Catalytic Enantioselective Hosomi-Sakurai Conjugate Allylation of Cyclic Unsaturated Ketoesters

Catalytic prenyl conjugate additions for synthesis of enantiomerically enriched PPAPs

Polycyclic polyprenylated acylphloroglucinols (PPAPs) are a class of >400 natural products with a broad spectrum of bioactivity, ranging from antidepressant and antimicrobial to anti-obesity and anticancer activity. Here, we present a scalable, regio-, site-, and enantioselective catalytic method for synthesis of cyclic β-prenyl ketones, compounds that can be used for efficient syntheses of many PPAPs in high enantiomeric purity. The transformation is prenyl conjugate addition to cyclic β-ketoesters promoted by a readily accessible chiral copper catalyst and involving an easy-to-prepare and isolable organoborate reagent. Reactions reach completion in just a few minutes at room temperature. The importance of this advance is highlighted by the enantioselective preparation of intermediates previously used to generate racemic PPAPs. We also present the enantioselective synthesis of nemorosonol (14 steps, 20% yield) and its one-step conversion to another PPAP, garcibracteatone (52% yield).

Forging the Tetracyclic Core Framework of Rhodomolleins XIV and XLII: A Ring-Distortion Approach

A ring distortion approach for the synthesis of an advanced intermediate en route to rhodomolleins XIV and XLII was described, which led to successful construction of the 5/8/5/5 tetracyclic core framework of the kalmane diterpenoids. Key steps of the strategy include an oxidative dearomatization-induced (ODI)-Diels-Alder cycloaddition, a Dowd-Beckwith rearrangement, and a bioinspired Wagner-Meerwein rearrangement.

Photochemical Organocatalytic Regio- and Enantioselective Conjugate Addition of Allyl Groups to Enals

We report the first catalytic enantioselective conjugate addition of allyl groups to α,β‐unsaturated aldehydes. The chemistry exploits the visible‐light‐excitation of chiral iminium ions to activate allyl silanes towards the formation of allylic radicals, which are then intercepted stereoselectively. The underlying radical mechanism of this process overcomes the poor regio‐ and chemoselectivity that traditionally affects the conjugate allylation of enals proceeding via polar pathways. We also demonstrate how this organocatalytic strategy could selectively install a valuable prenyl fragment at the β‐carbon of enals.

Copper(I)-catalyzed asymmetric 1,6-conjugate allylation

Catalytic asymmetric conjugate allylation of unsaturated carbonyl compounds is usually difficult to achieve, as 1,2-addition proceeds dominantly and high asymmetric induction is a challenging task. Herein, we disclose a copper(I)-NHC complex catalyzed asymmetric 1,6-conjugate allylation of 2,2-dimethyl-6-alkenyl-4H-1,3-dioxin-4-ones. The phenolic hydroxyl group in NHC ligands is found to be pivotal to obtain the desired products. Both aryl group and alkyl group at δ-position are well tolerated with the corresponding products generated in moderate to high yields and high enantioselectivity. Moreover, both 2-substituted and 3-substituted allylboronates serve as acceptable allylation reagents. At last, the synthetic utility of the products is demonstrated in several transformations by means of the versatile terminal olefin and dioxinone groups.

Catalytic asymmetric conjugate allylation of unsaturated carbonyl compounds is usually difficult to achieve due to competing 1,2-addition. Here, the authors report a copper(I)-catalyzed asymmetric 1,6-conjugate allylation of 2,2-dimethyl-6-alkenyl-4H-1,3-dioxin-4-ones with good scope and high enantioselectivity.

