Asymmetric bromine-lithium exchange: on the importance of both the diamine ligand and the organolithium reagent.

The asymmetric bromine-lithium exchange on a series of prochiral biphenyls was investigated. As in many asymmetric organolithium reactions, the chiral ligand (here a diamine) was considered to be the enantiocontrolling element. We show here that the organolithium reagent also plays an important role.

Formation of contiguous quaternary and tertiary stereocenters by sequential asymmetric conjugate addition of Grignard reagents to 2-substituted enones and Mg-enolate trapping.

Herein a comprehensive study is provided on the asymmetric conjugate addition (ACA) of Grignard reagents to α-substituted cyclic enones. After the elucidation of the optimal experimental conditions, the scope of Grignard reagents and Michael acceptors was examined. Whereas secondary Grignards gave better enantioselectivities with 2-cyclopentenones, both linear and branched Grignard reagents were tolerated for the ACA to 2-methylcyclohexenone. The sequential ACA-enolate trapping, which leads to quaternary stereocenters, was then studied. Thus, many electrophiles have been tested, thereby giving rise to highly functionalized cyclic ketones with contiguous α-quaternary and ß-tertiary centers. The present technique is believed to bring a new approach to versatile terpenoid-like skeletons of bioactive natural products. Straightforward derivatizations of enantioenriched saturated cyclic ketones further support the potential of the present methodology in synthesis.

Enantioselective halogenative semi-pinacol rearrangement: extension of substrate scope and mechanistic investigations.

The present Full Paper article discloses a survey of our recent results obtained in the context of the enantioselective halogenation-initiated semi-pinacol rearrangement. Commencing with the fluorination/semi-pinacol reaction first and moving to the heavier halogens (bromine and iodine) second, the scope and limitations of the halogenative phase-transfer methodology will be discussed and compared. An extension of the fluorination/semi-pinacol reaction to the ring-expansion of five-membered allylic cyclopentanols will be also described, as well as some preliminary results on substrates prone to desymmetrization will be given. Finally, the present manuscript will culminate with a detailed mechanistic investigation of the canonical fluorination/semi-pinacol reaction. Our mechanistic discussion will be based on in situ reaction progress monitoring, complemented with substituent effect, kinetic isotopic effect and non-linear behaviour studies.

Copper-catalyzed propargylic substitution of dichloro substrates: enantioselective synthesis of trisubstituted allenes and formation of propargylic quaternary stereogenic centers.

An easy and versatile Cu-catalyzed propargylic substitution process is presented. Using easily prepared prochiral dichloro substrates, readily available Grignard reagents together with catalytic amount of copper salt and chiral ligand, we accessed a range of synthetically interesting trisubstituted chloroallenes. Substrate scope and nucleophile scope are broad, providing generally high enantioselectivity for the desired 1,3-substitution products. The enantioenriched chloroallenes could be further transformed into the corresponding trisubstituted allenes or terminal alkynes bearing all-carbon quaternary stereogenic centers, through the copper-catalyzed enantiospecific 1,1/1,3-substitutions. The two successive copper-catalyzed reactions could be eventually combined into a one-pot procedure and different desired allenes or alkynes were obtained respectively with high enantiomeric excesses.

Enantioselective halogenative semi-pinacol rearrangement: a stereodivergent reaction on a racemic mixture.

An efficient, quantitative deracemization strategy for optically inactive allylic cycloalkanols has been achieved using the biphasic halogenative semi-pinacol reaction protocol. The resultant ß-halo spiroketones, containing three contiguous stereogenic centers, were easily recovered with high diastereomeric and enantiomeric purities following conventional silica gel chromatography. The optically active products could be further manipulated chemically, affording synthetically interesting scaffolds with complete preservation of stereoisomeric integrity.

Enantioselective catalytic fluorinative aza-semipinacol rearrangement.

An efficient and highly stereoselective fluorinative aza-semipinacol rearrangement is described. The catalytic reaction requires use of Selectfluor in combination with the chiral, enantiopure phosphate anion derived from acid L3. Under optimized conditions, cyclopropylamines A were transformed into ß-fluoro cyclobutylimines B in good yields and high levels of diastereo- and enantiocontrol. Furthermore, the optically active cyclobutylimines were reduced diastereoselectively with L-Selectride in the corresponding fluorinated amines C, compounds of significant interest in the pharmacological industry.

Domino asymmetric conjugate addition-conjugate addition.

Enantioenriched Al-, Mg-, and Zn-enolates undergo electrophilic trapping by nitroolefins and vinylsulfones to afford 1,4-diketones and 2-(bis(phenylsulfonyl)ethyl)ketones in good yield and excellent diastereoselectivity. A one-pot preparation of indenes and enantiopure syntheses of tetrahydrobenzofurans, tetrahydrobenzopyrroles, and azulenes are disclosed. A site-selective two-step sequence of three conjugate additions is also demonstrated.

Asymmetric conjugate addition to α-substituted enones/enolate trapping.

An NHC-Cu complex catalyzed the asymmetric conjugate addition (ACA) of various Grignard reagents to nonactivated α-substituted cyclic enones to give 2,3-dialkylated cyclopentanones and cyclohexanones. The Michael addition features the formation of a magnesium enolate intermediate. One-pot diastereoselective trapping of this enolate by alkyl, propargyl, allyl, and benzyl halides led to ketones with contiguous α-quaternary and ß-tertiary centers.

