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Here we describe a direct access to 2,2,3‐trisubstituted syn γ‐nitroaldehydes by addition of α‐branched aryl acetaldehydes to nitroolefins promoted by a cinchona based squaric acid‐derived amino acid peptide. Different α‐methyl arylacetaldehydes react with β‐aromatic and β‐alkyl nitroolefins to afford the Michael adducts in high enantioselectivity and syn‐selectivity. NMR experiments and DFT calculations predict the reaction to occur through the intermediacy of E‐enolate. The interaction between the substrates and the catalyst follows Pápai's model, wherein an intramolecular H‐bond interaction in the catalyst between the NH group of one of the tert‐leucines and the squaramide oxygen seems to be key for discrimination of the corresponding reaction transition states. [ABSTRACT FROM AUTHOR]

The high tendency of α‐amino aldehydes to undergo 1,2‐additions and their relatively low stability under basic conditions have largely prevented their use as pronucleophiles in the realm of asymmetric catalysis, particularly for the production of quaternary α‐amino aldehydes. Herein, it is demonstrated that the chemistry of α‐amino aldehydes may be expanded beyond these limits by documenting the first direct α‐alkylation of α‐branched α‐amino aldehydes with nitroolefins. The reaction produces densely functionalized products bearing up to two, quaternary and tertiary, vicinal stereocenters with high diastereo‐ and enantioselectivity. DFT modeling leads to the proposal that intramolecular hydrogen bonding between the NH group and the carbonyl oxygen atom in the starting α‐amino aldehyde is key for reaction stereocontrol. [ABSTRACT FROM AUTHOR]

A catalyst‐driven one‐pot reaction sequence is developed for the enantio‐ and diastereoselective synthesis of tetrasubstituted cyclohexenes from simple unsaturated ketones or thioesters. The method involves a tertiary amine/squaramide‐catalyzed α‐selective addition of transiently generated trienolates to nitroolefins, subsequent base‐catalyzed double bond isomerization, and an intramolecular (vinylogous) 1,6‐addition reaction, a rare key carbocyclization step that proceeded with essentially perfect stereocontrol. [ABSTRACT FROM AUTHOR]

A bifunctional amine/squaramide catalyst promoted direct aldol addition of an hydantoin surrogate to pyridine 2‐carbaldehyde N‐oxides to afford adducts bearing two vicinal tertiary/quaternary carbons in high diastereo‐ and enantioselectivity (d.r. up to >20:1; ee up to 98 %) is reported. Acid hydrolysis of adducts followed by reduction of the N‐oxide group yields enantiopure carbinol‐tethered quaternary hydantoin‐azaarene conjugates with densely functionalized skeletons. DFT studies of the potential energy surface (B3LYP/6–31+G(d)+CPCM (dichloromethane)) of the reaction correlate the activity of different catalysts and support an intramolecular hydrogen‐bond‐assisted activation of the squaramide moiety in the transition state of the catalytic reaction. [ABSTRACT FROM AUTHOR]

In this study, the unique capacity of bifunctional Brønsted bases to generate α‐branched ketone dienolates and control both site‐ and stereoselectivity of their addition reactions to representative classes of carbon electrophiles (i.e. vinyl sulfones, nitroolefins, formaldehyde) is documented. We demonstrate that by using selected chiral tertiary amine/squaramide catalysts, the reactions of β,γ‐unsaturated cycloalkanones proceed through the dienolate Cα almost exclusively and provide all‐carbon quaternary cyclic ketone adducts in good yields with very high enantioselectivities. A minor amount (<5 %) of γ‐addition is observed when nitroolefins are used as electrophiles. The parent acyclic ketone dienolates proved to be less reactive under these conditions, and thus still constitute a challenging class of substrates. Quantum chemical calculations correctly predict these differences in reactivity and explain the observed site‐specificity and enantioselectivity. [ABSTRACT FROM AUTHOR]

