Exercising regiocontrol in palladium-catalyzed asymmetric prenylations and geranylation: unifying strategy toward flustramines A and B

Metal-catalyzed regiodivergent organic reactions

This review discusses metal-catalysed regiodivergent additions, allylic substitutions, CH-activation, cross-couplings and intra- or intermolecular cyclisations.

Prenyl Praxis: A Method for Direct Photocatalytic Defluoroprenylation

The prenyl fragment is the quintessential constituent of terpenoid natural products, a diverse family which contains numerous members with diverse biological properties. In contrast, fluorinated and multifluorinated arenes make up an important class of anthropogenic molecules which are highly relevant to material, agricultural, and pharmaceutical industries. While allylation chemistry is well developed, effective prenylation strategies have been less forthcoming. Herein, we describe the photocatalytic defluoroprenylation, a powerful method that provides access to "hybrid molecules" that possess both the functionality of a prenyl group and fluorinated arenes. This approach involves direct prenyl group transfer under very mild conditions, displays excellent functional group tolerance, and includes relatively short reaction times (<4 h), which is the fastest photocatalytic C-F functionalization developed to date. Additionally, the strategy can be extended to include allyl and geranyl (10 carbon fragment) transfers. Another prominent finding is a reagent-dependent switch in regioselectivity of the major product from para to ortho C-F functionalization.

Thiourea-Catalyzed Enantioselective Malonate Addition onto 3-Sulfonyl-3'-indolyl-2-oxindoles: Formal Total Syntheses of (-)-Chimonanthine, (-)-Folicanthine, and (+)-Calycanthine

A general approach to bispyrroloindolines via a key thiourea-catalyzed addition of malonates to 3-sulfonyl-3'-indolyl-2-oxindoles is reported. The enantioselelective process is found to be highly effective (up to 94% ee), where a C-C bond formation leads to the synthesis of a number of 2-oxindoles with an all-carbon quaternary stereocenter.

Pd(0)-Catalyzed Chemoselective Deacylative Alkylations (DaA) of N-Acyl 2-Oxindoles: Total Syntheses of Pyrrolidino[2,3- b]indoline Alkaloids, (±)-Deoxyeseroline, and (±)-Esermethole

We report an efficient Pd(0)-catalyzed deacylative allylation of N-acyl 3-substituted 2-oxindoles via the coupling of in situ generated nucleophiles (3 and 4) with allyl electrophiles for the synthesis of a variety of 2-oxindoles with C3-quaternary centers. Gratifyingly, this alkylation process is found to be highly chemoselective in nature, where a C-C bond formation is completely predominant over a C-N bond formation. A variety of key intermediates were synthesized utilizing an aforementioned methodology.

A catalytic N-deacylative alkylation approach to hexahydropyrrolo[2,3-b]indole alkaloids

A versatile unprecedented strategy to diversely functionalized hexahydropyrrolo[2,3-b]indole alkaloids is described in high chemical yields. The synthesis features a key Pd(0)-catalyzed deacylative alkylation of N-acyl 3-substituted indoles using only 1 mol% of Pd(PPh3)4. The scope of this methodology is further defined in the asymmetric synthesis of pyrroloindolines using a diastereoselective approach.

Access to 3a-Benzoylmethyl Pyrrolidino[2,3- b]indolines via CuII-Catalyzed Radical Annulation/C3-Functionalization Sequence

A Cu^II-catalyzed radical annulation/C3-functionalization cascade of tryptamine derivatives with aryl ethylene is reported. The mild catalytic system enables the facile construction of 3a-benzoylmethylpyrrolidino[2,3- b]indolines with excellent chemo- and regioselectivities. Remarkably, this novel method utilizes earth-abundant and inexpensive cupric salt as the catalyst and air as the co-oxidant, rendering the process highly environmentally friendly and atom economic. Presumably, the reaction proceeds through Cu^II-initiated formation of pyrrolidino[2,3- b]indolines radical intermediate I, which is successively trapped by aryl ethylene and O₂ to form the product. An ¹⁸O₂-labeling experiment and several control experiments were designed to support the mechanistic proposal.