Copper-free asymmetric allylic alkylation of trisubstituted cyclic allyl bromides using Grignard reagents.

AAA: The asymmetric allylic alkylation (AAA) of trisubstituted cyclic allyl bromides with Grignard reagents is catalytic (2 mol % of ligand) and regioselective (SN 2'/SN 2=91:9→100:0). The quaternary carbon centers are formed with good to high enantioselectivity (e.r.=81.5:19.5→96:4).

Enantioselective organocatalytic iodination-initiated Wagner-Meerwein rearrangement.

The present manuscript describes a high-yielding enantioselective semipinacol transposition, initiated by an electrophilic iodination event. The title transformation makes use of the anionic phase-transfer catalysis (PTC) paradigm for chirality induction. Thus, when combined appropriately, the insoluble cationic iodinating reagent S9 and the lipophilic phosphoric acid L9 act as an efficient source of chiral iodine that performs the semipinacol transposition of strained allylic alcohols A(x) to ß-iodo spiroketones B(x) in good yields and with high levels of diastereo- and enantio-induction. The product ß-iodo spiroketones could be derivatized stereospecifically and without stereoerosion, giving rise to products inaccessible directly from a semipinacol rearrangement.

Formation of quaternary stereogenic centers by copper-catalyzed asymmetric conjugate addition reactions of alkenylaluminums to trisubstituted enones.

Alkenylaluminums undergo asymmetric copper-catalyzed conjugate addition (ACA) to ß-substituted enones allowing the formation of stereogenic all-carbon quaternary centers. Phosphinamine-copper complexes proved to be particularly active and selective compared with phosphoramidite ligands. After extensive optimization, high enantioselectivities (up to 96% ee) were obtained for the addition of alkenylalanes to ß-substituted enones. Two strategies for the generation of the requisite alkenylaluminums were explored allowing for the introduction of aryl- and alkyl-substituted alkenyl nucleophiles. Moreover, alkyl-substituted phosphinamine (SimplePhos) ligands were identified for the first time as highly efficient ligands for the Cu-catalyzed ACA.

Enantioselective 1,6-conjugate addition of dialkylzinc reagents to acyclic dienones catalyzed by Cu-DiPPAM complex-extension to asymmetric sequential 1,6/1,4-conjugate addition.

CC coupling: DiPPAM 1 and BINAP 2 ligands led to divergent behaviors in the asymmetric conjugate addition (ACA) of dialkylzinc reagents to linear aryldienones, which were applied to the development of a highly selective sequential asymmetric 1,6/1,4-ACA process (see scheme; Tf = triflate, DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene).

Asymmetric bromine-lithium exchange: application toward the synthesis of natural product.

Asymmetric bromine-lithium exchange has been successfully employed to synthesize bicoumarin chiral building blocks of (+)-isokotanin A and (-)-kotanin in good yields and with an excellent level of enantioselectivity. This is the first reported example of formal syntheses, using this direct methodology, leading to the single (M)-atropoisomer of (+)-isokotanin A and (-)-kotanin building blocks, without any resolution step.

Enantio- and regioselective conjugate addition of organometallic reagents to linear polyconjugated nitroolefins.

The copper-catalysed conjugate addition of trialkylaluminium and dialkylzinc reagents to polyconjugated nitroolefins (nitrodiene and nitroenyne derivatives) is reported. A reversed Josiphos ligand L7 allows for the selective 1,4- or 1,6-addition with high enantioselectivities.

Organocatalytic asymmetric annulation of 1,3-bis(alkoxycarbonyl)buta-1,3-dienes and aldehydes.

Asymmetric organocatalytic annulation of E/Z isomeric mixtures of bis(alkyl carboxylate)buta-1,3-dienes and aldehydes has been realized via enamine catalysis. In the presence of α,α-diphenyl-2-pyrrolidinemethanol trimethylsilyl ether, excellent stereo- and enantioselectivities were achieved for a broad spectrum of substrates.

Direct trapping of sterically encumbered aluminum enolates.

The formation of chiral and sterically congested cyclohexanone derivatives has been achieved through a multistep sequence with one single purification step. (n-Butoxymethyl)-diethylamine was identified as a highly efficient reagent for the direct trapping of aluminum enolates. The Lewis acidic character of aluminum suffices to activate the α-aminoether to form in situ an electrophilic iminium species. In return the aluminum enolate is rendered more nucleophilic by coordination of the butoxy group and formation of an aluminate.

Highly diastereo- and enantioselective organocatalytic domino Michael/aldol reaction of acyclic 3-halogeno-1,2-diones to α,ß-unsaturated aldehydes.

The first organocatalytic diastereo- and enantioselective domino Michael/aldol reaction of 3-halogeno-1,2-diones to α,ß-unsaturated aldehydes has been achieved. This transformation tolerates a large variety of electronically different substituents on both reactive partners and allows the synthesis of challenging cyclopentanone derivatives with four contiguous stereogenic centers in excellent diastereoselectivities (>20:1 dr) as well as good yields (69-97%), and enantioselectivities (up to 94% ee).

Creation of highly congested quaternary centers via Cu-catalyzed conjugate addition of alkenyl alanates to ß-substituted cyclic enones.

Easily prepared alkenylalanates proved to be excellent nucleophiles for the creation of highly congested quaternary centers via copper-catalyzed conjugate addition. In addition, functionalized cis-decaline systems can now be prepared in a simple two-step sequence involving Cu-catalyzed conjugate addition with functionalized alkenylalanates.