Modular access to functionalized 5-8-5 fused ring systems via a photoinduced cycloisomerization reaction

Visible light-driven dearomative ring expansion of (aza)arenes to access dihydrofuran-based polycyclic compounds

The dearomative expansion of aromatic rings has long been pursued by chemists due to its potential to provide tractable approaches for synthesizing valuable non-aromatic molecules. To circumvent the conventional use of hazardous and unstable diazo compounds, photochemical synthesis has recently emerged as a promising platform. However, protocols that can effectively handle both arenes and azaarenes remain scarce. Herein, we introduce a generic strategy that efficiently converts β-(aza)aryl-β-substituted enones into biologically significant cycloheptatriene derivatives, including their aza-variants. This method allows for the easy modulation of diverse functional groups on the product and demonstrates a wide substrate scope, evidenced by its excellent tolerance to various drug motifs and good compatibility with five-membered azaarenes undergoing ring expansion. Moreover, DFT calculations of plausible mechanisms have motivated the implementation of an important cascade diradical recombination strategy for 1,3-dienones, thus facilitating the synthesis of valuable 2-oxabicyclo[3.1.0]hex-3-ene derivatives.

Bioinspired Total Synthesis of Bipolarolides A and B

We have achieved the first total synthesis of bipolarolides A and B, which possess an intriguing and complex 5/6/6/6/5 caged pentacyclic skeleton with seven contiguous stereocenters. The synthesis features a lithium-halogen exchange/intermolecular nucleophilic addition to link two enantioenriched fragments, two ring-closing metathesis reactions to assemble the five- and eight-membered rings, and a bioinspired Prins reaction/ether formation cascade cyclization to construct the 5/6/6/6/5 caged skeleton.

A Stereoselective Photoinduced Cycloisomerization Inspired by Ophiobolin A

A stereoselective synthetic entry point to the 5-8-5 carbocyclic core of the ophiobolins was developed. This strategy exploits the chiral tertiary alcohol of ophiobolin A to guide assmebly of the 5-8-5 scaffold in a single step via a photoinitiated cycloisomerization. Mechanistic insights into the origin of stereocontrol in this reaction are described, as are efforts to elaborate the resultant fused 5-8-5 ring system to the pharmacophore of ophiobolin A.

Unified Total Synthesis of Tetracyclic Diquinane Lycopodium Alkaloids (+)-Paniculatine, (-)-Magellanine, and (+)-Magellaninone

A unified route for the total synthesis of three tetracyclic diquinane Lycopodium alkaloids (+)-paniculatine, (-)-magellanine, and (+)-magellaninone has been accomplished in 13-14 overall steps based on late-stage diverse transformations from an advanced tetracyclic common intermediate. In the established synthesis, quick formation of the two five-membered rings was efficiently achieved by an intramolecular reductive coupling of ketone-carbonyl and ester-carbonyl and an organocatalytic intramolecular Michael addition of aldehyde-derived enamine to an internal enone functionality with satisfactory redox and step economies and excellent stereoselectivities, providing the requisite tricyclic carbo-framework possessing multiple dense stereogenic centers, and an intramolecular reductive amination finally furnished the essential piperidine ring.

Site-Specific Alkene Hydromethylation via Protonolysis of Titanacyclobutanes

Methyl groups are ubiquitous in biologically active molecules. Thus, new tactics to introduce this alkyl fragment into polyfunctional structures are of significant interest. With this goal in mind, a direct method for the Markovnikov hydromethylation of alkenes is reported. This method exploits the degenerate metathesis reaction between the titanium methylidene unveiled from Cp2 Ti(μ-Cl)(μ-CH2 )AlMe2 (Tebbe's reagent) and unactivated alkenes. Protonolysis of the resulting titanacyclobutanes in situ effects hydromethylation in a chemo-, regio-, and site-selective manner. The broad utility of this method is demonstrated across a series of mono- and di-substituted alkenes containing pendant alcohols, ethers, amides, carbamates, and basic amines.

Total Synthesis of Fawcettimine-Type Alkaloid, Lycojaponicumin A

The efficient total synthesis of lycojaponicumin A (1) has been accomplished for the first time. The remarkable features of this novel strategy include the following: (1) rapid construction of tricyclic intermediate 4 through a regio- and stereoselective semipinacol ring expansion, which simplified the construction of rings A and B of 1; (2) the subsequent regio- and stereoselective formation of the highly strained rings C-E of 1 through a tandem oxa-hetero [3 + 2] cycloaddition/N-cycloalkylation.

Total Synthesis of (+)-6-epi-Ophiobolin A

The ophiobolin sesterterpenes are notable plant pathogens which have recently elicited significant chemical and biological attention because of their intriguing carbogenic frameworks, reactive functionalities, and emerging anticancer profiles. Reported herein is a total synthesis of (+)-6-epi-ophiobolin A in 14 steps, a task which addresses construction of the synthetically challenging spirocyclic tetrahydrofuran motif as well as several other key stereochemical problems. This work demonstrates a streamlined synthetic platform to complex ophiobolins leveraging disparate termination modes of a radical polycyclization cascade for divergent elaboration and functionalization.