Cu(ii)-tBu-PHOX catalyzed enantioselective malonate addition onto 3-hydroxy 2-oxindoles: application in the synthesis of dimeric pyrroloindoline alkaloids

An efficient Cu(ii)-PHOX-catalyzed malonate addition onto 3-hydroxy 3-indolyl-2-oxindoles is envisioned to afford excellent enantioselectivities (up to >99% ee) in high chemical yields. Detailed characterization techniques including X-ray, NMR, CV and EPR experiments suggest that a Cu(ii)-complex is involved as an active species in this process. Applying this strategy, an advanced intermediate of cyclotryptamine alkaloids has been synthesized in few steps for a general approach to bis-cyclotryptamine alkaloids.

Enantioselective Synthesis of Vicinal All-Carbon Quaternary Centers via Iridium-Catalyzed Allylic Alkylation

The development of the first enantioselective transition-metal-catalyzed allylic alkylation providing access to acyclic products bearing vicinal all-carbon quaternary centers is disclosed. The iridium-catalyzed allylic alkylation reaction proceeds with excellent yields and selectivities for a range of malononitrile-derived nucleophiles and trisubstituted allylic electrophiles. The utility of these sterically congested products is explored through a series of diverse chemo- and diastereoselective product transformations to afford a number of highly valuable, densely functionalized building blocks, including those containing vicinal all-carbon quaternary stereocenters.

Access to 3-Prenylated Oxindoles by α-Regioselective Prenylation: Application to the Synthesis of (±)-Debromoflustramine E

The development of a rapid, highly efficient, and one-pot synthesis of C3-α-prenylated oxindoles with simple reagents is described. The process is based on zinc-mediated α-regioselective prenylation of 3-acylidene-oxindole with commercially available prenyl bromide using inexpensive CeCl₃ as the catalyst. The new transformation tolerates a wide range of 3-acylidene-oxindoles, providing easy access to a variety of functionalized 3-prenylated oxindoles. The synthetic utility of the approach is verified by formal synthesis of the flustramine family alkaloid (±)-debromoflustramine E.

Divergent reactions of oxindoles with amino alcohols via the borrowing hydrogen process: oxindole ring opening vs. C3 alkylation

Oxindoles react with N-acetyl amino alcohols to form tryptamine-derived oxindoles, whereas analogous reactions with N-alkyl amino alcohols lead to lactam formation via a relatively mild borrowing hydrogen process.

Palladium-catalyzed three-component reaction for the synthesis of 3,3-disubstituted allylic alcohols with regio- and stereoselectivity

A novel three-component assembly of allenic alcohols, aryl iodides and 1,3-dicarbonyl compounds into 3,3-disubstituted allylic alcohols was promoted in the presence of a palladium source.

Regio- and Stereocontrolled Synthesis of 3-Substituted 1,2-Diazetidines by Asymmetric Allylic Amination of Vinyl Epoxide

Pd-catalyzed asymmetric allylic amination of rac-vinyl epoxide with unsymmetrical 1,2-hydrazines proceeds with excellent regio- and stereocontrol, which after further ring closure provides differentially protected 3-vinyl-1,2-diazetidines in good yields. The chirality at C-3 exerts stereocontrol over the nitrogen centers in the 1,2-diazetidine with all substituents orientating themselves trans to their neighbors. Efficient functionalization without rupture of the strained ring is demonstrated (e.g., by cross-metathesis), establishing the first general route to C-3-substituted 1,2-diazetidines in enantioenriched form.

Development of the Regiodivergent Asymmetric Prenylation of 3-Substituted Oxindoles

This paper describes our efforts to design a Pd-catalyzed asymmetric prenylation of 3-substituted oxindoles that affords access to both the linear and reverse-prenylated products. Both 3-alkyl- and 3-aryloxindoles performed well under our optimized reaction conditions. The regiodivergent alkylation of monoterpene-derived electrophiles using this methodology was also investigated. The utility of this methodology in natural product synthesis was demonstrated through the efficient total syntheses of four Flustra alkaloids, which also allowed the absolute stereochemistry of the prenylated oxindole products to be assigned. Surprisingly, the same enantiomer of ligand produced linear and branched regioisomers of opposite chirality.

