights into the underlying non-covalent interactions and their role for the catalysis.

[3,3]-Sigmatropic rearrangements of propargyl alkynyl ethers. Synthesis of complex dienoates and unsaturated lactones

In an extension of our studies on low-temperature rearrangements of 1-alkynyl ethers, we describe herein the [3,3]-sigmatropic rearrangement of in situ formed propargyl alkynyl ethers to allenyl ketenes, which furnish complex tert-butyl-(2E,4Z)-dienoates 2 in good yields upon tert-butanol addition. Similarly, sigmatropic rearrangements of in situ formed propargyl lithioalkynyl ethers yield methyl-(2Z,4Z)-dienoates 4 upon methanol addition or unsaturated lactones 6 upon aldehyde or ketone addition to the allenyl ynolate intermediate.

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.

seco-Labdanes: A Study of Terpenoid Structural Diversity Resulting from Biosynthetic C-C Bond Cleavage

The cleavage of a C-C bond is a complexity generating process, which complements oxidation and cyclisation events in the biosynthesis of terpenoids. This process leads to increased structural diversity in a cluster of related secondary metabolites by modification of the parent carbocyclic core. In this review, we highlight the diversifying effect of C-C bond cleavage by examining the literature related to seco-labdanes-a class of diterpenoids arising from such C-C bond cleavage events.

[1,3]-Claisen Rearrangement via Removable Functional Group Mediated Radical Stabilization

A thermal O-to-C [1,3]-rearrangement of α-hydroxy acid derived enol ethers was achieved under mild conditions. The 2-aminothiophenol protection of carboxylic acids facilitates formation of the [1,3] precursor and its thermal rearrangement via stabilization of a radical intermediate. Experimental and theoretical evidence for dissociative radical pair formation, its captodative stability via aminothiophenol, and a unique solvent effect are presented. The aminothiophenol was deprotected from rearrangement products as well as after derivatization to useful synthons.

Photoredox-enabled 1,2-dialkylation of α-substituted acrylates via Ireland-Claisen rearrangement

Herein, we report the 1,2-dialkylation of simple feedstock acrylates for the synthesis of valuable tertiary carboxylic acids by merging Giese-type radical addition with an Ireland-Claisen rearrangement. Key to success is the utilization of the reductive radical-polar crossover concept under photocatalytic reaction conditions to force the [3,3]-sigmatropic rearrangement after alkyl radical addition to allyl acrylates. Using readily available alkyl boronic acids as radical progenitors, this redox-neutral, transition-metal-free protocol allows the mild formation of two C(sp³)-C(sp³) bonds, thus providing rapid access to complex tertiary carboxylic acids in a single step. Moreover, this strategy enables the efficient synthesis of highly attractive α,α-dialkylated γ-amino butyric acids (GABAs) when α-silyl amines are used as radical precursors - a structural motif that was still inaccessible in related transformations. Depending on the nature of the radical precursors and their inherent oxidation potentials, either a photoredox-induced radical chain or a solely photoredox mechanism is proposed to be operative.

Asymmetric synthesis of multifunctional aryl allyl ethers by nucleophilic catalysis

Asymmetric allylic substitution of Morita–Baylis–Hillman (MBH) carbonates with less-nucleophilic phenols mediated by nucleophilic amine catalysis was successfully developed. A variety of substituted aryl allyl ethers were afforded with moderate to high yields with excellent enantioselectivities. The chiral MBH alcohol could be easily accessed from the corresponding aryl allyl ether.

Asymmetric allylic substitution of Morita–Baylis–Hillman (MBH) carbonates with less-nucleophilic phenols mediated by nucleophilic amine catalysis was successfully developed.

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.

