Total Syntheses of (−)-Englerins A/B, (+)-Orientalols E/F, and (−)-Oxyphyllol

Concise syntheses of (-)-quinocarcinol methyl ester and (-)-oxa-quinocarcinol methyl ester

A concise synthesis of (-)-quinocarcinol methyl ester was accomplished with an overall yield of 39% through a 9-step longest linear sequence (LLS). Our synthesis features a two-step ester reduction/reductive amination sequence, a stereoselective [3 + 2] intramolecular cross-cycloaddition for the construction of bicyclo[3.2.1]octane skeletons, four simultaneous hydrogenolysis reactions in a one-pot process, and a stereoselective Krapcho decarboxylation. By following this protocol, (-)-oxa-quinocarcinol methyl ester was also achieved.

Formal Syntheses of (-)-Quinocarcinamide and (-)-Quinocarcin

Concise and scalable formal syntheses of (-)-quinocarcinamide and (-)-quinocarcin have been achieved in 9 steps with 9% overall yield from simple commercially available chemicals. The synthetic strategy features an ortho-regioselective Pictet-Spengler cyclization for the construction of the tetrahydroisoquinoline skeleton, a stereoselective formal intramolecular [3 + 2] cross cycloaddition of cyclopropane 1,1-diester with an imine for the construction of the 3,8-diazabicyclo[3.2.1]octane skeleton.

Accessing Complex Tetrahydrofurobenzo-Pyran/Furan Scaffolds via Lewis-Acid Catalyzed Bicyclization of Cyclopropane Carbaldehydes with Quinone Methides/Esters

Herein, we report a straightforward one-pot synthesis of tetrahydrofurobenzopyran and tetrahydrofurobenzofuran systems via an in situ ring-expansion of the cyclopropane carbaldehydes followed by a [2 + n] cycloaddition with the quinone derivatives. The transformation not only unveils a new reaction mode of cyclopropane carbaldehydes with quinone methides/esters, but also promotes a step-efficient diastereoselective route to the sophisticatedly fused oxygen tricycles that can be further dehydrogenated to access the valued dihydro-2H-furo[2,3-b]chromene frameworks.

Asymmetric Total Syntheses of Hypoestin A, Albolic Acid, and Ceroplastol II

The first asymmetric total synthesis of bioactive diterpenoid hypoestin A with an unprecedented [5-8-5-3] tetracyclic skeleton is accomplished in 15 steps from commercially available (R)-limonene. Furthermore, the second asymmetric total syntheses of sesterterpenoids albolic acid and ceroplastol II in 21 steps are also reported. The synthetically challenging and highly functionalized [X-8-5] (X = 5 or 7) tricarbocyclic ring systems found in hypoestin A, albolic acid, ceroplastol II, and schindilactone A, as well as other natural products, are efficiently and directly constructed via a unique intramolecular Pauson-Khand reaction of an allene-yne. This work represents the first reported use of the Pauson-Khand reaction to access synthetically challenging eight-membered-ring systems in natural product synthesis.

A novel type of donor-acceptor cyclopropane with fluorine as the donor: (3 + 2)-cycloadditions with carbonyls

gem-Difluorocyclopropane diester is disclosed as a new type of donor–acceptor cyclopropane, which smoothly participates in (3 + 2)-cycloadditions with various aldehydes and ketones. This work represents the first application of gem-difluorine substituents as an unconventional donor group for activating cyclopropane substrates in catalytic cycloaddition reactions. With this method, a wide variety of densely functionalized gem-difluorotetrahydrofuran skeletons, which are otherwise difficult to prepare, could be readily assembled in high yields under mild reaction conditions. Computational studies show that the cleavage of the C–C bond between the difluorine and diester moieties occurs upon a S_N2-type attack of the carbonyl oxygen.

A new type of donor–acceptor cyclopropane with gem-difluorine as an unconventional donor group undergoes (3 + 2)-cycloadditions with various aldehydes/ketones, affording densely functionalized gem-difluorotetrahydrofurans.

Studies toward the Total Syntheses of Calyciphylline D-Type Daphniphyllum Alkaloids

An efficient construction of an aza-[5.7.6.5] tetracyclic core structure of calyciphylline D-type Daphniphyllum alkaloids has been achieved. The synthetic route features a diastereoselective cyclopropanation, efficient construction of the core bridged 8-aza-[3.2.1]octane skeleton through a [3 + 2] IMCC strategy, oxidative dearomatization of phenol, and gram-scale preparation in each step.