Allylsilane Reagent-Controlled Divergent Asymmetric Catalytic Reactions of 2-Naphthoquinone-1-methide

Herein, the first allylsilane reagent-controlled divergent asymmetric Hosomi-Sakurai conjugate allylation and hetero-Diels-Alder (HDA) with 2-naphthoquinone-1-methide (2-Nap-Q-1-M) under the catalysis of Sc^III /Feng ligand complex is reported. With these methods, a variety of uniquely substituted chiral allyl-functionalized diaryl compounds and naphthopyran products were obtained in a straightforward and highly stereoselective (up to 96.5:3.5 e.r.) manner under mild conditions. Moreover, it is demonstrated that 2-Nap-Q-1-M can serve as an efficient diene for a side asymmetric Diels-Alder (D-A) reaction. This principle can provide a straightforward access to hydrophenalene in an optically active form, which represents a structural core of various natural products and bioactive molecules.

Synthesis of Quaternary Carbon Stereogenic Centers by Diastereoselective Conjugate Addition of Boron-Stabilized Allylic Nucleophiles to Enones

A method for the site-selective and diastereoselective conjugate addition of boron-stabilized allylic nucleophiles to α,β-unsaturated ketones is disclosed. Transformations involve easily prepared γ,γ-disubstituted allyldiboron reagents and proceed in the presence of a fluoride activator at 80 °C. Reactions proceed with a wide variety of enones and allyldiboron reagents efficiently to deliver ketone products that contain otherwise difficult-to-access vicinal β-tertiary and γ-quaternary carbon stereogenic centers and an alkenylboron moiety. The utility of the method is highlighted by several transformations, including cross-coupling and carbocyclizations.

Catalytic Enantioselective Addition of an Allyl Group to Ketones Containing a Tri-, a Di-, or a Monohalomethyl Moiety. Stereochemical Control Based on Distinctive Electronic and Steric Attributes of C-Cl, C-Br, and C-F Bonds

We disclose the results of an investigation designed to generate insight regarding the differences in the electronic and steric attributes of C-F, C-Cl, and C-Br bonds. Mechanistic insight has been gleaned by analysis of variations in enantioselectivity, regarding the ability of electrostatic contact between a halomethyl moiety and a catalyst's ammonium group as opposed to factors lowering steric repulsion and/or dipole minimization. In the process, catalytic and enantioselective methods have been developed for transforming a wide range of trihalomethyl (halogen = Cl or Br), dihalomethyl, or monohalomethyl (halogen = F, Cl, or Br) ketones to the corresponding tertiary homoallylic alcohols. By exploiting electrostatic attraction between a halomethyl moiety and the catalyst's ammonium moiety and steric factors, high enantioselectivity was attained in many instances. Reactions can be performed with 0.5-5.0 mol % of an in situ generated boryl-ammonium catalyst, affording products in 42-99% yield and up to >99:1 enantiomeric ratio. Not only are there no existing protocols for accessing the great majority of the resulting products enantioselectively but also in some cases there are hardly any instances of a catalytic enantioselective addition of a carbon-based nucleophile (e.g., one enzyme-catalyzed aldol addition involving trichloromethyl ketones, and none with dichloromethyl, tribromomethyl, or dibromomethyl ketones). The approach is scalable and offers an expeditious route to the enantioselective synthesis of versatile and otherwise difficult to access aldehydes that bear an α-halo-substituted quaternary carbon stereogenic center as well as an assortment of 2,2-disubstituted epoxides that contain an easily modifiable alkene. Tertiary homoallylic alcohols containing a triazole and a halomethyl moiety, structural units relevant to drug development, may also be accessed efficiently with exceptional enantioselectivity.

Racemic Vinylallenes in Catalytic Enantioselective Multicomponent Processes: Rapid Generation of Complexity through 1,6-Conjugate Additions

Racemic vinylallenes are shown to be effective substrates for catalytic multicomponent diastereo- and enantioselective 1,6-conjugate addition of multifunctional allyl moieties to easily accessible α,β,γ,δ-unsaturated diesters. Reactions may be catalyzed by 5.0 mol % of a readily accessible NHC-Cu complex at ambient temperature, and other than a vinylallene, involve B₂ (pin)₂ and an α,β,γ,δ-unsaturated diester. A variety of vinylallenes were converted to products bearing a Z-trisubstituted alkenyl-B(pin) moiety, a vinyl group, a β,γ-unsaturated diester unit, and vicinal stereogenic centers in up to 67 % yield, 87:13 Z/E ratio, >98:2 d.r., and 98:2 e.r. Chemoselective modifications involving the alkenyl-B(pin), the vinyl, or the 1,2-disubstituted olefin moieties were carried out to demonstrate versatility and utility. Stereochemical models, based on mechanistic and DFT studies, demonstrate the dynamic behavior of intermediated Cu-allyl species and account for various selectivity profiles.