Chiral crotyl geminal bis(silane): a useful reagent for asymmetric Sakurai allylation by selective desilylation-enabled chirality transfer

Enantioselective synthesis of SiMe₃/SiPh₂Me-substituted crotyl geminal bis(silane) has been developed. This compound is a useful reagent for Ph₃C⁺B(C₆F₅)₄^--catalyzed asymmetric Sakurai allylation and one-pot Sakurai allylation/Prins cyclization processes. Chemoselective desilylation of SiPh₂Me leads to efficient chirality transfer.

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.

Total synthesis of (+)-herboxidiene/GEX 1A

An efficient synthesis of herboxidiene is granted from highly stereoselective aldol reactions from two lactate-derived ketones and an oxa-Michael cyclization.

Tandem Bond-Forming Reactions of 1-Alkynyl Ethers

Electron-rich alkynes, such as ynamines, ynamides, and ynol ethers, are functional groups that possess significant potential in organic chemistry for the formation of carbon-carbon bonds. While the synthetic utility of ynamides has recently been expanded considerably, 1-alkynyl ethers, which possess many of the reactivity features of ynamides, have traditionally been far less investigated because of concerns about their stability. Like ynamides, ynol ethers are relatively unhindered to approach by functional groups present in the same or different molecules because of their linear geometry, and they can potentially form up to four new bonds in a single transformation. Ynol ethers also possess unique reactivity features that make them complementary to ynamides. Research over the past decade has shown that ynol ethers formed in situ from stable precursors engage in a variety of useful carbon-carbon bond-forming processes. Upon formation at -78 °C, allyl alkynyl ethers undergo a rapid [3,3]-sigmatropic rearrangement to form allyl ketene intermediates, which may be trapped with alcohol or amine nucleophiles to form γ,δ-unsaturated carboxylic acid derivatives. The process is stereospecific, takes place in minutes at cryogenic temperatures, and affords products containing (quaternary) stereogenic carbon atoms. Trapping of the intermediate allyl ketene with carbonyl compounds, epoxides, or oxetanes instead leads to complex α-functionalized β-, γ-, or δ-lactones, respectively. [3,3]-Sigmatropic rearrangement of benzyl alkynyl ethers also takes place at temperatures ranging from -78 to 60 °C to afford substituted 2-indanones via intramolecular carbocyclization of the ketene intermediate. tert-Butyl alkynyl ethers containing pendant di- and trisubstituted alkenes and enol ethers are stable to chromatographic isolation and undergo a retro-ene/[2 + 2] cycloaddition reaction upon mild thermolysis (90 °C) to afford cis-fused cyclobutanones and donor-acceptor cyclobutanones in good to excellent yields and diastereoselectivities. This process, which takes place under neutral conditions and proceeds through an aldoketene intermediate, obviates the need to employ moisture-sensitive and/or unstable acid chlorides under basic conditions for intramolecular [2 + 2] cycloaddition reactions. Furthermore, Lewis acid-catalyzed intramolecular condensations of both ethyl and tert-butyl ynol ethers with tethered acetals efficiently provide protected five-, six-, and seven-membered cyclic Baylis-Hilman adducts. Metalated ethoxyacetylene can also participate in multiple bond-forming reactions that avoid isolation of the alkynyl ether intermediate. Lewis acid-promoted tandem additions employing epoxides/oxetanes and carbonyl compounds give rise to (Z)-α-alkylidene and α-benzylidene lactones stereoselectively in high overall yields. Three new carbon-carbon bonds and a ring are formed in this atom-economical single-flask transformation, resulting in a significant increase in molecular complexity. This Account provides a detailed overview of these useful transformations with the intention of stimulating further interest in and research on ynol ethers and their application in organic synthesis.

Stereoselective synthesis of quaternary carbons via the dianionic Ireland-Claisen rearrangement

A dianionic Ireland-Claisen rearrangement of chiral, nonracemic α-methyl-β-hydroxy allylic esters has been developed that proceeds with high diastereoselectivity and provides products containing three contiguous stereogenic carbons, including a quaternary center. The potential utility of the rearrangement for complex molecule synthesis is also demonstrated.