Development of catalytic deacylative alkylations (DaA) of 3-acyl-2-oxindoles: total synthesis of meso-chimonanthine and related alkaloids

We present an effective deacylative alkylation strategy for the construction of a variety of 2-oxindoles bearing an all-carbon quaternary center at the pseudobenzylic position. A wide variety of products with quaternary centers could be accessed by employing simple Pd(0) catalysis under mild reaction conditions. Importantly, the same strategy works equally well for the dimeric 2-oxindole system, furnishing products with a vicinal quaternary center in favour of meso-isomer as the major product. Eventual application to the total syntheses of meso-chimonanthine and meso-folicanthine very well demonstrates the synthetic potential of this strategy.

Regio- and Enantioselective Rhodium-Catalyzed Addition of 1,3-Diketones to Allenes: Construction of Asymmetric Tertiary and Quaternary All Carbon Centers

An unprecedented highly regio- and enantioselective rhodium-catalyzed addition of 1,3-diketones to terminal and 1,1-disubstituted allenes furnishing asymmetric tertiary and quaternary all-carbon centers is reported. By applying a Rh^I /phosphoramidite/TFA catalytic system under mild conditions, the desired chiral branched α-allylated 1,3-diketones could be obtained in good to excellent yields, with perfect regioselectivity and in high enantioselectivity. The reaction shows a broad functional-group tolerance on both reaction partners highlighting its synthetic potential.

A catalytic enantioselective approach to tetrol bearing vicinal all-carbon quaternary stereogenic centers

The first highly enantioselective catalytic protocol for selective manipulation of tetrol benzylidene acetals through chiral phosphoric acid mediated oxidative desymmetrization is reported.

Catalytic Enantioselective and Diastereoselective Allylic Alkylation with Fluoroenolates: Efficient Access to C3-Fluorinated and All-Carbon Quaternary Oxindoles

Synthetically versatile 3,3-disubstituted fluorooxindoles exhibiting vicinal chirality centers were obtained in high yields and with excellent enantio-, diastereo-, and regioselectivity through catalytic asymmetric fluoroenolate alkylation with allylic acetates. The reaction proceeds under mild conditions and can be scaled up without compromising the asymmetric induction. The unique synthetic usefulness of the products is highlighted by the incorporation of additional functionalities and the formation of 3-fluorinated oxindoles exhibiting an array of four adjacent centers of chirality. A new C-F bond functionalization path that provides unprecedented possibilities for the stereoselective generation of a chiral quaternary carbon center in the alkaloid scaffold is introduced.

Synthesis of 2-oxindoles via 'transition-metal-free' intramolecular dehydrogenative coupling (IDC) of sp(2) C-H and sp(3) C-H bonds

The synthesis of a variety of 2-oxindoles bearing an all-carbon quaternary center at the pseudo benzylic position has been achieved via a ‘transition-metal-free’ intramolecular dehydrogenative coupling (IDC). The construction of 2-oxindole moieties was carried out through formation of carbon–carbon bonds using KOt-Bu-catalyzed one pot C-alkylation of β-N-arylamido esters with alkyl halides followed by a dehydrogenative coupling. Experimental evidences indicated toward a radical-mediated path for this reaction.

Palladium-catalysed formation of vicinal all-carbon quaternary centres via propargylation

Construction of two vicinal all-carbon quaternary carbon centres is of great importance due to the common presence of such units in natural and unnatural molecules with attractive functions. However, it remains a significant challenge. Here, we have developed a palladium-catalysed general coupling for the efficient connection of two tertiary carbon atoms: Specifically, propargylic carbonate has been treated with a fully loaded soft functionalized nucleophile to connect such two fully loaded carbon atoms with a simple palladium catalyst. It is observed that the central chirality in the optically active tertiary propargylic carbonates has been remembered and transferred into the products with very high efficiency. The triple bond and the functional groups such as ester, cyano and unsaturated C–C bonds make this method a relatively general solution for such a purpose due to their high synthetic versatility.

Formation of vicinal, quaternary stereocentres is challenging, but such products are commonly encountered in nature. Here, the authors report a palladium catalysed process to form vicinal quaternary carbons via propargylation, including in enantiopure form when optically active starting materials are employed.