[4 + 2]-Cycloaddition and 1,4-Addition of ortho-Quinone Methides by a Chiral Crotyl Silane

Anhydrous FeCl₃ in the presence of 2,6-lutidine promotes the substrate-controlled enantioselective [4 + 2]-cycloaddition and crotylation reaction between an enantioenriched ( S, E)-crotyl silane and in situ generated ortho-quinone methides ( oQMs). The reaction produces both the chiral chroman and crotylation products in a ratio reflective of the electronic nature of the parent oQM with overall combined yields up to 96%. A ring-opening and elimination sequence was subsequently developed to provide direct access to the crotylation products, containing two contiguous tertiary carbon stereocenters, in good yields and enantioselectivities.

A highly efficient Hg(OTf)2-mediated Sakurai–Hosomi allylation of N-tert-butyloxycarbonylamino sulfones, aldehydes, fluoroalkyl ketones and α,β-unsaturated enones using allyltrimethylsilane

The cheap and easily available Hg(OTf)₂ can efficiently mediate the Sakurai–Hosomi reaction of N-Boc amino sulfones, aldehydes, α-fluoroalkyl ketones and α,β-unsaturated enones using allyltrimethylsilane with the catalyst loading down to 0.5–5.0 mol%.

Stereoconvergent 1,4-Addition Reaction of 5H-Oxazol-4-ones with E,Z Isomeric Mixture of Alkylidene β-Ketoesters Catalyzed by Chiral Guanidines

A 1,4-addition reaction of 5H-oxazol-4-ones to alkylidene β-ketoesters, which was catalyzed by using chiral guanidines via a dynamic kinetic resolution process that involved geometric isomerization of the alkylidene β-ketoesters, was developed. This method allowed using E,Z isomeric mixture of various acyclic alkylidene β-ketoesters to obtain the products stereoselectively. The relation between the geometry and the stereoselectivity of products was investigated, and the derivatization of the products to the corresponding α-acyl-γ-lactone was performed.

Enantioselective nickel-catalyzed arylative intramolecular 1,4-allylations

The enantioselective desymmetrization of allenyl cyclohexa-2,5-dienones by nickel-catalyzed arylative intramolecular 1,4-allylations is described.

Effect of protic additives in Cu-catalysed asymmetric Diels-Alder cycloadditions of doubly activated dienophiles: towards the synthesis of magellanine-type Lycopodium alkaloids

The pronounced beneficial effect of a precise amount of protic additive in an enantioselective Cu-catalysed Diels-Alder reaction is reported. This reaction, which employs a cyclic alkylidene β-ketoester as a dienophile, represents one of the first examples of a transformation where these extremely versatile, though highly unstable reaction partners participate effectively in catalytic asymmetric cycloaddition with a functionalised diene. The cycloadduct was used as an intermediate towards the synthesis of magellanine-type Lycopodium alkaloids featuring a Stille cross-coupling of a highly congested enol triflate and a unique Meinwald rearrangement/cyclopropanation sequence.

Bismuth-catalyzed allylation of para-quinone methides

An efficient 1,6-allylation addition of para-quinone methides with allylboronic acid pinacol ester was developed with 0.5–5 mol% bismuth(iii) triflate.

