CuH-catalyzed enantioselective intramolecular reductive aldol reactions generating three new contiguous asymmetric stereocenters

Treatment of beta,beta-disubstituted-alpha,beta-unsaturated ketones bearing a ketone residue with in situ generated, catalytic CuH ligated by a nonracemic ligand leads to three newly created adjacent chiral centers. Excellent de's and ee's are obtained for several examples studied.     

Oxidation of chiral α-phenylacetate derivatives: formation of dimers with contiguous quaternary stereocenters versus tertiary alcohols

The menthol esters of α-phenylacetate derivatives undergo diastereoselective oxidative dimerizations (α-cyano) in the presence of oxidants, and nitrosylation (α-amido) in the presence of CAN to form products containing adjacent functionalized quaternary stereocenters and tertiary alcohols, respectively.     

Highly enantio- and diastereoselective one-pot synthesis of acyclic epoxy alcohols with three contiguous stereocenters

Two highly enantio- and diastereoselective one-pot procedures for the synthesis of epoxy alcohols with up to three contiguous stereocenters are reported. Route one involves asymmetric addition of an alkylzinc reagent to an enal followed by diastereoselective epoxidation. Route two entails asymmetric vinylation of an aldehyde with divinylzinc reagents and subsequent diastereoselective epoxidation. The oxidant for the epoxidation is generated by exposure of the allylic alkoxide intermediate and the remaining organozinc reagent to dioxygen. Upon addition of catalytic titanium tetraisopropoxide, the directed epoxidation yields the epoxy alcohols with good to excellent yields.     

Memory of chirality in intramolecular conjugate addition of enolates: a novel access to nitrogen heterocycles with contiguous quaternary and tertiary stereocenters

Nitrogen heterocycles with contiguous quaternary and tertiary stereocenters have been prepared in high enantiomeric purity by intramolecular conjugate addition of enolates generated from alpha-amino acid derivatives via memory of chirality.     

Diastereoselection in the formation of contiguous quaternary carbon stereocenters by the intramolecular Heck reaction

The second in a series of two papers, this study examines the origins of diastereoselection in the second ring closure of the highly diastereoselective double Heck cyclization of cyclohexenes 1 and 3 that form contiguous quaternary stereocenters. Seven model substrates were synthesized and cyclized to examine the structural features responsible for imparting diastereoselection in the second intramolecular Heck reaction. These studies demonstrate that stereoselection in the formation of the second spirooxindole ring results from the avoidance of steric interactions in the insertion step with the spirooxindole formed in the first Heck cyclization. An axial substituent (carbonyl or arene) is required at the allylic position for high levels of diastereoselection to be realized.     

Toward the total synthesis of maoecrystal V: establishment of contiguous quaternary stereocenters

A synthetic strategy toward maoecrystal V has been identified. It has been shaped by the necessity to maneuver in sterically hindered molecular environments.     

Construction of three stereocenters via hydrogenative desymmetrization of 2,2,5-trisubstituted cyclohexane-1,3-diones

A Ru-catalyzed hydrogenative desymmetrization of 2,2,5-trisubstituted cyclohexane-1,3-diones was developed for construction of three stereocenters including two coherent and discontiguous chiral centers and a chiral quaternary carbon with excellent enantio-and diastereoselectivities.Stereodivergent synthesis of four stereoisomers could be conducted with high enantioselectivities.The desymmetrization could be achieved at gram scale without loss of reactivity and optical purity,and a formal synthesis of bioactive molecule(-)-isocelorbicol was completed.     

Catalytic alkene skeletal modification for the construction of fluorinated tertiary stereocenters

Herein we describe the first construction of fluorinated tertiary stereocenters based on an alkene C(sp²)–C(sp²) bond cleavage. The new process, that takes advantage of a Rh-catalyzed carbyne transfer, relies on a branched-selective fluorination of tertiary allyl cations and is distinguished by a wide scope including natural products and drug molecule derivatives as well as adaptability to radiofluorination.

