One-pot synthesis of α-amino phosphonates from α-amino acids and β-amino alcohols

Regioselective α-Phosphonylation of Unprotected Alicyclic Amines

Unprotected alicyclic amines undergo α-C-H bond phosphonylation via a two-stage one-pot process involving the oxidation of amine-derived lithium amides with simple ketone oxidants, generating transient imines which are then captured with phosphites or phosphine oxides. Amines with an existing α-substituent undergo regioselective α'-phosphonylation. Amine α-arylation and α'-phosphonylation can be combined, generating a difunctionalized product in a single operation.

Convergent Deboronative and Decarboxylative Phosphonylation Enabled by the Phosphite Radical Trap "BecaP"

Carbon-phosphorus bond formation is significant in synthetic chemistry because phosphorus-containing compounds offer numerous indispensable biochemical roles. While there is a plethora of methods to access organophosphorus compounds, phosphonylations of readily accessible alkyl radicals to form aliphatic phosphonates are rare and not commonly used in synthesis. Herein, we introduce a novel phosphorus radical trap "BecaP" that enables facile and efficient phosphonylation of alkyl radicals under visible light photocatalytic conditions. Importantly, the ambiphilic nature of BecaP allows redox neutral reactions with both nucleophilic (activated by single-electron oxidation) and electrophilic (activated by single-electron reduction) alkyl radical precursors. Thus, a broad scope of feedstock alkyl potassium trifluoroborate salts and redox active carboxylate esters could be employed, with each class of substrate proceeding through a distinct mechanistic pathway. The mild conditions are applicable to the late-stage installation of phosphonate motifs into medicinal agents and natural products, which is showcased by the straightforward conversion of baclofen (muscle relaxant) to phaclofen (GABAB antagonist).

Facile Conversion of α-Amino Acids into α-Amino Phosphonates by Decarboxylative Phosphorylation using Visible-Light Photocatalysis

Amino phosphonates exhibit potent inhibitory activity for a wide range of biological processes due to their specific structural and electronic properties, making them important in a plethora of applications, including as enzyme inhibitors, herbicides, antiviral, antibacterial, and antifungal agents. While the traditional synthesis of α-amino phosphonates has relied on the multicomponent Kabachnik-Fields reaction, we herein describe a novel and facile conversion of activated derivatives of α-amino acids directly to their respective α-amino phosphonate counterparts via a decarboxylative radical-polar crossover process enabled by the use of visible-light organophotocatalysis. The novel method shows broad applicability across a range of natural and synthetic amino acids, operates under mild conditions, and has been demonstrated to successfully achieve the late-stage functionalization of drug molecules.

“Cut and Paste” Processes in the Search of Bioactive Products: One-Pot, Metal-free O-Radical Scission-Oxidation-Addition of C, N or P-Nucleophiles

Hypervalent iodine reagents have been applied in many metal-free, efficient synthesis of natural products and other bioactive compounds. In particular, treatment of alcohols, acetals and acids with hypervalent iodine reagents and iodine results in O-radicals that can undergo a β-scission reaction. Under these oxidative conditions, derivatives of amino acids, peptides or carbohydrates are converted into cationic intermediates, which can subsequently undergo inter- or intramolecular addition of nucleophiles. Most reported papers describe the addition of oxygen nucleophiles, but this review is focused on the addition of carbon, nitrogen and phosphorous nucleophiles. The resulting products (nucleoside and alkaloid analogs, unnatural amino acids, site-selectively modified peptides) are valuable intermediates or analogs of bioactive compounds.