Catalytic enantioselective 1,6-conjugate additions of propargyl and allyl groups

Conjugate (or 1,4-) additions of carbanionic species to α,β-unsaturated carbonyl compounds are vital to research in organic and medicinal chemistry, and there are several chiral catalysts that facilitate the catalytic enantioselective additions of nucleophiles to enoates. Nonetheless, catalytic enantioselective 1,6-conjugate additions are uncommon, and ones that incorporate readily functionalizable moieties, such as propargyl or allyl groups, into acyclic α,β,γ,δ-doubly unsaturated acceptors are unknown. Chemical transformations that could generate a new bond at the C6 position of a dienoate are particularly desirable because the resulting products could then be subjected to further modifications. However, such reactions, especially when dienoates contain two equally substituted olefins, are scarce and are confined to reactions promoted by a phosphine-copper catalyst (with an alkyl Grignard reagent, dialkylzinc or trialkylaluminium compounds), a diene-iridium catalyst (with arylboroxines), or a bisphosphine-cobalt catalyst (with monosilyl-acetylenes). 1,6-Conjugate additions are otherwise limited to substrates where there is full substitution at the C4 position. It is unclear why certain catalysts favour bond formation at C6, and-although there are a small number of catalytic enantioselective conjugate allyl additions-related 1,6-additions and processes involving a propargyl unit are non-existent. Here we show that an easily accessible organocopper catalyst can promote 1,6-conjugate additions of propargyl and 2-boryl-substituted allyl groups to acyclic dienoates with high selectivity. A commercially available allenyl-boron compound or a monosubstituted allene may be used. Products can be obtained in up to 83 per cent yield, >98:2 diastereomeric ratio (for allyl additions) and 99:1 enantiomeric ratio. We elucidate the mechanistic details, including the origins of high site selectivity (1,6- versus 1,4-) and enantioselectivity as a function of the catalyst structure and reaction type, by means of density functional theory calculations. The utility of the approach is highlighted by an application towards enantioselective synthesis of the anti-HIV agent (-)-equisetin.

Catalytic Enantioselective Conjugate Additions of (pin)B-Substituted Allylcopper Compounds Generated in situ from Butadiene or Isoprene

Multicomponent catalytic enantioselective transformations that entail the combination of butadiene or isoprene (common feedstock), an enoate (prepared in one step) and B2 (pin)2 (commercially available) are presented. These processes constitute an uncommon instance of conjugate addition of an allyl moiety and afford the desired products in up to 83 % yield and 98:2 enantiomeric ratio. Based on DFT calculations stereochemical models and rationale for the observed profiles in selectivity are provided.

A highly enantioselective Hg(ii)-catalyzed Sakurai–Hosomi reaction of isatins with allyltrimethylsilanes

A chiral complex derived from (S)-difluorophos and Hg(OTf)₂ is identified as a powerful catalyst for the Sakurai–Hosomi reaction of isatins with allyltrimethylsilane, allowing the facile synthesis of valuable building blocks 3-allyl-3-hydroxyoxindoles in up to 97% ee, with only 0.5–1.0 mol% of catalyst loading.

Catalytic enantioselective carboannulation with allylsilanes

The first catalytic asymmetric carboannulation with allylsilanes is presented. The enantioselective [3+2] annulation is catalyzed using a scandium(III)/indapybox complex with tetrakis-[3,5-bis(trifluoromethyl)phenyl]-borate (BArF) to enhance catalytic activity and control stereoselectivity. Functionalized cyclopentanes containing a quaternary carbon center are derived from alkylidene oxindole, coumarin, and malonate substrates with high stereoselectivity. The enantioselective 1,4-conjugate addition and enantioselective lactone formation (by trapping of the β-silyl carbocation) is also described.

Zinc-mediated highly α-regioselective 1,4-addition of chalcones with prenyl bromide in THF

α-Regioselective addition to chalcones was realized by use of in situ generated prenylzinc reagents from zinc and prenyl bromide in the presence of SnCl4 in a 1,4-addition fashion. The approach uses the reagent combination of prenyl bromide, zinc, and SnCl4 in THF, all of which are inexpensive, readily available, and easily removable after the reaction.

