Asymmetric Brønsted Acid‐Catalyzed Nazarov Cyclization of Acyclic α‐Alkoxy Dienones

A Brønsted acid‐catalyzed asymmetric Nazarov cyclization of acyclic α‐alkoxy dienones has been developed. The reaction offers access to chiral cyclopentenones in a highly enantioselective manner. The reaction is complementary to our previously reported Brønsted acid‐catalyzed electrocyclization reactions, which provided differently substituted optically active cyclopentenones with a fused tetrahydropyrane ring in good yields and with excellent enantioselectivities.     

Enantioselective oxysulfenylation and oxyselenenylation of olefins catalyzed by chiral Brønsted acids

This paper describes Brønsted acid catalyzed enantioselective oxysulfenylation and oxyselenenylation of olefins. Enantiomerically enriched tetrahydrofurans are formed with up to 63% ee with dibenzoyl-d-tartaric acid and its derivatives as catalyst. Chiral β-carboxyl sulfides and selenides have also been obtained with up to 50% and 84% ee, respectively, via enantioselective desymmetrization of thiiranium and seleniranium ions in the presence of catalytic amounts of chiral binaphthol derived N-triflyl phosphoramide.     

Enantio- and diastereoselective stepwise cyclization of polyprenoids induced by chiral and achiral LBAs. A new entry to (-)-ambrox, (+)-podocarpa-8,11,13-triene diterpenoids, and (-)-tetracyclic polyprenoid of sedimentary origin

An enantio- and diastereoselective stepwise cyclization of polyprenoids induced by Lewis acid-assisted chiral Br?nsted acids (chiral LBAs) and achiral LBAs is described. In particular, the absolute stereocontrol in the initial cyclization of polyprenoids to form an A-ring induced by chiral LBAs and the importance of the nucleophilicity of the internal terminator in polyprenoids for the relative stereocontrol in subsequent cyclization are demonstrated. (-)-Ambrox was synthesized via the enantioselective cyclization of (E,E)-homofarnesyl triethylsilyl ether with tin(IV) chloride-coordinated (R)-2-(o-fluorobenzyloxy)-2'-hydroxy-1,1'-binaphthyl ((R)-BINOL-o-FBn) and subsequent diastereoselective cyclization with CF(3)CO(2)H.SnCl(4) as key steps. Protection of (E,E)-homofarnesol by a triethylsilyl group increased the enantioselectivity of chiral LBA-induced cyclization and both the chemical yield and diastereoselectivity in the subsequent cyclization. The enantioselective cyclization of homo(polyprenyl)arenes possessing an aryl group was also induced by (R)-BINOL-o-FBn.SnCl(4). Several optically active podocarpa-8,11,13-triene diterpenoids and (-)-tetracyclic polyprenoid of sedimentary origin were synthesized (75-80% ee) by the enantioselective cyclization of homo(polyprenyl)benzene derivatives induced by (R)-BINOL-o-FBn.SnCl(4) and subsequent diastereoselective cyclization induced by BF(3).Et(2)O/EtNO(2) or CF(3)CO(2)H .SnCl(4).     

Tin(IV) chloride-chiral pyrogallol derivatives as new Lewis acid-assisted chiral Brønsted acids for enantioselective polyene cyclization

[reaction: see text] New Lewis acid-assisted Br?nsted acids (LBAs), tin(IV) chloride-2,6-dialkoxyphenols, serve as artificial cyclases for biomimetic polyene cyclization. For example, the enantioselective cyclization of 4-(homogeranyl)toluene using tin(IV) chloride-2,6-di[(1'R,2'R)-trans-2'-(3' ',5' '-xylyl)cyclohexanoxy]phenol gave a trans-fused tricyclic compound with 85% ee.     

Palladium(II)/Brønsted Acid‐Catalyzed Enantioselective Oxidative Carbocyclization–Borylation of Enallenes

An enantioselective oxidative carbocyclization–borylation of enallenes that is catalyzed by palladium(II) and a Brønsted acid was developed. Biphenol‐type chiral phosphoric acids were superior co‐catalysts for inducing the enantioselective cyclization. A number of chiral borylated carbocycles were synthesized in high enantiomeric excess.     

Development of Chiral Organosuperbase Catalysts Consisting of Two Different Organobase Functionalities

In the field of chiral Brønsted base catalysis, a new generation of chiral catalysts has been highly anticipated to overcome the intrinsic limitation of pronucleophiles that are applicable to the enantioselective reactions. Herein, we reveal conceptually new chiral Brønsted base catalysts consisting of two different organobase functionalities, one of which functions as an organosuperbase and the other as the substrate recognition site. Their prominent activity, which stems from the distinctive cooperative function by the two organobases in a single catalyst molecule, was demonstrated in the unprecedented enantioselective direct Mannich‐type reaction of α‐phenylthioacetate as a less acidic pronucleophile. The present achievement would provide a new guiding principle for the design and development of chiral Brønsted base catalysts and significantly broaden the utility of Brønsted base catalysis in asymmetric organic synthesis.     

