Synthesis of beta-substituted alpha-amino acids via Lewis acid promoted radical conjugate additions to alpha,beta-unsaturated alpha-nitro esters and amides

[reaction: see text] Beta-substituted alpha,beta-unsaturated alpha-nitro esters and amides undergo radical conjugate additions when treated with an appropriate Lewis acid. Deuterium studies revealed that the acidic alpha-stereocenter of the alpha-nitro ester products does not racemize under strictly controlled workup conditions. The alpha-nitro amides did racemize significantly during chromatography, but this could be greatly minimized by subjecting the crude adducts to subsequent transformations. The conjugate addition products can be elaborated into beta-substituted alpha-amino acids in two simple steps.     

Second-generation DBFOX ligands for the synthesis of beta-substituted alpha-amino acids via enantioselective radical conjugate additions

A set of second-generation DBFOX ligands possessing extended aryl or benzyl-type groups was synthesized. The requisite amino alcohols were either commercially available (DBFOX/Bn) or constructed via Sharpless asymmetric aminohydroxylation (DBFOX/Nap, DBFOX/ t-BuPh, DBFOX/Pip) or phase-transfer-catalyzed asymmetric alkylation (DBFOX/MeNap). Complexes of the ligands with Mg(NTf2)2 were evaluated as promoters of enantioselective radical conjugate additions to alpha,beta-unsaturated alpha-nitro amides and esters. Reactions employing the DBFOX/Nap ligand exhibited improved enantioselectivity relative to previously published additions mediated by DBFOX/Ph. However, the relatively modest increase in diastereomeric ratio suggests that our substrate-Lewis acid binding model, which was formulated based on results from DBFOX/Ph-promoted radical conjugate additions, is in need of revision.     

Lanthanide Lewis acid-mediated enantioselective conjugate radical additions

[reaction: see text] Lanthanide triflates along with proline-derived ligands have been found to be efficient catalysts for enantioselective conjugate addition of nucleophilic radicals to enoates. N-Acyl oxazolidinones, when used as achiral additives, gave meaningful enhancements in the ees for the product.     

Synthesis of cyclopropene alpha-amino acids via enantioselective desymmetrization

[reaction: see text] The preparation of cyclopropene alpha-amino acids via the enantioselective desymmetrization of cyclopropene bis-carboxylic acid derivatives is described. The amino acids are stable to harsh reaction conditions, and a derivative has been incorporated into a tripeptide using conventional methods for peptide synthesis.     

Synthesis of beta-substituted alpha-amino acids with use of iridium-catalyzed asymmetric allylic substitution

The asymmetric synthesis of beta-substituted alpha-amino acids with use of iridium-catalyzed allylic substitution was described. The Ir-catalyzed allylic substitution of diphenylimino glycinate with allylic phosphates proceeded smoothly even at 0 degrees C and gave branch products with high enantioselectivity (up to 97% ee), when chiral bidentate phosphite bearing the 2-ethylthioethyl group was employed. In addition, both diastereomers of the branch products were synthesized stereoselectively by simply switching the base employed. These methods were also applied to the asymmetric synthesis of quaternary alpha-amino acids.     

The enantioselective synthesis of alpha-amino acid derivatives via organoboranes

Optically active (S)-alpha-amino acids are prepared in 54-95% ee (12 cases) by reaction of the Schiff base acetate of glycine tert-butyl ester with B-alkyl-9-BBN derivatives in the presence of the Cinchona alkaloid, cinchonidine, and base. The enantiomeric (R)-alpha-amino acids are available in 59-92% ee (3 cases) by using cinchonine as the chiral control element.     

Organocatalytic enantioselective conjugate additions to enones

Conjugate addition reactions of a wide range of nucleophilic enol species with enones, catalyzed by a cinchona alkaloid derived thiourea organocatalyst, have been developed with attending good yields and high enantioselectivities under a mild reaction condition. The general method provides an efficient approach to the preparation of versatile building blocks possessing various functional groups.     

Readily accessible chiral diene ligands for Rh-catalyzed enantioselective conjugate additions of boronic acids

Enabled by the broad scope of the Pd-catalyzed asymmetric alkylation of meso- and d,l-divinylethylene carbonate, several chiral diene ligands were prepared in two steps from commercial materials. Subsequently, these ligands were evaluated in the Rh-catalyzed asymmetric conjugate addition of boronic acids to enones.     

A heterodinuclear asymmetric catalyst for conjugate additions of alpha-hydroxyketones to beta-substituted nitroalkenes

[Structure: see text] The bis-ProPhenol ligand was designed to facilitate formation of hetereodinuclear complexes based upon the large difference in pKa of the one phenolic OH group to the tertiary OH groups. In exploring the first example of hydroxyacetophenones as donors in asymmetric Michael reactions with nitroalkene acceptors, the best stereocontrol was observed with a zinc/magnesium dinuclear complex where enantiomeric excesses ranged up to 92% for the major anti diastereomer.     

Lewis acid promoted phenylseleno group transfer tandem radical cyclization reactions

[reaction: see text] A new Lewis acid promoted phenylseleno group transfer tandem radical cyclization method was developed. In the presence of Lewis acids such as Yb(OTf)3 or Mg(ClO4)2, under photolysis condition at low temperature (-45 degrees C), various unsaturated alpha-phenylseleno beta-keto amides underwent radical cyclization reactions to give monocyclic or bicyclic products in a highly efficient, regioselective, and stereoselective manner.     

Highly enantioselective Michael-cyclization cascade promoted by synergistic asymmetric aminocatalysis and Lewis acid catalysis

The novel dual cooperative asymmetric aminocatalysis and Lewis acid catalysis has been successfully developed for promoting cascade Michael-cyclization reaction with high enantio-, regio- and chemo-selectivity. The simple and practical process affords a one-pot approach to synthetically useful cyclopentenes.     

A catalytic and iterative route to beta-substituted esters via highly enantioselective conjugate addition of dimethylzinc to unsaturated malonates

Using the chiral phosphoramidite ligand (S,R,R)-L1 in the conjugate addition of dimethylzinc to acyclic unsaturated malonates, enantioselectivities of up to 98% have been obtained for the first time. An iterative and stereodivergent route to 3,5-dimethyl esters that takes advantage of this asymmetric catalysis has been developed.     

Synthesis of 2H-indazoles via Lewis acid promoted cyclization of 2-(phenylazo)benzonitriles

Lewis-acid promoted "coarctate" cyclization of 10 2-(phenylazo)benzonitrile derivatives furnishes the isoindazole ring system in ca. 65-95% yield. A plausible mechanism for this unusual transformation is proposed.     

A practical method for enantioselective synthesis of all-carbon quaternary stereogenic centers through NHC-Cu-catalyzed conjugate additions of alkyl- and arylzinc reagents to beta-substituted cyclic enones

We present the first examples of Cu-catalyzed enantioselective conjugate additions of alkyl- and arylzinc reagents to unactivated cyclic beta-substituted enones. Transformations are promoted in the presence of 2.5-15 mol % of a readily available chiral NHC-based Cu complex, affording the desired products bearing all-carbon quaternary stereogenic centers in 67-->98% yield and in up to 97% ee. Catalytic enantioselective reactions can be carried out on a benchtop, with undistilled solvent and commercially available (not further purified) Cu salts. Mechanistic models, accounting for the observed levels and trends in enantioselectivity are provided.