Various sets of enolizable alkynyl ketones (including methyl ynones with α‐aryl, α‐alkenyl, and α‐alkoxy groups) were able to react smoothly with nitroolefins with the assistance of bifunctional Brønsted base/H‐bond catalysts to provide adducts with two consecutive tertiary stereocenters in a highly diastereo‐ and enantioselective fashion. Further transformation of the obtained adducts into optically active acyclic and polycyclic molecules, including some with intricate carbon skeletons, was also demonstrated. Donor ynones: under very mild conditions linear ynones with no additional electron‐withdrawing group at Cα get activated by tertiary amine/squaramide chiral catalysts and react with nitroolefins in a highly diastereo‐ and enantioselective fashion. In this way, an easy access to branched alkyl alkynyl ketones with two contiguous stereocenters and derivatives thereof is provided. [ABSTRACT FROM AUTHOR]

Abstract: The potential of β‐alkoxy α‐keto amides as pronucleophiles in the enantioselective Mannich type reaction with p‐nosyl imines is presented. The proper combination of β‐alkoxy α‐keto amides and a squaramide‐based Brønsted base catalyst produced highly enantioenriched Mannich adducts, which may be transformed into functionalized amino diols. [ABSTRACT FROM AUTHOR]

A new method for the enantioselective synthesis of 5,5-disubstituted (quaternary) hydantoins was developed on the basis of an organocatalytic Michael reaction approach involving the use of 2-benzylthio-3,5-dihydroimidazol-4-ones as key hydantoin surrogates. The method is general with respect to the substitution pattern at the hydantoin N1 (alkyl, aryl, acyl), N3 (aryl), and C5 (linear/branched alkyl, aryl) positions and affords essentially single diastereomeric products with enantioselectivities higher than 95%ee in most cases. Among the bifunctional Brønsted base/H-bond catalysts examined, a known squaramide–tertiary amine catalyst and a newly prepared squaramide–tertiary amine catalyst provide the highest selectivity so far with either nitroolefins or vinyl ketones as the acceptor components. Kinetic measurements support a first-order rate dependence on both reaction partners, the donor template and the Michael acceptor, whereas competitive 1H NMR spectroscopy experiments reveal the high ability of the template for catalyst binding. [ABSTRACT FROM AUTHOR]

A divergent synthesis of optically active quaternary Δ4- and Δ5-dehydro prolines is developed based on the first catalytic enantioselective conjugate addition of α-substituted isocyano(thio)acetates to vinyl ketones that is general for both α-aryl and α-alkyl isocyano(thio)acetates. The new tetrasubstituted C−N stereocenter is formed without the need of any metal salt due to a bifunctional tertiary amine/squaramide catalyst, featuring a bulky polyaryl sidearm and an unusually short squaramide diamide H⋅⋅⋅H interatomic distance in the solid state. [ABSTRACT FROM AUTHOR]

The first regio-, diastereo-, and enantioselective direct Michael reaction of β,γ-unsaturated ketones with nitroolefins is enabled by Brønsted base/hydrogen-bonding bifunctional catalysis. A squaramide-substituted tertiary amine catalyzes the reaction of a broad range of β,γ-unsaturated ketones to proceed at the α-site exclusively, giving rise to adducts with two consecutive tertiary carbon stereocenters in diastereomeric ratios of up to >20:1 and enantioselectivities generally in the 90-98 % ee range. [ABSTRACT FROM AUTHOR]

A catalytic enantioselective entry to β2, 2-amino acids enabling their direct coupling with nucleophiles is described. The approach is based upon an effective bifunctional Brønsted base catalyzed construction of a quaternary carbon stereocenter at C4 position of pyrrolidin-2,3-diones. Subsequent regioselective Baeyer-Villiger oxidation of the resultant adducts gives β2, 2-amino acid N-carboxyanhydrides as the reactive species, which can further react with nucleophiles. Following this strategy both, β2, 2-amino acid derivatives with different functionalities at the newly created stereocenter, and spirocyclic structures can be efficiently prepared. [ABSTRACT FROM AUTHOR]