We report a previously unknown disconnection approach to valuable fluorinated tertiary stereocenters based on the skeletal modification of 1,1-disubstituted alkenes by a Rh-catalyzed carbyne transfer.     

Highly enantio- and diastereoselective tandem generation of cyclopropyl alcohols with up to four contiguous stereocenters

Three highly enantio- and diastereoselective one-pot procedures for the synthesis of cyclopropyl and iodocyclopropyl alcohols with up to four contiguous stereocenters are reported. Route 1 involves asymmetric addition of an alkylzinc reagent to an enal followed by diastereoselective cyclopropanation. Route 2 parallels route 1, except that iodoform is used to generate the zinc carbenoid, and the products are iodocyclopropyl alcohols. Route 3 entails asymmetric vinylation of an aldehyde with divinylzinc reagents and subsequent diastereoselective cyclopropanation.     

Highly diastereoselective synthesis of vicinal quaternary and tertiary stereocenters using the iodo-aldol cyclization

The intramolecular iodo-aldol cyclization of alpha-substituted enoate aldehydes and ketones is described. Using prochiral starting materials, the reaction produces hetero- and carbocycles containing quaternary centers adjacent to secondary or tertiary centers. The reactions occur in good yields and are highly selective for the trans-products, having the hydroxyl and iodomethyl groups on opposite faces of the ring system.     

Aziridines in one step from hydantoins via Red-Al mediated ring-contraction

We have developed a new method to synthesize aziridines from their corresponding hydantoins in one step using an excess of Red-Al overnight in refluxing toluene. This allows direct access to N-H aziridines without the need for protecting group strategies.     

Construction of adjacent spiro-quaternary and tertiary stereocenters through phosphine-catalyzed asymmetric [3+2] annulation of allenoates with alkylidene azlactones

A novel axially chiral spiro-phosphine-catalyzed highly regio-, diastereo- and enantioselective [3+2] cycloaddition of alkylidene azlactones with various allenic esters has been developed, affording the corresponding functionalized spirocyclic products in moderate to excellent yields under mild conditions. These spirocyclic products as masked amino acids can be easily transformed into aspartic amino acid analogues.     

Cu(I)-catalyzed decarboxylative aldol-type and Mannich-type reactions for asymmetric construction of contiguous trisubstituted and quaternary stereocenters

The catalytic asymmetric decarboxylative aldol-type reaction between aldehydes and cyanocarboxylic acids and Mannich-type reaction between aldimines and cyanocarboxylic acids were developed. α,α,β-Trisubstituted-β-hydroxy nitriles bearing contiguous all-carbon quaternary and trisubstituted stereocenters were produced with moderate enantio- and diastereoselectivity in the presence of 10 mol % CuOAc–TANIAPHOS (or DTBM-SEGPHOS) complex in the aldol-type reaction. α,α,β-Trisubstituted-β-amino nitriles containing contiguous all-carbon quaternary and trisubstituted stereocenters were produced with moderate to high enantio- and diastereoselectivity using 5 mol % CuOAc–(R)-DTBM-SEGPHOS complex in the Mannich-type reaction. These reactions proceed through Cu(I)-catalyzed decarboxylative nucleophile generation, followed by the addition of the resulting chiral Cu-ketenimide to aldehydes or imines. Because the reactions proceed under very mild conditions at nearly neutral pH, the reactions are applicable to a wide range of substrate combinations, including both aromatic and aliphatic substrates. Finally, α,α,β-trisubstituted-β-amino nitriles were converted to β^2,2,3-amino acid derivatives through simple acidic hydrolysis without any racemization and epimerization.     

Diastereoselective conjugate addition/cyclization/bromination: Access to four stereocenters in a single step

A stereoselective tandem conjugate addition reaction with a chiral amine-derived nucleophile is reported in which the enolate intermediate is quenched with 1,2-dibromotetrachloroethane as a mild brominating reagent. X-ray analysis of a subsequent derivative was used to prove the configuration at each of the four newly formed stereocenters. The resulting α-bromoester underwent selective transesterification catalyzed by mild base to allow selective manipulation of the two ester groups of the product.