The synthetic protocol for α-bromocarbonyl compounds via brominations

α-Bromocarbonyl compound easily generates α-radicals in the presence of a proper initiator, such as copper salt. Recently, tertiary-alkylation reaction using α-bromocarbonyl compound as a tertiary alkyl source is recognized as one of the most important alkylation reactions. The reactions using α-bromocarbonyl compound are increasing, whereas synthetic methods for various functionalized α-bromocarbonyl compounds are not summarized. Generally, α-bromocarbonyl compounds can be synthesized from the corresponding carboxylic acids via α-bromo acid bromide, but the brominations of carboxylic acids are sometimes problematic. In this paper, we will report some technical information for a bromination and synthetic examples of representative α-bromocarbonyl compounds.     

Direct generation of nucleophilic chiral palladium enolate from 1,3-dicarbonyl compounds: catalytic enantioselective Michael reaction with enones

Generation of chiral palladium enolates from 1,3-dicarbonyl compounds with the palladium aqua complex and its application to the highly efficient catalytic enantioselective Michael reaction with enones are described. The palladium aqua complexes are likely to supply Br?nsted base and Br?nsted acid successively during the reaction. The former activates the carbonyl compounds to give chiral palladium enolates, and the latter cooperatively activates enones. Using a catalytic amount (2-10 mol %) of the palladium complexes, the various 1,3-dicarbonyl compounds including diketones and beta-ketoesters were converted to the desired Michael adducts in good yields (69-92%) with excellent enantiomeric excesses (89-99% ee).     

Cross-coupling reaction of alpha-chloroketones and organotin enolates catalyzed by zinc halides for synthesis of gamma-diketones

The reaction of tin enolates 1 with alpha-chloro- or bromoketones 2 gave gamma-diketones (1,4-diketones) 3 catalyzed by zinc halides. In contrast to the exclusive formation of 1,4-diketones 3 under catalytic conditions, uncatalyzed reaction of 1 with 2 gave aldol-type products 4 through carbonyl attack. NMR study indicates that the catalyzed reaction includes precondensation between tin enolates and alpha-haloketones providing an aldol-type species and their rearrangement of the oxoalkyl group with leaving halogen to produce 1,4-diketones. The catalyst, zinc halides, plays an important role in each step. The carbonyl attack for precondensation is accelerated by the catalyst as Lewis acid and the intermediate zincate promotes the rearrangement by releasing oxygen and bonding with halogen. Various types of tin enolates and alpha-chloro- and bromoketones were applied to the zinc-catalyzed cross-coupling. On the other hand, the allylic halides, which have no carbonyl moiety, were inert to the zinc-catalyzed coupling with tin enolates. The copper halides showed high catalytic activity for the coupling between tin enolates 1 and organic halides 7 to give gamma,delta-unsaturated ketones 8 and/or 9. The reaction with even chlorides proceeded effectively by the catalytic system.     

A one-pot silyl-Reformatsky olefination

A novel one-pot olefination reaction has been developed, involving the stereoselective formation of (E)-α,β-unsaturated esters/ketones from the reaction of α-bromocarbonyl compounds with aromatic aldehydes. The reactions use a reagent combination of trichlorosilane, and triethylamine and may proceed via the in situ formation of a trichlorosilyl ketene acetal. The general procedure offers key advantages (high conversions, low quantities of organic soluble by-products) over the conventional Wittig reaction.     

Cross-coupling reactions of two different activated alkenes through tetrabutylammonium fluoride (TBAF) promoted deprotonation/activation strategy: a regioselective construction of quaternary carbon centers

A novel TBAF-promoted intermolecular crossed-conjugate addition has been developed. For a range of cyclic β-halo-α,β-unsaturated carbonyl compounds, the vinylogous enolates generated by deprotonation at the γ-position preferentially reacted with Michael acceptors at the α-position to deliver cross-coupling products in good yields.     

Asymmetric synthesis of α,β-diamino acid derivatives with an aziridine-, azetidine- and γ-lactone-skeleton via Mannich-type additions across α-chloro-N-sulfinylimines

The efficient asymmetric synthesis of new chiral γ-chloro-α,β-diamino acid derivatives via highly diastereoselective Mannich-type reactions of N-(diphenylmethylene) glycine esters across a chiral α-chloro-N-p-toluenesulfinylimine was developed. The influence of the base, LDA or LiHMDS, used for the formation of the glycine enolates, was of great importance for the anti-/syn-diastereoselectivity of the Mannich-type reaction. The γ-chloro-α,β-diamino acid derivatives proved to be excellent building blocks for ring closure towards optically pure anti- and syn-β,γ-aziridino-α-amino esters, and subsequent ring transformation into trans-3-aminoazetidine-2-carboxylic acid derivatives and α,β-diamino-γ-butyrolactones.     

