Regioselective lithiation of aryl benzyl ethers

The lithiation of a series of aryl benzyl ethers containing -F and -OMe groups has been studied. It was found that the presence of one or two fluorine atoms in the meta position relative to the oxygen atom prevents the lithiation at the benzyl carbon atom. 3-(3′,5′-Dimethoxybenzyloxy)fluorobenzene was lithiated and next reacted with DMF to give 2-(3′,5′-dimethoxybenzyloxy)-6-fluorobenzaldehyde. A three-step procedure was applied to remove three hydrogen atoms from 1,3-difluoro-5-(3′,5′-dimethoxybenzyloxy)benzene and replace them with methyl groups by reactions of the lithiated compounds with MeI.     

Highly regioselective lithiation of inter-ring carbon of bis(thien-2yl)methane: a general meso-elaboration protocol

Bis(thien-2yl)methane is regioselectively lithiated at the inter-ring methylene carbon using dimsyl anion in THF at 0 °C; quenching with appropriate electrophile furnishes meso-elaborated derivatives, exclusively with synthetic advantage.     

Formation of mixed aryl–, alkyl–lithium aggregates in the heteroatom assisted lithiation of α,α′-dialkyl substituted 1,3-bis[(dimethylamino)methyl]benzene

The heteroatom assisted lithiation of 1,3-bis[1-(dimethylamino)ethyl]benzene with n-BuLi afforded 2,6-bis[1-(dimethylamino)ethyl]phenyllithium. An X-ray crystal structure determination revealed a dimeric aggregate in which the four benzylic chiral centers are identical, pointing to stereoselective crystallization. In contrast, reaction of 1,3-bis[1-(dimethylamino)propyl]benzene with n-BuLi afforded a dimeric aggregate comprising the parent lithiated compound and n-BuLi in a 1:1 molar ratio. The four Li atoms and the four bridging carbon atoms are arranged in a unique ladder-type C–Li₂–C₂′–Li₂–C framework.     

A practical synthesis of protected β-homolysine

Protected β-homolysine of high enantiomeric purity (ee>99.5%) is prepared utilizing the stereoselective conjugate addition of lithiated (S)-(α-methylbenzyl)benzylamide to (E)-7-(tosyloxy)hept-2-enoic acid tert-butyl ester, followed by subsequent ammonia substitution, Boc protection and removal of the auxiliary.     

Stereoselective lithiation and alkylation and aldol reactions of chiral 5-methyl-3-(o-aryl)-oxazolidinones

Alkylation and aldol reactions carried out on axially chiral, diastereoenriched oxazolidinedione derivatives were found to be highly stereoselective with respect to substitution at the 5-position of the oxazolidindione ring. Thus, diastereoenriched 5-methyl-3-(o-iodophenyl)-2-thioxo-4-oxazolidinone and 5-methyl-3-(o-aryl)-2,4-oxazolidinediones were prepared and lithiated at C-5 of the oxazolidine ring and the subsequent electrophilic quench with alkyl halides or benzaldehyde yielded products with the same diastereomer ratios as the starting materials.     

Synthesis of (+)-laurencin via ring expansion of a C-glycoside derivative

Laurencin was efficiently synthesized from a C-glycoside derivative based on ring expansion of the oxane part of the starting compound into an eight-membered cyclic ether via a ring-cleavage/ring-closing olefin metathesis process, stereoselective introduction of a bromo group at C4, and convergent construction of the side-chain part using a lithiated enyne unit.     

Acyclic ene-carbamate. A useful tool for an original synthesis of phosphine-containing α-amino acids bearing a quaternary carbon

A novel and efficient approach for the synthesis of phosphine-containing α-amino acids bearing quaternary carbon is described. The key step involves the original nucleophilic addition of lithiated phosphines onto acyclic ene-carbamates concomitant with a spontaneous internal (N→C) alkyloxycarbonyl migration.     

Unexpected Products from Carbonylation of Lithiated Quinazolin-4(3H)-one Derivatives

Doubly lithiated 3-pivaloylaminoquinazolin-4(3H)-one reacts with carbon(II) oxide at 0°C to give 77% of a mixture of azetidinone and indole derivatives, each incorporating a diisopropylamide unit from lithium diisopropylamide used for lithiation. No analogous reaction occurs with doubly lithiated 3-acetylaminoquinazolin-4(3H)-one and 3-acyl-2-alkylquinazolin-4(3H)-one. Carbonylation of doubly lithiated 2-alkyl-3-aminoquinazolin-4(3H)-ones at 0°C results in deamination to give 2-alkylquinazolin-4(3H)-ones in good yields.     