Asymmetric Synthesis of Heterocyclic γ‐Amino‐Acid and Diamine Derivatives by Three‐Component Radical Cascade Reactions

An enantioselective three‐component radical reaction of quinolines or pyridines with enamides and α‐bromo carbonyl compounds by dual photoredox and chiral Brønsted acid catalysis is presented. A range of valuable chiral γ‐amino‐acid derivatives are accessible in high chemo‐, regio‐, and enantioselectivity from simple, readily available starting materials under mild reaction conditions. Using the same strategy, the asymmetric synthesis of 1,2‐diamine derivatives is also reported.     

Chiral Lewis base-assisted Brønsted acid (LBBA)-catalyzed enantioselective cyclization of 2-geranylphenols

Chiral Lewis base-assisted Br?nsted acids (Chiral LBBAs) have been designed as new organocatalysts for biomimetic enantioselective cyclization. A salt of a chiral phosphonous acid diester with FSO(3)H catalyzes the enantioselective cyclization of 2-geranylphenols to give the desired trans-fused cyclized products with high diastereo- and enantioselectivities (up to 98:2 dr and 93% ee).     

Efficient enantioselective synthesis of (+)-sclareolide and (+)-tetrahydroactinidiolide: chiral LBA-induced biomimetic cyclization

An efficient enantioselective synthesis of the lactones (+)-sclareolide and (+)-tetrahydroactinidiolide has been achieved through Lewis acid-assisted chiral Brønsted acid-induced enantioselective cyclization of terpenic carboxylic acids. The reaction sequence involved the [2,3] sigmatropic rearrangement of an allylic alcohol and biomimetic cyclization of terpenic acid in the presence of (R)-2-benzyloxy-2′-hydroxy-1,1′-binaphthyl and tin tetrachloride as key steps. The cyclization gave lactones in good yield and with high enantiomeric excess.     

Binolam-AlCl: a two-centre catalyst for the synthesis of enantioenriched cyanohydrin O-phosphates

The enantioselective synthesis of cyanohydrin O‐phosphates by using in situ generated bifunctional catalysts (R)‐ or (S)‐3,3′‐bis(diethylaminomethyl)‐1,1′‐binaphthol–aluminium chloride (binolam–AlCl) is reported. The reaction, which can be described as an overall cyano‐O‐phosphorylation of aldehydes, has a wide scope and applicability. Evidence is also provided, including ab initio and DFT calculations, in support of supported by the Lewis acid/Brønsted base (LABB) dual role of the catalyst in inducing first the key enantioselective hydrocyanation, which is then followed by O‐phosphorylation. A brief screening of the synthetic usefulness of the resulting cyanohydrin O‐phosphates unveiles some interesting applications. Among them, chemoselective hydrolysis, reduction and palladium‐catalysed nucleophilic allyl substitution, thereby leading to enantiomerically enriched α‐O‐phosphorylated α‐hydroxy esters, β‐amino alcohols and γ‐cyanoallyl alcohols, respectively. Naturally occurring (?)‐tembamide and (?)‐aegeline are synthesised accordingly.     

Lewis Base/Brønsted Acid Co‐catalyzed Enantioselective Sulfenylation/Semipinacol Rearrangement of Di‐ and Trisubstituted Allylic Alcohols

An enantioselective sulfenylation/semipinacol rearrangement of 1,1‐disubstituted and trisubstituted allylic alcohols was accomplished with a chiral Lewis base and a chiral Brønsted acid as cocatalysts, generating various β‐arylthio ketones bearing an all‐carbon quaternary center in moderate to excellent yields and excellent enantioselectivities. These chiral arylthio ketone products are common intermediates with many applications, for example, in the design of new chiral catalysts/ligands and the total synthesis of natural products. Computational studies (DFT calculations) were carried out to explain the enantioselectivity and the role of the chiral Brønsted acid. Additionally, the synthetic utility of this method was exemplified by an enantioselective total synthesis of (?)‐herbertene and a one‐pot synthesis of a chiral sulfoxide and sulfone.     

BINOL-phosphoric acid catalyzed asymmetric Mannich addition of β-ketoesters to indolenines generated in situ by DDQ

An asymmetric Mannich addition of β-ketoesters to indolenines that were generated in situ from 3-indolinone-2-carboxylates by DDQ has been developed using a catalytic amount of chiral BINOL-phosphoric acid. The corresponding chiral 3-indolinone-2-carboxylate derivatives were obtained in good yields with excellent ee (up to 99%). This is the first report on the enantioselective addition of β-ketoesters to indolenines using a chiral Brønsted acid.     

