Synthesis of natural products containing highly strained trans-fused bicyclo[3.3.0]octane: historical overview and future prospects

Palladium-catalyzed selective C-C bond cleavage of keto-vinylidenecyclopropanes: construction of structurally rich dihydrofurans and tetrahydrofurans

Palladium-catalyzed selective cleavage of the distal C-C bond and proximal C-C bond of keto-vinylidenecyclopropanes by altering the sterically bulky phosphine ligands has been realized. The proximal C-C bond cleavage can be achieved by using dtbpf as a phosphine ligand, affording bicyclic products containing dihydrofuran skeletons in good yields along with broad substrate scope. In proximal C-C bond cleavage reactions, the eight-membered cyclic palladium intermediate plays a key role in the reaction. The [3 + 2] cycloaddition of keto-vinylidenecyclopropanes through the distal C-C bond cleavage can be effectively accomplished with t BuXPhos as a phosphine ligand and ZnCl2 as an additive, delivering bicyclic products containing tetrahydrofuran skeletons in good yields. The further transformation of these bicyclic products has been demonstrated, and the reaction mechanisms of two different C-C bond cleavage reactions have been investigated by control experiments and DFT calculations.

Synthesis of the [6-6-7-5-5] Pentacyclic Core of Calyciphylline N

A new approach for the concise 11-step synthesis of the [6-6-7-5-5] BCDEF pentacyclic core of calyciphylline N is described. A type II [5 + 2] cycloaddition was employed to construct the strained BCD skeleton, which encompasses the challenging bicyclo[2.2.2] and bicyclo[4.3.1] ring systems. With a regio- and diastereoselective Lu's [3 + 2] cycloaddition, followed by intramolecular aldol cyclization and elimination, the desired [5-5]-fused EF ring system has been successfully installed, resulting in the complete carbocyclic skeleton of calyciphylline N.

Mechanistic Studies on Rhodium-Catalyzed Chemoselective Cycloaddition of Ene-Vinylidenecyclopropanes: Water-Assisted Proton Transfer

Rhodium-catalyzed cycloaddition reactions are a powerful tool for the construction of polycyclic compounds. Combined experimental and DFT studies were used to investigate the temperature-controlled chemoselectivity of cationic rhodium-catalyzed intramolecular cycloaddition reactions of ene-vinylidenecyclopropanes. After a series of mechanistic studies, it was found that trace amounts of water in the reaction system play an important role in generating the product with endo double bond located on a five-membered ring and revealed that trace amounts of water in the reaction system, including the rhodium catalyst, substrate and solvent, were sufficient to promote the formation of the product with endo double bond located on a five-membered ring, and additional water could not further accelerate the reaction. DFT calculation results show that the addition of water indeed significantly lowers the energy barrier of the proton transfer step, making the formation of the product with endo double bond located on a five-membered ring more likely to occur and confirming the rationality of water-assisted proton transfer occurring in the selective access to the product with endo double bond located on a five-membered ring.

Decarboxylative Intramolecular [3 + 2] Cycloaddition of Cyclic Enol Carbonates: Construction of a Bicyclo[3.3.0]octanone Skeleton

Stereoselective synthesis of bicyclic cyclopentanones was achieved by sequential Tf2O-catalyzed decarboxylation and intramolecular [3 + 2] cycloaddition reactions of cyclic enol carbonates bearing an alkene unit. Four stereogenic centers in the obtained cyclopentanone were stereoselectively constructed. This method could be applied to the synthesis of various fused bicyclic products in moderate-to-good yields.

Molecular insights into the catalytic promiscuity of a bacterial diterpene synthase

Diterpene synthase VenA is responsible for assembling venezuelaene A with a unique 5-5-6-7 tetracyclic skeleton from geranylgeranyl pyrophosphate. VenA also demonstrates substrate promiscuity by accepting geranyl pyrophosphate and farnesyl pyrophosphate as alternative substrates. Herein, we report the crystal structures of VenA in both apo form and holo form in complex with a trinuclear magnesium cluster and pyrophosphate group. Functional and structural investigations on the atypical 115DSFVSD120 motif of VenA, versus the canonical Asp-rich motif of DDXX(X)D/E, reveal that the absent second Asp of canonical motif is functionally replaced by Ser116 and Gln83, together with bioinformatics analysis identifying a hidden subclass of type I microbial terpene synthases. Further structural analysis, multiscale computational simulations, and structure-directed mutagenesis provide significant mechanistic insights into the substrate selectivity and catalytic promiscuity of VenA. Finally, VenA is semi-rationally engineered into a sesterterpene synthase to recognize the larger substrate geranylfarnesyl pyrophosphate.

Four-Step Synthesis of (-)-4-epi-Presilphiperfolan-8α-ol by Intramolecular Iron Hydride Atom Transfer-Mediated Ketone-Alkene Coupling and Studies to Access trans-Hydrindanols with a Botryane Scaffold

From an (R)-(+)-pulegone-derived building block that incorporates the stereo-defined tertiary carbon bearing a methyl group, as found in the targeted sesquiterpenoid, a four-step synthesis of (-)-4-epi-presilphiperfolan-8-α-ol was achieved. The key processes involved are a ring-closing metathesis leading to a bridged alkene-tethered ketone and its subsequent Fe^III -mediated metal-hydride atom transfer (MHAT) transannular cyclization. This synthetic method, implying an irreversible addition of a carbon-centered radical upon a ketone by means of a hydrogen atom transfer upon the alkoxy radical intermediate, was also applied in the synthesis of trans-fused hydrindanols structurally related to botrydial compounds.

Palladium-Catalyzed Cascade Cyclization for the Synthesis of Fused Benzo-Aza-Oxa-[5-6-5] Tetracycles

A novel and expedient cascade strategy has been demonstrated for the synthesis of fused benzo-aza-oxa-[5-6-5] tetracycles in high yields and diastereoselectivities (up to 20 : 1 dr). The strategy was fulfilled through palladium-catalyzed oxidative convergent assembly of functionally divergent anilines and 3-butenoic acid with five chemical bonds constructed. Coupled with control experiments and deuterium labelled studies, DFT calculations were performed for the proposed mechanism. The utility of the illustrated strategy is emphasized by gram-scale syntheses, late-stage functionalization, and the transformation to a key core of natural products such as martinellic acid and seneciobipyrrolidine.

Photoinduced Intramolecular Cyclization of Acylsilanes Bearing a Boronate Moiety: Construction of a Highly Strained trans-Fused Bicyclo[3.3.0]octane Skeleton

A reliable strategy for the construction of trans-fused bicyclo[n.3.0] skeletons was explored by utilizing photoinduced cyclization of acylsilanes bearing a boronate. The substrates having an acylsilane and a boronate in a 1,2-trans relationship were prepared via hydroboration of cycloalkene derivatives. The desired cyclization was efficiently promoted by photoirradiation to give the trans-fused bicyclo[n.3.0] derivatives as a single diastereomer. It is noteworthy that this methodology enables the efficient construction of a highly strained trans-5-5 ring system.

The synthesis of seven- and eight-membered rings by radical strategies

Radical strategies for preparation of seven- or eight-membered rings.