Isoprene functionalisation : Chlorination of C10-isoprenoid sulfones with hexachloro-ethane

Lithiation of the sulfones 1–3 in THF at ?78° with lithium diisopropyl amide (LDA) is regioselective, since deuteration of the α-lithiosulfone 4 results in formation of the α-monodeuteriated sulfone 7. Higher temperature causes an intramolecular 1,4-addition of the lithiated sulfone 4 to the lithiated cyclic sulfone 19 and an intermolecular 1,4-addition of the lithiated sulfones 5 and 6. The cyclisation of Z-sulfone 1 has been used for the isolation of the isomeric E-sulfone 2 from mixtures of 1 and 2. The lithiated sulfones 4–6 are chlorinated with hexachloroethane (HCE). Due to acid/base reactions the α,α-dichlorinated cyclic sulfone 23 and α,α'-dichlorinated butadienyl sulfones 13–15 are formed in small amounts.     

Synthesis of P-chiral phosphines via chiral metal template promoted asymmetric furan Diels-Alder reaction

The organoplatinum complex containing ortho-metalated (S)-(1-(dimethylamino)ethyl)-naphthalene as the chiral auxiliary has been used to promote the asymmetric [4+2] Diels-Alder reaction between phenyldivinylphosphine and 2-diphenylphosphinofuran. The reaction was complete in 6 days at room temperature, with the formation of four isomeric diphosphino-substituted oxanorbornene metal complexes in the ratio of 4:2:2:1. Only the exo-cycloaddition products were formed. The formation of stereogenic carbon centers within the oxanorbornene skeleton are highly stereoselective, with all four cycloadducts adopting the same absolute configurations. However, the stereocontrol at the external phosphorus stereogenic center is less efficient (S_p:R_p = 2:1 for the template cycloadducts). The chiral naphthylamine auxiliary could be removed chemoselectively by treatment with concentrated hydrochloric acid, and further ligand liberation of the dichloro complexes with aqueous cyanide gave the diphosphino-substituted oxanorbornene ligands. Hydrogenation of the double bonds in the cycloadduct stabilizes the phosphorus stereogenic center of the free diphosphine ligand which otherwise undergoes inversion of absolute configuration.     

Selective lithiation of bis(furan-2-yl)methane: an efficient protocol for novel meso-functionalised synthons

Bis(furan-2-yl)methane can be lithiated at the inter-ring carbon atom (meso-position) to give carbanions, which react with a variety of electrophiles to yield meso-elaborated derivatives in high yield and regioselectivity. This constitutes the first general approach to the title compounds.     

Lithiation of 2-(chloroaryl)-2-methyl-1,3-dioxolanes and application in synthesis of new ortho-functionalized acetophenone derivatives

2-(4-Chlorophenyl)-2-methyl-1,3-dioxolane 2a was lithiated ortho to the ketal group by treatment with butyllithium in THF at 0°C. Related 2-aryl-2-methyl-1,3-dioxolanes possessing a chlorine substituent at the meta position of the aryl group 2b,c were lithiated with butyllithium in THF at −78°C at the position between the two directing groups. The lithio species thus generated were treated with various electrophiles to give ortho-functionalized acetophenone derivatives.     

An easy access to functionalized allyl dithiocarbamates from Baylis-Hillman adducts in water

A facile and direct highly stereoselective synthesis of [E]- and [Z]-allyl dithiocarbamates has been accomplished from acetates of Baylis-Hillman (BH) adducts in catalyst-free one-pot three-component coupling reactions of carbon disulfide and amine in water under a mild and green procedure with high yields. The reaction pathway involves the nucleophilic displacement (SN2′) of BH acetates by dithiocarbamate anions. The utility of these allyl dithiocarbamates has also been demonstrated in heterocyclic chemistry. © 2009 Elsevier Science. All rights reserved     

A convenient approach to an advanced intermediate for (+)-lactacystin synthesis

A fully stereoselective preparation of the advanced intermediate 24 for the synthesis of (+)-lactacystin from known 1,2:5,6-di-O-isopropylidene-α-d-gulofuranose (2), as the source of chirality, has been achieved. The C-5 methyl group was introduced via a Wittig olefination followed by Pd/C-mediated hydrogenation of the conformationally restricted alkene 11 in a highly stereoselective manner. The stereogenic tetrasubstituted carbon centre at C-3, with an amino group, was installed stereoselectively via an Overman rearrangement, which was efficiently controlled by a saccharide environment.     

Nucleophilic additions of lithiated allylphenylsulfone to nitrones: experimental and theoretical investigations

The nucleophilic addition of lithiated allylphenylsulfone to nitrones at −80 °C proceeds exclusively α to the phenylsulfonyl group affording anti adducts in high yield. At 0 °C isoxazolidines are obtained with complete all-trans selectivity. The formation of these compounds involves isomerization of the allylsulphonyl moiety to give a transient vinylsulfone that then undergoes a subsequent intramolecular Michael addition. The addition to several nitrones has been studied and theoretical calculations have been refined to accurately explain the selectivity of the allylation reaction.     