Enantioselective synthesis of 2-substituted-1,4-diketones from (S)-mandelic acid enolate and α,β-enones

An approach for the synthesis of chiral non-racemic 2-substituted-1,4-diketones from (S)-mandelic acid and α,β-enones has been developed. The reaction of lithium enolate of the 1,3-dioxolan-4-one derived from optically active (S)-mandelic acid and pivalaldehyde with α,β-unsaturated carbonyl compounds proceeds readily to give the corresponding Michael adducts in good yields and with high diastereoselectivities. The addition of HMPA (3 equiv) reverses and strongly enhances the diastereoselectivity of the reaction. A change in the reaction mechanism from a lithium catalyzed to the one where catalysis has been suppressed by coordination of HMPA to lithium is proposed to explain these results. Subsequent basic hydrolysis of the 1,3-dioxolan-4-one moiety yields the corresponding α-hydroxy acids (in hemiacetal form), which after decarboxylation with oxygen in the presence of pivalaldehyde and the Co(III)Me₂opba complex as catalyst give chiral 2-substituted 1,4-dicarbonyl compounds with very high enantiomeric excesses. In this approach, (S)-mandelic acid acts as an umpoled chiral equivalent of the benzoyl anion.     

The tin(II) enolate addition reactions to α, β-unsaturated ketones and quinones

The reaction of tin(II) enolates with various α ,β-unsaturated carbonyl compounds is examined. When TMSCl is added as an activator of the α,β-unsaturated ketones, the Michael addition reaction proceeds smoothly to give the corresponding 1,4-adduct in good yield. When 1,4-benzoquinone and its mono-imino derivative are employed as acceptors in conjunction with dichloromethylsilane- DMAP activator, a novel addition-reduction reaction takes place to afford the α-arylcarbonyl compounds.     

Axially chiral guanidine as enantioselective base catalyst for 1,4-addition reaction of 1,3-dicarbonyl compounds with conjugated nitroalkenes

A new strategy for designing chiral guanidine molecules is presented, which features the introduction of an axially chiral binaphthyl backbone. The axially chiral guanidine catalysts thus developed facilitated the highly enantioselective 1,4-addition reaction of 1,3-dicarbonyl compounds with a broad range of conjugated nitroalkenes and showed extremely high catalytic activity.     

Synthesis of 3-acyl-5,6-dihydro-1,4-oxathiines through ring expansion of thiiranes

Synthesis of 3-acyl-5,6-dihydro-1,4-oxathiines has been achieved via reactions of thiiranes and α-diazo-β-1,3-dicarbonyl compounds under microwave and copper sulfate-assisted conditions. The current method provides a direct and simple strategy in efficient preparation of 3-acyl-1,4-oxathiines from readily available thiiranes and trans-3-acyl-5,6-dihydro-1,4-oxathiines as stereospecific products for 1,2-disubstituted cis-thiiranes. The reaction mechanism was also proposed.     

Bifunctional transition metal-based molecular catalysts for asymmetric syntheses

The discovery and development of conceptually new chiral bifunctional transition metal-based catalysts for asymmetric reactions is described. The chiral bifunctional Ru catalyst was originally developed for asymmetric transfer hydrogenation of ketones and imines and is now successfully applicable to enantioselective C-C bond formation reaction with a wide scope and high practicability. The deprotonation of 1,3-dicarbonyl compounds with the chiral amido Ru complexes leading to the amine Ru complexes bearing C- or O-bonded enolates, followed by further reactions with electrophlies gives C-C bond formation products. The present bifunctional Ru catalyst offers a great opportunity to open up new fundamentals for stereoselective molecular transformation including enantioselective C-H and C-C as well as C-O, C-N bond formation.     

Conversion of 1,3- into 1,4-dicarbonyl compounds by means of α-phosphoryl sulphides. Total synthesis of dihydrojasmone and methylenomycin B.

An effective synthesis of 1,4-dicarbonyl systems involving the Horner—Wittig reaction of α-phosphoryl sulphides with the half-protected 1,3-dicarbonyl compounds followed by hydrolysis of the vinyl sulphides formed is described. The total synthesis of dihydrojasmone and methylenomycin B employing this approach is reported.     

