Search results for "synthesis"
showing 10 items of 2844 documents
Enantioselective Protonation of the Lithium Transient Enolate of2-Methyltetralone with 2-Sulfinyl Alcohols
2005
A new catalytic cycle for the enantioselective protonation of cyclic ketone enolates with sulfinyl alcohols has been developed. An enol trifluoroacetate that can be easily obtained from the corresponding ketone is used for the first time as an enolate precursor of a cyclic ketone enolate. In this method, the achiral alcohol plays two roles: it is involved, as is usual in catalytic asymmetric protonation reactions, in the turnover of the chiral proton source and also in the generation of a transient enolate through the reaction of its corresponding alkoxide with the enol trifluoroacetate precursor. Stereoselectivity is highly dependent on the structure of the achiral alcohol. High levels of …
Highly Enantioselective Protonation of the 3,4-Dihydro-2- methylnaphthalen-1(2H)-one Li-Enolate by TADDOLs
2000
A series of nine TADDOLs (=α,α,α′,α′-tetraaryl-1,3-dioxolane-4,5-dimethanols) 1a – 1i, have been tested as proton sources for the enantioselective protonation of the Li-enolate of 2-methyl-1-tetralone (=3,4-dihydro-2-methylnaphthalen-1(2H)-one). The enolate was generated directly from the ketone (with LiN(i-Pr)2 (LDA)/MeLi) or from the enol acetate (with 2 MeLi) or from the silyl enol ether (with MeLi) in CH2Cl2 or Et2O as the solvent (Scheme). The Li-enolate (associated with LiBr/LDA, or LiBr alone) was combined with 1.5 – 3.0 equiv. of the TADDOL at −78° by addition of the latter or by inverse addition. 2-Methyl-1-tetralone of (S)-configuration is formed (≤80% yield) with up to 99.5% sele…
An improved method for the asymmetric protonation of enolates with chiral α-sulfinyl alcohols/trifluoroethanol
2001
Abstract Enantioselective protonation of 2-methyl tetralone enolate using a stoichiometric amount of 2-sulfinyl alcohol ( S , R s )- 1 as a chiral proton donor yields the corresponding chiral ketone with a high level of enantioselectivity, provided that the chiral proton donor (CPD) is regenerated with an achiral proton source (APS). In contrast, stereoselectivity was only moderate if catalytic CPD (0.2 equiv.) and an APS are used.
Double Diastereoselection in Aldol Reactions Mediated by Dicyclohexylchloroborane between Chiral Aldehydes and a Chiral Ethyl Ketone Derived from l-E…
2005
Both matched and mismatched diastereoselections have been observed in the aldol reactions of a range of chiral aldehydes with the dicyclohexylboron enolate of a chiral ethyl ketone related to l-erythrulose. As was previously observed in the corresponding aldol reactions with l-erythrulose derivatives, the Felkin−Anh model provides an adequate explanation for the stereochemical outcome of reactions with chiral α-methyl aldehydes. However, a satisfactory account of the results observed with α-oxygenated aldehydes was only possible with the Cornforth model. As a practical application of the methodology described herein, a C1−C9 fragment of the structure of the antifungal macrolide soraphen A1α…
Synthesis of Optically Active 15-epi-9,14-Methylene Lipoxin A4
2018
The synthesis of lipoxin A4 and B4 analogues (LXA4, LXB4) to gain access to stabilized inflammation resolving compounds is an important field of research. Starting from known structural requirements of the natural compounds displaying biological activity and a broad investigation of their rapid metabolism, various LXA4 derivatives have been developed and tested. Focusing on variation and stabilization of the conjugated E,E,Z,E C7–C14 tetraene moiety of natural LXA4, a methylene bridge introduced between C9 and C14 might suppress any Z/E isomerization of the C11–C12 olefin. Intending to enable at least known structure variations in connection with the C1–C7 and the C15–C20 fragments, a conve…
Synthesis of Optically Active N-(4-Hydroxynon-2-enyl)pyrrolidines: Key Building Blocks in the Total Synthesis of Streptomyces coelicolor Butanolide 5…
2021
AbstractStarting from 5-methylhexanal and (S)-configured N-propargylprolinol ethers, coupling delivered N-(4-hydroxynon-2-ynyl)prolinol derivatives as mixtures of C4 diastereomers. Resolution of the epimers succeeded after introduction of an (R)-mandelic ester derivative and subsequent HPLC separation. Alternatively, suitable oxidation gave the corresponding alkynyl ketone. Midland reagent controlled diastereoselective reduction afforded a defined configured propargyl alcohol with high selectivity. LiAlH4 reduction and Mosher analyses of the allyl alcohols enabled structure elucidation. The suitably protected products are used as key intermediates in enantioselective Streptomyces γ-butyrola…
Synthesis of Functionalized Indoles with an α-Stereogenic Ketone Moiety Through an Enantioselective Friedel-Crafts Alkylation with (E)-1,4-Diaryl-2-b…
2009
Chiral complexes of BINOL-based ligands with hafnium tert-butoxide catalyze the enantioselective Friedel-Crafts alkylation of indoles with (E)-1,4-diaryl-2-butene-1,4-diones at room temperature, with good yields and ee up to 94%. Hafnium(IV) was found to be a more effective Lewis acid than other frequently used metal ions such as titanium(IV) or zirconium(IV). Unlike the enantioselective Friedel-Crafts alkylation of indoles with α,β-unsaturated compounds where the stereogenic center is generated in the β-position to a carbonyl group, the Friedel-Crafts alkylation with 2-butene-1,4-diones described here generates an α-stereogenic center with respect to one of the carbonyl groups. This can be…
Stereoselective Synthesis of (+)-Boronolide
2002
The delta-lactone boronolide (+)-1, a pharmacologically active, naturally occurring product, has been synthesized in enantiopure form with L-erythrulose as the chiral starting material. The key steps of the synthesis were a highly stereoselective aldol-reduction one-pot sequence, an indium-mediated diastereoselective aldehyde allylation, and a ring-closing metathesis.
ChemInform Abstract: Catalytic Enantioselective Addition of Terminal Alkynes to Aromatic Aldehydes Using Zinc-Hydroxyamide Complexes.
2010
A mandelamide ligand, derived from (S)-mandelic acid and (S)-phenylethanamine, catalyzes the addition of aryl-, alkyl- and silyl-alkynylzinc reagents to aromatic and heteroaromatic aldehydes with good yields and good to high enantioselectivities.
Catalytic enantioselective addition of terminal alkynes to aromatic aldehydes using zinc-hydroxyamide complexes
2009
[EN] A mandelamide ligand, derived from (S)-mandelic acid and (S)-phenylethanamine, catalyzes the addition of aryl-, alkyl-and silyl-alkynylzinc reagents to aromatic and heteroaromatic aldehydes with good yields and good to high enantioselectivities.