Application of Polyamines and Amino Acid Derivatives Based on 2-Azabicycloalkane Backbone in Enantioselective Aldol Reaction

Carbon–carbon bond forming reactions, such as aldol reaction and condensation, belong to extremely desired transformations as manifested by >25,000 entries in SciFinder. Their stereoselective variant requires the use of an appropriate catalyst with a strictly defined structure. Hence, chiral 2-azabicycloalkane-based catalysts were designed, synthesized and tested in a stereoselective aldol reaction between cyclic/acyclic ketone and p-nitrobenzaldehyde both in organic and aqueous media. Among catalysts containing a chiral bicyclic backbone, amide based on 2-azabicyclo[3.2.1]octane and pyrrolidine units showed the best catalytic activity and afforded aldol product in excellent chemical yields (up to 95%) and good diastereo- and enantioselectivity (dr 22:78, ee up to 63%).     

3-(o-Trifluoroacetamidoaryl)-1-propargylic esters: common intermediates for the palladium-catalyzed synthesis of 2-aminomethyl-, 2-vinylic, and 2-alkylindoles

3-(o-Trifluoroacetamidoaryl)-1-propargylic esters have been used as common synthetic intermediates for the preparation of a variety of 3-unsubstituted 2-substituted indoles. Treating ethyl 3-(o-trifluoroacetamidoaryl)-1-propargylic carbonates unsubstituted or containing an aryl substituent at the propargylic carbon with piperazines and Pd(PPh₃)₄ in THF at 80 °C affords 2-(piperazin-1-ylmethyl)indoles in excellent yields. Good to excellent yields of 2-aminomethylindoles are also obtained with other secondary amines. Ethyl 3-(o-trifluoroacetamidoaryl)-1-propargylic carbonates bearing an alkyl substituent at the propargylic carbon and ethyl 3-(o-trifluoroacetamidoaryl)-1-propargylic acetates disubstituted at the propargylic carbon give 2-vinylic indoles with the Pd(OAc)₂/PPh₃ combination and Et₃N in THF at 80 °C. Formation of 2-vinylic indoles is quite stereoselective, generating trans vinylic derivatives, at least with the substrates that we have investigated. In the presence of formic acid, Et₃N, and Pd(PPh₃)₄ in MeCN at 80 °C, ethyl 3-(o-trifluoroacetamidoaryl)-1-propargylic carbonates afford 2-alkylindoles in good to excellent yields.     

Stereospecific nucleophilic substitution at tertiary and quaternary stereocentres

Nucleophilic substitution reactions have always been considered as one of the most powerful reactions for the creation of carbon–carbon or carbon–heteroatom bonds in organic synthesis. In contrast to secondary carbons, the steric shielding of tertiary carbons retards a concerted, stereospecific nucleophilic substitution, and ionizing pathways often lead to nonselective substitution due to ion pair dissociation. In this minireview, we will detail pioneering contributions and more recent achievements emphasizing the feasibility of nucleophilic substitution on tertiary stereocentres under certain conditions, with inversion of configuration. The development of these transformations at tertiary centres are of remarkable added value to practitioners in the field of complex molecule synthesis. A stereoselective substitution at a quaternary carbon stereocentre with inversion of configuration is also discussed in the case of a three-membered ring.

In this minireview, we summarized the fascinating and rich area of stereoselective or stereospecific nucleophilic substitution at tertiary and quaternary carbon centres.     

One-pot sequential cross-metathesis/hydride reduction: highly stereoselective synthesis of primary (E)-allylic alcohols from terminal olefins

Several di- and trisubstituted primary (E)-allylic alcohols have been prepared from the corresponding terminal olefins in a highly stereoselective manner (>20:1 E/Z) by sequencing olefin cross-metathesis (CM) with hydride reduction (DIBAL-H) in good yields utilizing only commercially available reagents in a one-pot fashion. The method is a reliable alternative to the direct CM of terminal olefins with allyl alcohol, which is not always stereoselective but rather highly substrate dependent.     

A new approach to a vitamin D ring C/D building block from the Hajos dione, involving epoxide opening at the more substituted carbon atom

A new approach to a trans-hydrindane building block for 1α,25-dihydroxyvitamin D₃ (calcitriol) synthesis from the Hajos dione has been developed. The α,β-unsaturated carbonyl group in the Hajos dione has been protected and the carbonyl group in the five-membered ring has been used for the side chain attachment. Key steps in the six-membered ring transformation include: reduction of the carbonyl group, stereoselective epoxidation of the double bond in the allylic alcohol, opening of the epoxide at the more substituted carbon atom and regioselective deoxygenation.