Organoiodine‐Catalyzed Enantioselective Alkoxylation/Oxidative Rearrangement of Allylic Alcohols

An enantioselective catalytic alkoxylation/oxidative rearrangement of allylic alcohols has been established by using a Brønsted acid and chiral organoiodine. The presence of 20 mol % of an (S)‐proline‐derived C₂‐symmetric chiral iodine led to enantioenriched α‐arylated β‐alkoxylated ketones in good yields and with high levels of enantioselectivity (84–94 % ee).     

Brønsted Acid Catalyzed Asymmetric Hydroamination of Alkenes: Synthesis of Pyrrolidines Bearing a Tetrasubstituted Carbon Stereocenter

The first highly enantioselective Brønsted acid catalyzed intramolecular hydroamination of alkenes enables the efficient construction of a series of chiral (spirocyclic) pyrrolidines with an α‐tetrasubstituted carbon stereocenter with excellent functional group tolerance. A unique feature of this strategy is the use of a thiourea group acting as both the activating and the directing group through cooperative multiple hydrogen bonding with a Brønsted acid and the double bond. The utility of this method is highlighted by the facile construction of chiral synthetic intermediates and important structural motifs that are widely found in organic synthesis.     

Enantioselective Aza‐Ene‐type Reactions of Enamides with Gold Carbenes Generated from α‐Diazoesters

Carbophilic gold carbenes generated from the decomposition of α‐diazoesters show high reactivity towards enamides, leading to an unprecedented aza‐ene‐type reaction. The presence of 0.1 mol % of a chiral Brønsted acid co‐catalyst is sufficient to give synthetically relevant γ‐keto esters in excellent yields and selectivities (up to 99 % yield, 97 % ee ).     

Catalytic Enantioselective Direct Aldol Addition of Aryl Ketones to α‐Fluorinated Ketones

The catalytic enantioselective synthesis of α‐fluorinated chiral tertiary alcohols from (hetero)aryl methyl ketones is described. The use of a bifunctional iminophosphorane (BIMP) superbase was found to facilitate direct aldol addition by providing the strong Brønsted basicity required for rapid aryl enolate formation. The new synthetic protocol is easy to perform and tolerates a broad range of functionalities and heterocycles with high enantioselectivity (up to >99:1 e.r.). Multi‐gram scalability has been demonstrated along with catalyst recovery and recycling. ¹H NMR studies identified a 1400‐fold rate enhancement under BIMP catalysis, compared to the prior state‐of‐the‐art catalytic system. The utility of the aldol products has been highlighted with the synthesis of various enantioenriched building blocks and heterocycles, including 1,3‐aminoalcohol, 1,3‐diol, oxetane, and isoxazoline derivatives.     

Organocatalytic Enantioselective Protonation for Photoreduction of Activated Ketones and Ketimines Induced by Visible Light

The first catalytic asymmetric photoreduction of 1,2‐diketones and α‐keto ketimines under visible light irradiation is reported. A transition‐metal‐free synergistic catalysis platform harnessing dicyanopyrazine‐derived chromophore (DPZ) as the photoredox catalyst and a non‐covalent chiral organocatalyst is effective for these transformations. With the flexible use of a chiral Brønsted acid or base in H⁺ transfer interchange to control the elusive enantioselective protonation, a variety of chiral α‐hydroxy ketones and α‐amino ketones were obtained with high yields and enantioselectivities.     

Enantioselective α-chlorination of β-keto esters and amides catalyzed by chiral imidodiphosphoric acids

Chiral imidodiphosphoric acids were employed as catalysts for the enantioselective α-chlorination of β-keto esters and amides using NCS as the chlorine source, providing a series of optically active products with good to high enantioselectivities (74–95% ee) and excellent yields (92–99%). This represents the first report of the Brønsted acid catalyzed enantioselective α-chlorination of cyclic β-keto derivatives.     

Enantioselective Addition Reaction of Azlactones with Styrene Derivatives Catalyzed by Strong Chiral Brønsted Acids

An enantioselective intermolecular addition reaction of azlactones, as carbon nucleophiles, with styrene derivatives, as simple olefins, was demonstrated using a newly developed chiral Brønsted acid catalyst, namely, F₁₀BINOL‐derived N‐triflyl phosphoramide. Addition products having vicinal tetrasubstituted carbon centers, one of which is an all‐carbon quaternary stereogenic center, were formed in good yields with moderate to high stereoselectivities. Extremely high acidity of the new chiral Brønsted acid was confirmed by its calculated pK_a value based on DFT studies and is the key to accomplishing not only high catalytic activity but also efficient stereocontrol in the intermolecular addition.     

Catalytic Enantioselective Cloke–Wilson Rearrangement

Racemic cyclopropyl ketones undergo enantioselective rearrangement to deliver the corresponding dihydrofurans in the presence of a chiral phosphoric acid as the catalyst. The reaction involves activation of the donor‐acceptor cyclopropane substrate by the chiral Brønsted acid catalyst to promote the ring‐opening event, thus generating a carbocationic intermediate that subsequently undergoes cyclization. Computational studies and control experiments support this mechanistic pathway.