Catalytic enantioselective synthesis of benzocyclobutenols and cyclobutanols via a sequential reduction/C-H functionalization

Sex Pheromone of the Papaya Mealybug

Sex pheromones of mealybugs are reported from more than 20 species and have generally been shown to be monoterpene alcohols esterified with short-chain carboxylic acids. Here, however, we discovered and isolated an alcohol without an acid moiety as a pheromone compound released from adult females of the papaya mealybug, Paracoccus marginatus. By means of gas chromatography - mass spectrometry and nuclear magnetic resonance spectroscopy analyses, the structure was identified to be trans-2-(2-isopropenyl-1-methylcyclobutyl)ethanol, a monoterpene with a unique cyclobutene skeleton, commonly known as fragranol. We then completely separated synthetic (±)-fragranol into each enantiomer by means of preparative high-performance liquid chromatography using a chiral resolution column, and (-)-(1S,2S)-fragranol was definitely confirmed to be the natural pheromone and to attract many males in the field when used as a pheromone trap lure. (±)-Fragranol showed attractiveness comparable to that of the pure (-)-(1S,2S)-enantiomer. This study provides not only useful information for the monitoring and management of P. marginatus but also an interesting exception underlining the great diversity of mealybug pheromone structures.

Photoredox/Cobalt-Catalyzed Chemo-, Regio-, Diastereo- and Enantioselective Reductive Coupling of 1,1-Disubstituted Allenes and Cyclobutenes

A dual photoredox/cobalt-catalyzed protocol for chemo-, regio-, diastereo- and enantioselective reductive coupling of 1,1-disubstituted allenes and cyclobutenes through chemo-, regio-, diastereo- and enantioselective oxidative cyclization followed by stereoselective protonation promoted by a chiral phosphine-cobalt complex is presented. Such process represents an unprecedented reaction pathway for cobalt catalysis that enables selective transformation of the less sterically congested alkenes of 1,1-disubstituted allenes with cyclobutenes, incorporating a broad scope of tetrasubstituted alkenes into the cyclobutane scaffolds in up to 86 % yield, >98 : 2 chemo- and regioselectivity, >98 : 2 dr and >99.5:0.5 er. Functionalization delivered a variety of enantioenriched cyclobutanes that are otherwise difficult to access. Preliminary mechanistic studies revealed that the reactions proceeded through oxidative cyclization followed by protonation and protonation might be the rate-determining step.

Pd/C-Catalyzed Stereoselective Arene Hydrogenation of Benzocyclobutenes Enabled by π-Bond Localization

We developed here a Pd/C-catalyzed diastereoselective cis-hydrogenation of benzocyclobutene derivatives under mild conditions to deliver an array of bicyclo[4.2.0]octane scaffolds with up to five stereocenters. The π-bond localization enabled hydrogenation of the arene moiety to occur even at room temperature under 1 atm of a H2 atmosphere.

Manganese/Enzyme Sequential Catalytic Pathway for the Production of Optically Active γ-Functionalized Alcohols

A brief, practical catalytic process for the production of optically active γ-functionalized alcohols from relevant alkenes has been developed by using a robust Mn(III)/air/(Me2SiH)2O catalytic system combined with lipase-catalyzed kinetic resolution. This approach demonstrates exceptional tolerance toward proximal functional groups present on alkenes, enabling the achievement of high yields and exclusive enantioselectivity. Under this sequential catalytic system, the chiral alkene precursors can also be converted into γ-functionalized alcohols and related acetates as separable single enantiomers.

Asymmetric Transfer Hydrogenation of Cyclobutenediones

Four-membered carbocycles are fundamental substructures in bioactive molecules and approved drugs and serve as irreplaceable building blocks in organic synthesis. However, developing efficient protocols furnishing diversified four-membered ring compounds in a highly regio-, diastereo-, and enantioselective fashion remains challenging but very desirable. Here, we report the unprecedented asymmetric transfer hydrogenation of cyclobutenediones. The reaction can selectively afford three types of four-membered products in high yields with high stereoselectivities, and the highly functionalized products enable a series of further transformations to form more diversified four-membered compounds. Asymmetric synthesis of di-, tri-, and tetrasubstituted bioactive molecules has also been achieved. Systematic mechanistic studies and theoretical calculations have revealed the origin of the regioselectivity, the key hydrogenation transition state models, and the sequence of the double and triple hydrogenation processes. The work provides a new choice for the catalytic asymmetric synthesis of cyclobutanes and related structures and demonstrates the robustness of asymmetric transfer hydrogenation in the accurate selectivity control of highly functionalized substrates.

