Conjugate addition of lithium N-tert-butyldimethylsilyloxy-N-(α-methylbenzyl)amide: asymmetric synthesis of β2,2,3-trisubstituted amino acids

Imbuing an Old Heterocycle with the Power of Modern Catalysis: An Isoxazolidin-5-one Story

Isoxazolidin-5-ones have been regarded as β-amino acid surrogates owing to their labile N-O bond. While many efforts have been devoted to the catalytic enantioselective synthesis of the core of this heterocycle, its further transformation has been less explored, especially in the context of catalysis. This review summarizes the author's research on the development of catalytic reactions using isoxazolidin-5-ones as substrates. Asymmetric catalysis has proven effective for C-C bond formation at the carbonyl α-carbon. Catalytic asymmetric allylation and direct Mannich-type reactions have been developed. Further, the resulting products have been readily converted into the corresponding quaternary β^2,2-amino acids. Moreover, isoxazolidin-5-ones have been identified as alkyl nitrene precursors in the presence of a suitable metal catalyst. The generated metallonitrene undergoes either the electrophilic amination of the aromatic ring or aliphatic C-H insertion, affording a series of cyclic β-amino acids. A remarkable difference in chemoselectivity between rhodium and copper alkyl nitrenes has also been demonstrated, highlighting the unique nature of the underexplored reactive intermediates. The various linear and cyclic β-amino acids obtained through the study are likely to find great utility in a broad range of chemical sciences.

Synthesis of Unprotected Spirocyclic β-Prolines and β-Homoprolines by Rh-Catalyzed C-H Insertion

A series of unprotected spirocyclic β-prolines and β-homoprolines are prepared by Rh-catalyzed C-H insertion. The key intermediate, a Rh nitrenoid, is generated by the N-O bond cleavage of a substituted isoxazolidin-5-one. The reaction proceeds on a gram scale with a catalyst loading of as little as 0.1 mol %, affording spirocyclic β-amino acids that are otherwise difficult to obtain. The building blocks prepared in this work will likely find applications in medicinal chemistry.

Lewis Base Assisted Lithium Brønsted Base Catalysis: A New Entry for Catalytic Asymmetric Synthesis of β2,2-Amino Acids

A new catalytic system comprising chiral Ag complex and Li aryloxide/bisphosphine oxide is developed for the synthesis of β^2,2-amino acids via direct asymmetric Mannich-type reaction of 4-subsituted isoxazolidin-5-ones. The Mannich adduct is a direct precursor of β-peptidic compounds otherwise difficult to obtain.

Traceless Electrophilic Amination for the Synthesis of Unprotected Cyclic β-Amino Acids

Electrophilic aminations involve an umpolung of a nitrogen atom, providing an alternate, distinctive synthetic strategy. The recent advent of various designed O-substituted hydroxylamines has significantly advanced this research field. An underappreciated issue is atom economy of the transformations: The necessary activating group on the oxygen atom is left in coproduced waste. Herein, we describe Rh-catalyzed electrophilic amination of substituted isoxazolidin-5-ones for the synthesis of unprotected, cyclic β-amino acids featuring either benzo-fused or spirocyclic scaffolds. Using the cyclic hydroxylamines allows for retaining both nitrogen and oxygen functionalities in the product. The traceless, redox neutral process proceeds on a gram scale with as little as 0.1 mol % catalyst loading. In contrast to related electrophilic aminations in the literature, a series of mechanistic experiments suggests a unique pathway involving spirocyclization, followed by the skeletal rearrangement. The insights provided herein shed light on a nuanced reactivity of the active species, Rh-nitrenoid generated from the activated hydroxylamine, and extend the knowledge on electrophilic aromatic substitutions.

Preparation of conformationally restricted β(2,2)- and β(2,2,3)-amino esters and derivatives containing an all-carbon quaternary center

β-Amino acids are routinely incorporated into peptidic drugs to increase their stability and to incur conformational biases. However, the synthesis of highly substituted β-amino acids still represents a great challenge. A new approach to their preparation is reported involving a Vilsmeier-Haack reaction with nonaromatic carbon nucleophiles. The highly challenging preparation of contiguous tertiary and all-carbon quaternary centers was successfully used to generate several β(2,2,3)-amino esters, such as derivatives of homoproline, homoalanine, and homopipecolinic esters.

To be or not to be butterfly: New mechanistic insights in the Aza-Michael asymmetric addition of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide

The asymmetric Aza-Michael addition of homochiral lithium benzylamides to α,β-unsaturated esters represents an extended protocol to obtain enantioenriched β-amino esters. An exhaustive mechanistic revision of the originally proposed mechanism is reported, developing a quantum mechanics/molecular mechanics protocol for the asymmetric Aza-Michael reaction of homochiral lithium benzylamides. Explicit and implicit solvent schemes were considered, together with a proper account of long-range dispersion forces, evaluated through a density functional theory benchmark of different functionals. Theoretical results showed that the diastereoselectivity is mainly controlled by the N-α-methylbenzyl moiety placing, deriving a Si/Re 99:1 diastereoselective ratio, in good agreement with reported experimental results. The main transition state geometries are two transition state conformers in a "V-stacked" orientation of the amide's phenyl rings, differing in the tetrahydrofuran molecule arrangement coordinated to the metal center. Extensive conformational sampling and quantum-level refinement give reasonable good speed/accuracy results, allowing this protocol to be extended to other similar Aza-Michael reaction systems.

Organocatalytic asymmetric allylic amination of Morita-Baylis-Hillman carbonates of isatins

The investigation of a Lewis base catalyzed asymmetric allylic amination of Morita–Baylis–Hillman carbonates derived from isatins afforded an electrophilic pathway to access multifunctional oxindoles bearing a C3-quaternary stereocenter, provided with good to excellent enantioselectivity (up to 94% ee) and in high yields (up to 97%).