The catalytic asymmetric synthesis of both α-substituted and α,α-disubstituted (quaternary) β-tetralones through direct α-functionalization of the corresponding β-tetralone precursor remains elusive. A designed Brønsted base-squaramide bifunctional catalyst promotes the conjugate addition of either unsubstituted or α-monosubstituted β-tetralones to nitroalkenes. Under these reaction conditions, not only enolization, and thus functionalization, occurs at the α-carbon atom of the β-tetralone exclusively, but adducts including all-carbon quaternary centers are also formed in highly diastereo- and enantioselective manner. [ABSTRACT FROM AUTHOR]

An efficient, highly diastereo- and enantioselective assembly of acyclic carbonyl fragments possessing nonadjacent all-carbon tertiary/quaternary stereoarrays is reported based on a Brønsted base catalyzed Michael addition/α-protonation sequence involving α-cyanoacetates and 2,4-dimethyl-4-hydroxypenten-3-one as novel methacrylate equivalent. [ABSTRACT FROM AUTHOR]

The first enantioselective direct cross-aldol reaction of α-keto amides with aldehydes, mediated by a bifunctional ureidopeptide-based Brønsted base catalyst, is described. The appropriate combination of a tertiary amine base and an aminal, and urea hydrogen-bond donor groups in the catalyst structure promoted the exclusive generation of the α-keto amide enolate which reacted with either non-enolizable or enolizable aldehydes to produce highly enantioenriched polyoxygenated aldol adducts without side-products resulting from dehydration, α-keto amide self-condensation, aldehyde enolization, and isotetronic acid formation. [ABSTRACT FROM AUTHOR]

The addition of cyanoalkyl moieties to imines is a very attractive method for the preparation of β-amino nitriles. We present a highly efficient organocatalytic methodology for the stereoselective synthesis of β-amino nitriles, in which the key to success is the use of ureidopeptide-based Brønsted base catalysts in combination with (arylsulfonyl)acetonitriles as synthetic equivalents of the acetonitrile anion. The method gives access to a variety of β-amino nitriles with good yields and excellent enantioselectivities, and broadens the stereoselective Mannich-type methodologies available for their synthesis. [ABSTRACT FROM AUTHOR]

Fully loaded: The ureidopeptide‐based bifunctional Brønsted base 1 efficiently promotes the first direct catalytic Michael reaction of α‐mercapto carboxylate surrogates with nitroolefins involving a fully substituted α‐carbon atom construction. [ABSTRACT FROM AUTHOR]

Catalytic and asymmetric Michael reactions constitute very powerful tools for the construction of new C-C bonds in synthesis, but most of the reports claiming high selectivity are limited to some specific combinations of nucleophile/electrophile compound types, and only few successful methods deal with the generation of all-carbon quaternary stereocenters. A contribution to solve this gap is presented here based on chiral bifunctional Brønsted base (BB) catalysis and the use of α'-oxy enones as enabling Michael acceptors with ambivalent H-bond acceptor/donor character, a yet unreported design element for bidentate enoate equivalents. It is found that the Michael addition of a range of enolizable carbonyl compounds that have previously demonstrated challenging (i.e., α-substituted 2-oxindoles, cyanoesters, oxazolones, thiazolones, and azlactones) to α'-oxy enones can afford the corresponding tetrasubstituted carbon stereocenters in high diastereo- and enantioselectivity in the presence of standard BB catalysts. Experiments show that the α'-oxy ketone moiety plays a key role in the above realizations, as parallel reactions under identical conditions but using the parent α,β-unsaturated ketones or esters instead proceed sluggish and/or with poor stereoselectivity. A series of trivial chemical manipulations of the ketol moiety in adducts can produce the corresponding carboxy, aldehyde, and ketone compounds under very mild conditions, giving access to a variety of enantioenriched densely functionalized building blocks containing a fully substituted carbon stereocenter. A computational investigation to rationalize the mode of substrate activation and the reaction stereochemistry is also provided, and the proposed models are compared with related systems in the literature. [ABSTRACT FROM AUTHOR]