Recent advances towards catalytic asymmetric Conia-ene-type reactions

Synthesis of cis-fused cyclopentenone-pyrrolidine scaffolds via sequential aza-Piancatelli and Conia-ene type reactions in one pot

A novel one-pot protocol that enables sequential execution of an aza-Piancatelli rearrangement and a Conia-ene type reaction has been developed under Lewis acid catalysis. Here, a combination of B(C6F5)3 and Cu(OTf)2, triethylamine, and triphenylphosphine yielded a wide range of cis-fused cyclopentenone-pyrrolidine scaffolds in one pot with good yields and diastereoselectivity.

Total Synthesis of Penicibilaenes Enabled by a Tandem Double Conia-ene Type Reaction

The total syntheses of penicibilaenes A and B are described. The key step is the tBuOK/DMSO-mediated tandem 5-exo-dig Conia-ene type reaction and 6-exo-dig Conia-ene type reaction to install the tricyclic [6.3.1.01,5] dodecane core of penicibilaenes from dibutynyl cyclohexanone in a single step, together with a sequence of copper-mediated conjugate addition and Crabtree's hydrogenation to forge the stereogenic centers at C5 and C2, respectively.

Recent Advances in Palladium-Catalyzed [4 + n] Cycloaddition of Lactones, Benzoxazinanones, Allylic Carbonates, and Vinyloxetanes

Palladium-catalyzed allylation cyclization reaction has recently emerged as an efficient and powerful synthetic platform for the construction of diverse and valuable carbo- and heterocycles. Thus the development of new allylic motifs for achieving this type of transformations in high reactivity and selectivity is of great importance. Generally, these substrates have been utilized as 1,3-, 1,4-, 1,5-, 1,6-dipoles in many reactions, which are applied to prepare highly functionalized products with complete control of chemo-, regio-, diastereo-, and enantioselectivity. In this review, we focus our attention on the development of palladium-catalyzed [4 + n] cycloaddition of allylic motifs and describe a comprehensive and impressive advances in this area. Meanwhile, the related mechanism and the application of these annulation strategies in natural product total synthesis will be highlighted in detail.

Annulation of O-silyl N,O-ketene acetals with alkynes for the synthesis of dihydropyridinones and its application in concise total synthesis of phenanthroindolizidine alkaloids

The formation of N-heterocycles with multiple substituents is important in organic synthesis. Herein, we report a novel method for the construction of functionalized dihydropyridinone rings through the annulation of an amide α-carbon with a tethered alkyne moiety. The reaction of the amide with the alkyne was achieved via O-silyl N,O-ketene acetal formation and silver-mediated addition. Furthermore, the developed method was applied for the total synthesis of phenanthroindolizidine and phenanthroquinolizidine alkaloids. By varying the coupling partners, a concise and collective total synthesis of these alkaloids was achieved.

Base-promoted Conia-ene cyclization of propargyl amides

We report a tBuOK-promoted synthesis of 1,3-dihydro-2H-pyrrol-2-one and 4-methylenepyrrolidin-2-one systems via Conia-ene like intramolecular cyclization. The method features extremely short reaction times (5 min) and mild reaction conditions (rt), enabling the trapping of a propargyl unit by an amide enolate. An intriguing anionic chain mechanism is at work, which can trigger the isomerization of an exo-alkene giving access to the otherwise elusive endo-product.

Rapid access to C2-quaternary 3-methyleneindolines via base-mediated post-Ugi Conia-ene cyclization

Highly efficient synthesis of diverse 2,2-disubstituted 3-methyleneindoline derivatives through a one-pot base-promoted post-Ugi 5-exo-dig "Conia-ene"-type cyclization has been disclosed. The mechanism study indicates that an intramolecular hydrogen bond may play a vital role in this process. The antiproliferative evaluation of cancer cell lines reveals that this protocol provides practical use in the green synthesis of bioactive compound libraries.