A mild and efficient direct α-amination of β-dicarbonyl compounds using iodosobenzene and p-toluenesulfonamide catalyzed by perchlorate zinc hexahydrate

A direct α-amination of β-dicarbonyl compounds has been achieved by using iodosobenzene (PhIO) as an oxidant and p-toluenesulfonamide (TsNH(2)) as an aminating reagent in the presence of a catalytic amount of perchlorate zinc hexahydrate. The present amination reaction proceeds quickly at rt (<30 min needed for most tested substrates) to provide the corresponding α-N-tosylamido β-dicarbonyl compounds in high to excellent yields.     

An effective new synthesis of 2-aminopyrrole-4-carboxylates

Efficient syntheses of 2-aminopyrroles are presented starting from β-dicarbonyl compounds, bromoacetonitrile, and amines. Alkylation of β-dicarbonyl compounds with bromoacetonitrile furnished α-cyanomethyl-β-dicarbonyl compounds. The condensation reaction of α-cyanomethyl-β-dicarbonyl compounds with amines catalyzed by p-TsOH affords the corresponding enamines in good yields. Base catalyzed cyclization via the addition of an amine moiety to the carbon-nitrogen triple bond of nitrile furnished 2-aminopyrroles in high yields.     

Trihaloisocyanuric acids as convenient reagents for regioselective halogenation of β-dicarbonyl compounds

The reaction of β-dicarbonyl compounds (β-ketoesters and β-diketones) with 0.34 mol equiv of trichloro- and tribromoisocyanuric acids produced regioselectively the corresponding α-monohalo β-dicarbonyl compound. On the other hand, utilization of 0.68 mol equiv of the trihaloisocyanuric acid produced the α,α-dihalo β-dicarbonyl compound.     

A new method for synthesis of α,α-disubstituted carbonyl compounds from carbonyl compounds with one-carbon homologation through β-oxido carbenoid rearrangement as the key reaction

The addition reaction of carbonyl compounds with lithium α-sulfinyl carbanions of 1-chloroalkyl p-tolyl sulfoxides gave adducts, α-chloro β-hydroxy sulfoxides, in high to quantitative yields. The adducts were first treated with a base to give alkoxides, which were treated with i-PrMgCl or t-BuLi to give β-oxido carbenoids via a sulfoxide-metal exchange reaction. The β-oxido carbenoid rearrangement then took place to afford the enolates with one-carbon elongation. The enolate intermediates were found to be able to be trapped with electrophiles such as aldehydes, ethyl chloroformate, benzoyl chloride, haloalkanes to give α,α-disubstituted carbonyl compounds in moderate to good yields. This method provides a new and efficient way for synthesis of α,α-disubstituted carbonyl compounds from carbonyl compounds with one-carbon homologation in only two synthetic operations.     

Diastereo- and Enantioselective 1,4-Difunctionalization of Borylenynes by Catalytic Conjunctive Cross-Coupling

Enantioselective conjunctive cross-coupling of enyne-derived boronate complexes occurs with 1,4 addition of the electrophile and migrating group across the π system. This reaction pathway furnishes α-boryl allenes as the reaction product. In the presence of a chiral catalyst, both the central and axial chirality of the product can be controlled during product formation.     

Stereoselective kinetic protonation of chiral γ-lactone enolates

When lithium enolates generated from five chiral α, γ-disubstituted γ-lactones are treated with proton sources, formation of the α, γ-syn epimers always predominates over the a,y-anti epimers in ratios of 10.2–2.9:1. Higher syn/anti ratios are obtained via silyl enolates in some cases. Chirality discrimination by chiral proton source is also observed in particular cases.     

Inorganic chemistry of vitamin B12; by J.M. Pratt. Academic Press,London and New York, 1972, £6.00 net

Action of dimethyllithium cuprate on α,α-dichloro esters at room temperature leads to the formation of α-chloro α-methyl esters together with non chlorinated α-methyl esters; the same reaction performed at ?70° produces α-chloro ester enolates which upon heating to room temperature are transformed into α-methyl ester enolates. The reaction is applied to β-functional α,α-dichloro esters and one α,α-dichloro ketone.     

Enantioselective gem-chlorofluorination of active methylene compounds using a chiral spiro oxazoline ligand

Highly enantioselective gem-chlorofluorination of active methylene compounds was carried out by using a copper(II) complex of a chiral spiro pyridyl monooxazoline ligand. This reaction yielded α-chloro-α-fluoro-β-keto esters and α-chloro-α-fluoro-β-keto phosphonates with up to 92% ee. The resulting dihalo β-keto ester was converted into various α-fluoro-α-heteroatom-substituted carbonyl compounds via nucleophilic substitution without loss of optical purity. A fully protected β-amino acid with a gem-chlorofluoromethylene function was also synthesized.