Search results for "selective synthesis"
showing 10 items of 430 documents
Mukaiyama–Michael Reactions with Acrolein and Methacrolein: A Catalytic Enantioselective Synthesis of the C17–C28 Fragment of Pectenotoxins
2013
Enantioselective iminium-catalyzed reactions with acrolein and methacrolein are rare. A catalytic enantioselective Mukaiyama-Michael reaction that readily accepts acrolein or methacrolein as substrates, affording the products in good yields and 91-97% ee, is presented. As an application of the methodology, an enantioselective route to the key C17-C28 segment of the pectenotoxin using the Mukaiyama-Michael reaction as the key step is described.
Asymmetric synthesis of α,β-diamino acid derivatives with an aziridine-, azetidine- and γ-lactone-skeleton via Mannich-type additions across α-chloro…
2012
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-diami…
Asymmetric synthesis of chloroisothreonine derivatives via syn-stereoselective Mannich-type additions across N-sulfinyl-α- chloroimines
2014
Mannich-type reactions of O-Boc glycolic esters across chiral N-sulfinyl-α-chloroaldimines resulted in the efficient and syn-stereoselective synthesis of new γ-chloro-α-hydroxy-β-amino esters (dr > 99:1). The α-coordinating ability of the chlorine atom was of great importance for the diastereoselectivity of the Mannich-type reaction and overruled the chelation of the sulfinyl oxygen with the lithium ion of the incoming E-enolate in the transition state model. These novel chloroisothreonine derivatives proved to be excellent building blocks in asymmetric synthesis of novel syn-β,γ-aziridino-α-hydroxy esters and biologically relevant trans-oxazolidinone carboxylic esters.
Enantioselective zinc-mediated conjugate alkynylation of saccharin-derived 1-aza-butadienes
2020
The enantioselective 1,4-alkynylation of conjugated imines derived from saccharin with aryl- and alkyl-substituted terminal alkynes has been achieved. The reaction mediated by diethylzinc in the presence of a catalytic amount of a bis(hydroxy)malonamide chiral ligand provides the corresponding imines bearing a propargylic stereocenter with moderate yields and fair to excellent enantioselectivities.
Asymmetric Synthesis of Spirocyclic β-Lactams through Copper-Catalyzed Kinugasa/Michael Domino Reactions
2018
The first copper-catalyzed highly chemo-, regio-, diastereo-, and enantioselective Kinugasa/Michael domino reaction for the desymmetrization of prochiral cyclohexadienones is described. In the presence of a chiral copper catalyst, alkyne-tethered cyclohexadienones couple with nitrones to generate the chiral spirocyclic lactams with excellent stereoselectivity (up to 97 % ee, >20:1 dr). The new method provides direct access to versatile highly functionalized spirocyclic β-lactams possessing four contiguous stereocenters, including one quaternary and one tetra-substituted stereocenter.
Enantioselective Synthesis of Tetrahydroprotoberberines and Bisbenzylisoquinoline Alkaloids from a Deprotonated α-Aminonitrile
2011
Under controlled conditions, 6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline-1-carbonitrile can be quantitatively deprotonated in the α-position. Its alkylation directly furnishes 3,4-dihydroisoquinolines which can serve as starting materials for the preparation of various alkaloids. Here, the preparation of the benzylisoquinolines (+)-laudanidine, (+)-armepavine, and (+)-laudanosine as well as the tetrahydroprotoberberines (-)-corytenchine and (-)-tetrahydropseudoepiberberine using Noyori's asymmetric transfer hydrogenation are described. The dimeric alkaloids (+)-O-methylthalibrine and (+)-tetramethylmagnolamine were obtained from nonracemic precursors in Ullmann diaryl ether syntheses.
Enantioselective Mannich reaction of β-keto esters with aromatic and aliphatic imines using a cooperatively assisted bifunctional catalyst
2014
An efficient urea-enhanced thiourea catalyst enables the enantioselective Mannich reaction between β-keto esters and N-Boc-protected imines under mild conditions and minimal catalyst loading (1–3 mol %). Aliphatic and aromatic substituents are tolerated on both reaction partners, affording the products in good enantiomeric purity. The corresponding β-amino ketones can readily be accessed via decarboxylation without loss of enantiomeric purity.
Enantioselective synthesis of fluorinated alpha-amino acids and derivatives in combination with ring-closing metathesis: intramolecular pi-stacking i…
2001
[reaction: see text]. Hydride reduction of C=N bonds stereocontrolled by intramolecular pi-stacking interactions of 1-naphthylsulfinyl and N-aryl groups, nonoxidative Pummerer rearrangement, and ring-closing metathesis are efficiently combined in a highly stereoselective entry to enantiomerically pure cyclic and acyclic fluorinated beta-amino alcohols and alpha-amino acid derivatives, respectively.
Microwave-Assisted Tandem Organocatalytic Peptide-Coupling Intramolecular aza-Michael Reaction: α,β-UnsaturatedN-Acyl Pyrazoles as Michael Acceptors
2014
Conjugated N-acyl pyrazoles have been successfully employed in the organocatalytic enantioselective intramolecular aza-Michael reaction as ester surrogates. Bifunctional squaramides under microwave irradiation provided the best results in this transformation. Furthermore, this protocol has been combined with a peptide-coupling reaction in a tandem sequence. The final products were easily converted into the corresponding ethyl esters.
Recent Advances in Enantioselective Desymmetrizations of Prochiral Oxetanes
2021
Abstract Strain relief of oxetanes offers a plethora of opportunities for the synthesis of chiral alcohols and ethers. In this context, enantioselective desymmetrization has been identified as a powerful tool to construct molecular complexity and this has led to the development of elegant strategies on the basis of transition metal, Lewis acid, and Brønsted acid catalysis. This review highlights recent examples that harness the inherent reactivity of prochiral oxetanes and offers an outlook on the immense possibilities for synthetic application.