Regioselective control of allyllithiums of some heterocycles: α vs γ condensation with aldehydes.

A procedure for directing the condensation of allyllithiums of benzothiazole, benzoxazole and pyrimidine such as 2 with aldehydes to the α or γ position is described. Erythro diasteroselection is observed in the α regioisomers.     

Asymmetric synthesis of β2-tryptophan analogues via Friedel-Crafts alkylation of indoles with a chiral nitroacrylate

The asymmetric Friedel-Crafts alkylation of various indoles with a chiral nitroacrylate provides optically active β-tryptophan analogues after reduction of the nitro group and removal of the chiral auxiliary. This reaction generally occurs in good yield and high diastereoselectivity (up to 90:10).     

A general stereoselective enamine mediated alkylation of α-substituted aldehydes

We herein present a general enamine-mediated α-alkylation of α-substituted aldehydes with carbenium ions for the stereoselective construction of quaternary stereogenic centers.     

Highly stereocontrolled alkylation of protected ‘diacetone hexulose aldehydes’

Claisen–Schmidt aldol condensation (with acetone, catalysed by l- and d-proline), Knoevenagel reaction with acetoacetic acid and the modified Reformatsky reaction with bromoacetone of the ‘diacetone hexulose aldehydes’ 2 and 7 gave the corresponding β-hydroxyketones 3–4 and 8–9 as well as the (E)-α,β-enones 5 and 10, respectively. The highest chemo- and stereoselectivities were obtained with the Knoevenagel procedure using l-proline as the catalyst.     

Asymmetric Michael reactions of α,α-disubstituted aldehydes with maleimides using a primary amine thiourea organocatalyst

Primary amine thiourea organocatalyst 8 was used to promote Michael additions of bulky α,α-disubstituted aldehydes, such as isobutyraldehyde with maleimides to afford the corresponding adducts in high to excellent yields and with up to 91% ee.     

Asymmetric organocatalytic oxy-Michael addition of alcohols to α,β-unsaturated aldehydes

A 1,4-addition of alcohols to α,β-unsaturated aldehydes was found to be efficiently promoted by biphenyldiamine-based catalyst 3 without formation of the acetals. An asymmetric variant of this reaction has also been performed by designing a novel axially chiral organocatalyst (R)-10c.     

Condensation of polyfluorinated β-diimines with aromatic aldehydes

Polyfluorinated 2-arylpyrimidines were synthesized by the reaction of polyfluorinated -diimines with aromatic aldehydes.     

Interaction of hydroxyaryl aldehydes with α-hydroxyethyl radicals

The effect of some hydroxylated aromatic aldehydes on the radiation-chemical transformations of deaerated ethanol during continuous radiolysis has been studied. The data obtained show that these compounds effectively inhibit radiation-induced processes involving α-hydroxyethyl radicals (α-HER). Benzaldehyde and its hydroxylated derivatives (II, III) predominantly oxidize, and compounds containing the cinnamic moiety, (IV–VI), add α-HER to a carbonyl group or -C=C bond.     

Asymmetric synthesis of γ-lactones through reaction of sulfoxonium ylides,aldehydes, and ketenes

A method for the enantioselective synthesis of γ-lactones through the reaction of enantioenriched sulfoxonium ylides, aldehydes, and ketenes was developed. Enantioenriched (98% ee) aminosulfoxonium ylide was subjected to reaction with a variety of aldehydes (both aromatic and aliphatic) and disubstituted ketenes, leading to the formation of α,β-substituted γ-lactones in moderate to very good diastereoselectivity (dr up to 95:5) and with enantiomeric excesses of up to 79% ee. Best levels of enantioselectivity were observed in the reactions of enantioenriched aminosulfoxonium ylide with isobutyraldehyde and various alkylarylketenes.     

Facile alkylation of o-hydroxyarylaldehydes and α-formylketones with trialkylboranes

Trialkylboranes readily undergo alkylation reactions with o-hydroxyarylaldehydes and α-formylketones via the dialkylboryl complexes.     

Asymmetric Michael addition of aldehydes to nitroolefins catalyzed by a pyrrolidine–pyrazole

An effective protocol for the stereoselective Michael addition of aldehydes to nitroolefins using pyrrolidine–pyrazole as an organocatalyst is described. The catalytic cycle was found to be productive in terms of yield and selectivity, when performed under solvent-free reaction conditions and employing 15 mol % of catalyst and 10 mol % of benzoic acid at room temperature.     

Reactions of carbocations with unsaturated hydrocarbons: electrophilic alkylation or hydride abstraction?

Benzhydryl cations were used as reference electrophiles to determine the hydride donor reactivities of unsaturated hydrocarbons. The kinetics of the reactions were followed by UV-vis spectroscopy and conductivity measurements, and it was found that the second-order rate constants for the hydride transfer processes were almost independent of the solvents or counterions employed. The rate constants correlate linearly with the previously published empirical electrophilicity parameters E of the benzhydrylium ions. Therefore, the linear free energy relationship log k(20 degrees C) = s(E + N) could be employed to characterize the hydride reactivities of the hydrocarbons by the nucleophilicity parameters N and s. The similarity of the slopes s for hydride donors and pi-nucleophiles allows a direct comparison of the reactivities of these different functional groups based on their nucleophilicity parameters N. Since nucleophilicity parameters of -5 < N < 0 have been found for a large variety of allylic and bisallylic hydride donors, a rule of thumb is derived that hydride transfer processes may compete with carbon-carbon bond-forming reactions when carbocations are combined with olefins of pi-nucleophilicity N < 0.     

Enantioselective allylation of aldehydes with chiral allyl organolanthanide reagents

The first example of enantioselective allylation of aldehydes with chiral allyl organolan-thanide reagents has been achieved in high chemical yield and moderate optical purity (up to 54% e.e.).     

Cooperative thiourea-Brønsted acid organocatalysis: enantioselective cyanosilylation of aldehydes with TMSCN

We report a new thiourea-Br?nsted acid cooperative catalytic system for the enantioselective cyanosilylation of aldehydes with yields up to 90% and enantioselectivities up to 88%. The addition of an achiral acid was found to be crucial for high asymmetric induction. Mechanistic investigations using a combination of NMR, ESI-MS, and density functional theory computations (including solvent corrections) at the M06/6-31G(d,p) level of theory suggest that the key catalytic species results from the cooperative interaction of bifunctional thioureas and an achiral acid that form well-defined chiral hydrogen-bonding environments.     

The NHCs-mediated cross-coupling of aromatic aldehydes with benzyl halides: synthesis of α-aryl ketones

A new NHCs-mediated synthetic method was found to produce α-aryl ketones in 22-63% yields in one-pot process from the corresponding aromatic aldehydes and benzyl halides. This method is the first example of the NHCs-mediated intermolecular nucleophilic acylation of aromatic aldehydes with benzyl halides.