Enantiospecific Total Synthesis of (-)-Japonicol C

An efficient and convergent first total syntheses of (±)-japonicol B and (-)-japonicol C have been completed. The notable points of the synthetic route are Lewis-acid-catalyzed Friedel-Crafts reaction for one pot C-C and C-O bond formations resulting in construction of the tricyclic meroterpenoid skeleton, one pot Pd(OH)₂/C-catalyzed isomerization/hydrogenation, and site selective sp³ C-H oxidation.

cis-Selective, Enantiospecific Addition of Donor-Acceptor Cyclopropanes to Activated Alkenes: An Iodination/Michael-Cyclization Cascade

We present a versatile method for the enantiospecific, cis-diastereoselective intermolecular and intramolecular cycloaddition of donor-acceptor cyclopropanes to electron-poor alkenes with cyclic acceptor groups to afford highly substituted spirocyclopentanes in good to excellent yields. The reaction can be applied to biologically interesting scaffolds, including barbiturates and isoxazolones. Mechanistic investigations were undertaken to explain the unusual diastereoselectivity and enantiospecificity; these suggest an iodination/Michael-cyclization cascade.

Semisynthesis of Plant-Derived Englerin A Enabled by Microbe Engineering of Guaia-6,10(14)-diene as Building Block

Herein, we report a short semisynthesis of the potent transient receptor potential canonical (TRPC) channel agonist englerin A (EA) and the related guaianes oxyphyllol and orientalol E. The guaia-6,10(14)-diene starting material was systematically engineered in Escherichia coli and Saccharomyces cerevisiae using the CRISPR/Cas9 system and was produced with high titers. The potentially scalable approach combines the advantages of synthetic biology and chemical synthesis providing an efficient and economical method for producing EA and analogues.

Allylative Approaches to the Synthesis of Complex Guaianolide Sesquiterpenes from Apiaceae and Asteraceae

With hundreds of unique members isolated to date, guaianolide lactones represent a particularly prolific class of terpene natural products. Given their extensive documented therapeutic properties and fascinating chemical structures, these metabolites have captivated the synthetic chemistry community for many decades. As a result of divergent biosynthetic pathways, which produce a wide array of stereochemical and oxidative permutations, a unifying synthetic pathway to this broad family of natural products is challenging. Herein we document the evolution of a chiral-pool-based synthetic program aimed at accessing an assortment of guaianolides, particularly those from the plant family Apiaceae as well as Asteraceae, members of which possess distinct chemical substructures and necessitate deviating synthetic platforms. An initial route employing the linear monoterpene linalool generated a lower oxidation state guaianolide but was not compatible with the majority of family members. A double-allylation disconnection using a carvone-derived fragment was then developed to access first an Asteraceae-type guaianolide and then various Apiaceae congeners. Finally, using these findings in conjunction with a tandem polyoxygenation cascade, we developed a pathway to highly oxygenated nortrilobolide. A variety of interesting observations in metal-mediated aldehyde allylation and alkene polyoxygenation are reported and discussed.

Diastereoselective synthesis of α-dicarbonyl cyclopropanes via a lanthanide amide-catalysed reaction

Lanthanide bis(trimethylsilyl)amides, [(Me₃Si)₂N]₃Ln(μ-Cl)Li(THF)₃, were used as efficient catalysts for a one-pot reaction of α-ketoesters, dialkyl phosphite, and activated alkenes to produce α-dicarbonyl cyclopropanes in moderate to high yields. The reaction was stereoselective and the two adjacent carbonyls linked to the cyclopropane were in the cis-configuration. The high efficiency of the lanthanide amide in catalysing the reaction is the result of the cooperation between the lanthanide metal centre and the N(SiMe₃)₂ anion.

Synthesis of Oxa-Bridged Medium-Sized Carbocyclic Rings via Prins Cyclization

Herein, we report a new method for the synthesis of oxa-bridged carbocyclic units based on intramolecular Prins reaction of dioxinones. Our new synthetic approach is flexible and practical and has been successfully applied to the preparation of highly functionalized seven-, eight-, and nine-membered carbocycles. The potential utility of this approach has also been demonstrated in a model study toward construction of the 7,8-fused ring system presented in neoabyssomicin D.

Expanding the Reactivity of Donor-Acceptor Cyclopropanes: Synthesis of Benzannulated Five-Membered Heterocycles via Intramolecular Attack of a Pendant Nucleophilic Group

Lewis-acid-induced domino transformations of donor-acceptor cyclopropanes, possessing a nucleophilic center embedded in a donor group, into functionalized 2,3-dihydrobenzo[ b]furans and 2,3-dihydrobenzo[ b]thiophenes are reported herein. An unusual switch of the electrophilic center in the three-membered ring, from the atom bearing a donor substituent to an unsubstituted carbon atom, was achieved by a judicious choice of Lewis acid, which induces the isomerization of a cyclopropane to an electrophilic alkene, and the length of linker, connecting a nucleophilic moiety and the small ring.