Asymmetric Divergent Synthesis of ent-Kaurane-, ent-Atisane-, ent-Beyerane-, ent-Trachylobane-, and ent-Gibberellane-type Diterpenoids

A unified strategy toward asymmetric divergent syntheses of nine C8-ethano-bridged diterpenoids A1-A9 (candol A, powerol, sicanadiol, epi-candol A, atisirene, ent-atisan-16α-ol, 4-decarboxy-4-methyl-GA₁₂, trachinol, and ent-beyerane) has been developed based on late-stage transformations of common synthons having ent-kaurane and ent-trachylobane cores. The expeditious assembly of crucial advanced ent-kaurane- and ent-trachylobane-type building blocks is strategically explored through a regioselective and diastereoselective Fe-mediated hydrogen atom transfer (HAT) 6-exo-trig cyclization of the alkene/enone and 3-exo-trig cyclization of the alkene/ketone, showing the multi-reactivity of densely functionalized polycyclic substrates with π_C═C and π_C═O systems in HAT-initiated reactions. Following the rapid construction of five major structural skeletons (ent-kaurane-, ent-atisane-, ent-beyerane-, ent-trachylobane-, and ent-gibberellane-type), nine C8-ethano-bridged diterpenoids A1-A9 could be accessed in the longest linear 8 to 11 steps starting from readily available chiral γ-cyclogeraniol 1 and known chiral γ-substituted cyclohexenone 2, in which enantioselective total syntheses of candol A (A1, 8 steps), powerol (A2, 9 steps), sicanadiol (A3, 10 steps), epi-candol A (A4, 8 steps), ent-atisan-16α-ol (A6, 11 steps), and trachinol (A8, 10 steps) are achieved for the first time.

Organo/Silver Dual Catalytic (3 + 2)/Conia-Ene Type Cyclization: Asymmetric Synthesis of Indane-Fused Spirocyclopenteneoxindoles

Developing efficient strategies to synthesize spirocyclopenteneoxindoles is an attractive target due to their potential biological activity. This work described the thiourea/silver dual catalytic (3 + 2)/Conia-ene type reaction of 2-(2-oxoindolin-3-yl)malononitrile with ortho-ethynyl substituted nitrostyrene. The reaction features mild conditions and good atom- and step-economy. Three new C-C bonds were formed within one synthetic step, providing the indane-fused spirocyclopenteneoxindoles in good yields, with excellent chemo-, regio-, and stereoselectivity.

Chiral N-Triflylphosphoramide-Catalyzed Asymmetric Hydroamination of Unactivated Alkenes: A Hetero-Ene Reaction Mechanism

A highly enantioselective intramolecular hydroamination reaction catalyzed by chiral N-triflylphosphoramide (NTPA) that covers an exceptionally broad substrate scope of isolated unactivated alkenes was recently reported by some of us. Herein...

Cooperative Palladium-Catalyzed and P(NEt2)3-Mediated (4+1) Annulation of Isatins with 2-Hydroxymethylallylcarbonates

P(NR2)3-promoted transformations of isatins with electron-withdrawing alkenes have received consistently attention over the last decades. However, no example involving chemical transformations of nonactivated or weakly activated alkenes is discovered in...

Gold(I)-Catalyzed 7-exo-dig Cyclization: A Key Step to Access the Bicyclo[4.2.1]nonane Skeleton of Vibsatin A, a Neurotrophic Diterpenoid

Vibsatin A is a new neurotrophic vibsane-type diterpenoid comprising a bridged bicyclo[4.2.1]nonane skeleton. Inspired by Sawamura's works, we generated the bicyclic backbone through a Conia-ene-derived 7-exo-dig cyclization from an enantiomerically enriched TIPS-based silyl enol ether. The reaction, catalyzed by a sensitive gold(I) complex, was efficiently performed on a large scale by glovebox free techniques. Furthermore, the shape of this system was exploited for subsequent installation of all of the stereogenic centers.