Pattern recognition analysis in complex molecule synthesis and the preparation of iso-Diels-Alder motifs

The identification of synthesizable substructural domains within more complex structural targets is of significant value in designing a workable plan of synthesis. We term this process "pattern recognition analysis" (PRA). In this paper we continued to build on the theme of PRA as a potential resource in retrosynthetic blueprints to reach highly challenging targets. The paper operates at two levels. First, there is provided a clear sense of definitions of categories by which patterns are related to hypothetical reaction types. Although the required reaction type may for the moment not exist, we believe that this method of analysis is likely to promote innovation that identifies unmet needs and opportunities to advance the cause of complex target synthesis. In addition, we describe reductions to practice in expanding the menu of achievable patterns. It is likely that the future value of PRA will be associated with its utility in leading the way to new and exploitable chemical innovation.

Enantioselective aza-Michael addition of imides by using an integrated strategy involving the synthesis of a family of multifunctional catalysts, usage of multiple catalysis, and rational design of experiment

A challenging asymmetric reaction (aza-Michael addition of imides to enones) has been optimized through an integrated approach involving the synthesis of a family of organocatalysts, multiple catalysis (usage of additives), and finally with rational exploration of the chemical space by the application of the experiment design.

Catalytic asymmetric [4 + 2] cycloadditions and Hosomi-Sakurai reactions of α-alkylidene β-keto imides

Highly enantioselective catalytic asymmetric reactions of rationally designed α-alkylidene β-keto imides are described. The [4 + 2] cycloadditions and Hosomi-Sakurai reactions of α-alkylidene β-keto imides proceed with high enantioselectivity and yield. The [4 + 2] cycloadditions of the imides with various dienes afford products bearing an all-carbon quaternary stereogenic center at the ring junction. α-Alkylidene β-keto imides should be useful for the enantioselective total synthesis of natural products and other catalytic asymmetric applications.

Asymmetric Diels-Alder and Ficini reactions with alkylidene β-ketoesters catalyzed by chiral ruthenium PNNP complexes: mechanistic insight

Hydride abstraction from the β-position of the enolato ligand of the previously reported complex [Ru(3a-H)(PNNP)]PF(6) (5a; 3a-H is the enolate of 2-tert-butoxycarbonylcyclopentanone) with (Ph(3)C)PF(6) gives the dicationic complex [Ru(6a)(PNNP)](2+) (7a) as a single diastereoisomer, which contains the unsaturated β-ketoester 2-tert-butoxycarbonyl-2-cyclopenten-1-one (6a) as a chelating ligand. The methyl analogue 2-methoxycarbonylcyclopentanone (3b) gives [Ru(3b-H)(PNNP)]PF(6) as a mixture of noninterconverting diastereoisomers (ester group of 3b trans to P, 5b; or to N, 5c), which were separated by column chromatography. Hydride abstraction from 5b (or 5c) yields diastereomerically pure [Ru(6b)(PNNP)](2+) (7b or 7c). Complexes 7b and 7c do not interconvert at room temperature in CD(2)Cl(2) and form opposite enantiomers of the Diels-Alder adduct upon reaction with Dane's diene (1 equiv). X-ray studies of 7a, 5b, and 5c give insight into the origin of enantioselection and the sense of asymmetric induction in the previously reported asymmetric Diels-Alder and Ficini cycloaddition reactions with 2,3-disubstituted butadienes and ynamides, respectively. Stoichiometric reactions (substrate coordination, cycloaddition, and product displacement) between [Ru(OEt(2))(2)(PNNP)](2+) (2), 6b (or 6a), and Dane's diene (15, to give estrone derivatives) or N-benzyl-N-(cyclohexylethynyl)-4-methylbenzenesulfonamide (17, to give cyclobutenamides) suggest that product displacement from the catalyst is turnover limiting.

Enantioselective conjugate allylation of cyclic enones

Enantioselective organocatalytic 1,2-allylation of a cyclic enone followed by anionic oxy-Cope rearrangement delivered the ketone as a mixture of diastereomers. This appears to be a general method for the net enantioselective conjugate allylation of cyclic enones.