En Route to D-Ring Inverted Phorbol Esters

Unearthing the Subtleties of Rhodium(II)-Catalyzed Carbenoid Cycloadditions to Furans with an N-Sulfonyl-1,2,3-triazole Probe

The rhodium(II)-catalyzed reaction of a model alkenyl donor/acceptor N-sulfonyltriazole with a wide selection of furans is reported. This investigation unearthed a range of structurally diverse carbocyclic and ring-opened products, in good to excellent yields. The products obtained are proposed to arise selectively via cyclopropanation or zwitterionic rearrangement pathways, which are highly dependent on both the structural and electronic features of the furan substrate.

Highly efficient construction of an oxa-[3.2.1]octane-embedded 5-7-6 tricyclic carbon skeleton and ring-opening of the bridged ring via C-O bond cleavage

We report herein a highly efficient strategy for construction of a bridged oxa-[3.2.1]octane-embedded 5–7–6 tricyclic carbon skeleton through [3 + 2] IMCC (intramolecular [3 + 2] cross-cycloaddition), and the substituents and/or stereochemistries on C-4, C-6, C-7 and C-10 fully match those in the rhamnofolane, tigliane and daphnane diterpenoids. Furthermore, ring-opening of the bridged oxa-[3.2.1]octane via C–O bond cleavage was also successfully achieved.

We reported a highly efficient construction of an oxa-[3.2.1]octane-embedded 5–7–6 tricyclic carbon skeleton with a full match of the substituents and stereochemistries on C-4/-6/-7/-10 with those in the rhamnofolane/tigliane/daphnane diterpenoids.

Dearomative [4 + 3] cycloaddition of furans with vinyl-N-triftosylhydrazones by silver catalysis: stereoselective access to oxa-bridged seven-membered bicycles

A practical dearomative [4 + 3] cycloaddition of furans with vinylcarbenes to access oxa-bridged seven-membered carbocycles, with complete and predictable stereoselectivity, is achieved by merging silver catalysis and vinyl-N-triftosylhydrazones.

Total Synthesis of Dysifragilones A and B and Dysidavarone C

The concise synthesis of dysifragilones A and B and dysidavarones has been accomplished for the first time in a divergent way from a common intermediate. The synthetic route features an intramolecular reductive Heck reaction to construct the 6/5/6/6/-tetracycle of dysifragilones A and B and an intramolecular palladium-catalyzed α-arylation of a sterically hindered ketone to forge the tetracyclo[7.7.1.0^2,7.0^10,15]heptadecane core structure of dysidavarone C. The late-stage introduction of amino and ethoxy groups is effective.

Unified Total Syntheses of Rhamnofolane, Tigliane, and Daphnane Diterpenoids

Rhamnofolane, tigliane, and daphnane diterpenoids are structurally complex natural products with multiple oxygen functionalities, making them synthetically challenging. While these diterpenoids share a 5/7/6-trans-fused ring system (ABC-ring), the three-carbon substitutions at the C13- and C14-positions on the C-ring and appending oxygen functional groups differ among them, accounting for the disparate biological activities of these natural products. Here, we developed a new, unified strategy for expeditious total syntheses of five representative members of these three families, crotophorbolone (1), langduin A (2), prostratin (3), resiniferatoxin (4), and tinyatoxin (5). Retrosynthetically, 1-5 were simplified into their common ABC-ring 6 by detaching the three-carbon units and the oxygen-appended groups. Intermediate 6 with six stereocenters was assembled from four achiral fragments in 12 steps by integrating three powerful transformations, as follows: (i) asymmetric Diels-Alder reaction to induce formation of the C-ring; (ii) π-allyl Stille coupling reaction to set the trisubstituted E-olefin of the B-ring; and (iii) Eu(fod)₃-promoted 7-endo cyclization of the B-ring via the generation of a bridgehead radical. Then 6 was diversified into 1-5 by selective installation of the different functional groups. Attachment of the C14-β-isopropenyl and isopropyl groups led to 1 and 2, respectively, while oxidative acetoxylation and C13,14-β-dimethylcyclopropane formation gave rise to 3. Finally, formation of an α-oriented caged orthoester by C13-stereochemical inversion and esterification with two different homovanillic acids delivered 4 and 5 with a C13-β-isopropenyl group. This unified synthetic route to 1-5 required only 16-20 total steps, demonstrating the exceptional efficiency of the present strategy.

Balancing skeleton and functional groups in total syntheses of complex natural products: a case study of tigliane, daphnane and ingenane diterpenoids

Total synthesis of natural products has greatly contributed to natural product research, organic synthesis and drug discovery and development. However, in most cases, the efficiency of total synthesis is far from sufficient for direct practical industrial application. Thus, designing a concise and efficient synthetic route with balanced efforts between building the complex skeleton and introducing functional groups is highly desirable. In this critical review, we first present an introduction of this issue and a philosophical framework that cover possible synthetic approaches. Next, we have chosen the biogenetically closely related, biologically important and synthetically extremely challenging natural products, tiglianes, daphnanes and ingenanes as the particular case for the discussion, since in the past 40 years many synthetic approaches have been reported. The successes and pitfalls included therefore serve as the basis to draw some conclusions that may inspire future development in this area.

Total synthesis of crotophorbolone

As a natural diterpenoid, crotophorbolone possesses a challenging trans,trans-5/7/6 framework decorated with six contiguous stereogenic centers and is structurally and biogenetically related to tigliane-type diterpenoids with intriguing bioactivities such as phorbol and prostratin. Based on the convergent strategy, we completed an eighteen-step total synthesis of crotophorbolone starting from (−)-carvone and (+)-dimethyl-2,3-O-isopropylidene-l-tartrate. The key elements of the synthesis involve expedient installation of the six-membered ring and the five-membered ring with multiple functional groups at an early stage, cyclization of the seven-membered ring through alkenylation of the ketone between the five-membered ring and the six-membered ring, functional group-sensitive ring-closing metathesis and final selective introduction of hydroxyls at C₂₀ and C₄.

Convergent total synthesis of crotophorbolone was accomplished in 18 longest linear steps. Observation of unexpected thermodynamic stability of a cis,trans-5/7/6 tricycle would benefit synthetic design of tigliane- and daphnane-related diterpenoids.

Facile synthesis of the daphnane and tigliane framework by semi-flow tube-based-bubbling photooxidation and diastereoselective conjugate addition

The common [5-7-6] tricyclic framework in tigliane and daphnane natural diterpenes containing five contiguous stereocenters has been synthesized in 9 steps from readily available starting materials in a completely stereocontrolled manner.