Asymmetric Synthesis of Scillascillin-Type Homoisoflavonoid

The first asymmetric synthesis of a scillascillin-type homoisoflavonoid was reported. Key reactions for the asymmetric synthesis of benzocyclobutene include catalytic reductive desymmetrization of malonic ester and an intramolecular C-H activation of the methyl group.

Cobalt-Catalyzed Diastereo- and Enantioselective Carbon-Carbon Bond Forming Reactions of Cyclobutenes

Catalytic enantioselective functionalization of cyclobutenes constitutes a general and modular strategy for construction of enantioenriched complex cyclobutanes bearing multiple stereogenic centers, as chiral four-membered rings are common motifs in biologically active molecules and versatile intermediates in organic synthesis. However, enantioselective synthesis of cyclobutanes through such a strategy remained significantly limited. Herein, we report a series of unprecedented cobalt-catalyzed carbon-carbon bond forming reactions of cyclobutenes that are initiated through enantioselective carbometalation. The protocols feature diastereo- and enantioselective introduction of allyl, alkynyl, and functionalized alkyl groups. Mechanistic studies indicated an unusual 1,3-cobalt migration and subsequent β-carbon elimination cascade process occurred in the allyl addition. These new discoveries established a new elementary process for cobalt catalysis and an extension of diversity of nucleophiles for enantioselective transformations of cyclobutenes.

Enhanced thermal and photo-stability of a para-substituted dicumyl ketone intercalated in a layered double hydroxide

A ketodiacid, 4,4′-dicarboxylate-dicumyl ketone (3), has been intercalated into a Zn, Al layered double hydroxide (LDH) by a coprecipitation synthesis strategy. The structure and chemical composition of the resultant hybrid material (LDH-KDA3) were characterized by powder X-ray diffraction (PXRD), FT-IR, FT-Raman and solid-state 13C{1H} NMR spectroscopies, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), thermogravimetric analysis (TGA), and elemental analysis (CHN). PXRD showed that the dicarboxylate guest molecules assembled into a monolayer to give a basal spacing of 18.0 Å. TGA revealed that the organic guest starts to decompose at a significantly higher temperature (ca. 330°C) than that determined for the free ketodiacid (ca. 230°C). Photochemical experiments were performed to probe the photoreactivity of the ketoacid in the crystalline state, in solution, and as a guest embedded within the photochemically-inert LDH host. Irradiation of the bulk crystalline ketoacid results in photodecarbonylation and the exclusive formation of the radical-radical combination product. Solution studies employing the standard myoglobin (Mb) assay for quantification of released CO showed that the ketoacid behaved as a photoactivatable CO-releasing molecule for transfer of CO to heme proteins, although the photoreactivity was low. No photoinduced release of CO was found for the LDH system, indicating that molecular confinement enhanced the photo-stability of the hexasubstituted ketone. To better understand the behavior of 3 under irradiation, a more comprehensive study, involving excitation of this compound in DMSO-d6 followed by 1H NMR, UV-Vis and fluorescence spectroscopy, was undertaken and further rationalized with the help of time-dependent density functional theory (TDDFT) electronic quantum calculations. The photophysical study showed the formation of a less emissive compound (or compounds). New signals in the 1H NMR spectra were attributed to photoproducts obtained via Norrish type I α-cleavage decarbonylation and Norrish type II (followed by CH3 migration) pathways. TDDFT calculations predicted that the formation of a keto-enol system (via a CH3 migration step in the type II pathway) was highly favorable and consistent with the observed spectral data.

Palladium-Catalyzed Access to Benzocyclobutenone-Derived Ketonitrones via C(sp2)-H Functionalization

The palladium-catalyzed C(sp2)-H functionalization of bromoaryl aldonitrones leading to benzocyclobutenone-derived ketonitrones is described. This method allows for the preparation of a wide range of strained, four-membered ketonitrones with broad functional group tolerance. Downstream transformations of the formed products were readily demonstrated, illustrating the synthetic utility of the obtained benzocyclobutenone-derived nitrones for the construction of polycyclic nitrogen-containing scaffolds.

Enantioselective Synthesis of Indanes with a Quaternary Stereocenter via Diastereoselective C(sp3)-H Functionalization

A practical synthesis of enantioenriched indane derivatives with quaternary stereocenters was developed via sequential enantioselective reduction and C-H functionalization. Good to excellent enantioselectivity could be achieved by either the CuH-catalyzed asymmetric reduction or the Corey-Bakshi-Shibata (CBS) reduction of indanone derivatives. The subsequent diastereospecific and regioselective rhodium-catalyzed silylation of the methyl C-H bond led to indane derivatives with quaternary centers. This strategy was further applied in syntheses of (nor)illudalane and botryane sesquiterpenoids.