Synthesis of Functionalized Diethyl(pyrrolidin-2-yl)phosphonate and Diethyl(5-oxopyrrolidin-2-yl)phosphonate

Short and efficient syntheses of functionalized (pyrrolidin-2-yl)phosphonate and (5-oxopyrrolidin-2-yl)phosphonate have been developed. The synthetic strategy involved the diastereospecific 1,3-dipolar cycloaddition of N-benzyl-C-(diethoxyphosphoryl)nitrone to cis-1,4-dihydroxybut-2-ene and dimethyl maleate, respectively. O,O-Diethyl 3-carbamoyl-4-hydroxy(5-oxopyrrolidin-2-yl)phosphonate was obtained from O,O-diethyl 2-benzyl-4,5-dimethoxycarbonyl(isoxazolidin-3-yl)phosphonate by hydrogenation and subsequent treatment with ammonia, whereas transformation of O,O-diethyl 2-benzyl-4,5-dihydroxymethyl(isoxazolidin-3-yl)phosphonate into O,O-diethyl 3-aminomethyl-4-hydroxy(pyrrolidin-2-yl)phosphonate was accomplished by mesylation followed by hydrogenolysis to undergo intramolecular cyclization and the introduction of amino group via ammonolysis. Stereochemistry of the isoxazolidine cycloadducts, as well as the final functionalized (pyrrolidin-2-yl)- and (5-oxopyrrolidin-2-yl)phosphonates were established based on conformational analyses using vicinal H-H, H-P, and C-P couplings and supported by the observed diagnostic NOESY correlation signals.

New approaches towards the synthesis of 1,2,3,4-tetrahydro isoquinoline-3-phosphonic acid (TicP)

Two new strategies for the efficient synthesis of racemic 1,2,3,4-tetrahydroisoquinoline-3-phosphonic acid (Tic^P) (±)-2 have been developed. The first strategy involves the electron-transfer reduction of the easily obtained α,β-dehydro phosphonophenylalanine followed by a Pictet-Spengler cyclization. The second strategy involves a radical decarboxylation-phosphorylation reaction on 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (Tic). In both strategies, the highly electrophilic N-acyliminium ion is formed as a key intermediate, and the target compound is obtained in good yield using mild reaction conditions and readily available starting materials, complementing existing methodologies and contributing to the easy accessibility of (±)-2 for further research.

Small peptide diversification through photoredox-catalyzed oxidative C-terminal modification

A photoredox-catalyzed oxidative decarboxylative coupling of small peptides is reported, giving access to a variety of N,O-acetals. They were used as intermediates for the addition of phenols and indoles, leading to novel peptide scaffolds and bioconjugates. Amino acids with nucleophilic side chains, such as serine, threonine, tyrosine and tryptophan, could also be used as partners to access tri- and tetrapeptide derivatives with non-natural cross-linking.

Organo or Metal Complex Catalyzed Synthesis of Five-membered Oxygen Heterocycles

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The reactions involving the formation of C-O bond using metal as a catalyst have emerged to be one of the most influential reactions for the synthesis of heterocycles in modern organic chemistry. Catalysis by metals offers diverse opportunities to invent new organic reactions with a promising range of selectivities such as chemoselectivity, regioselectivity, diastereoselectivity, and enantioselectivity. The methodologies used earlier for synthesis were less approachable to the organic chemist because of their high cost, highly specified instrumentation and inconvenient methods. For both stereoselective and regioselective formation of five-membered O-containing heterocycles, cyclic reactions that are metal and non-metal-catalyzed have known to be very efficient. The present review article covers the applications of metal and non-metal as a catalyst for the synthesis of five-membered O-containing heterocycles.

Practical and Efficient Synthesis of α-Aminophosphonic Acids Containing 1,2,3,4-Tetrahydroquinoline or 1,2,3,4-Tetrahydroisoquinoline Heterocycles

We report here a practical and efficient synthesis of α-aminophosphonic acid incorporated into 1,2,3,4-tetrahydroquinoline and 1,2,3,4-tetrahydroisoquinoline heterocycles, which could be considered to be conformationally constrained analogues of pipecolic acid. The principal contribution of this synthesis is the introduction of the phosphonate group in the N-acyliminium ion intermediates, obtained from activation of the quinoline and isoquinoline heterocycles or from the appropriate δ-lactam with benzyl chloroformate. Finally, the hydrolysis of phosphonate moiety with simultaneous cleavage of the carbamate afforded the target compounds.

The sunny side of chemistry: green synthesis by solar light

The storage of solar energy is one of the main challenges in the near future. A rather unexploited way to fulfil this goal is the solar light induced formation of new chemical bonds, i.e. the synthesis of chemicals. Solar photons can be considered the ideal green reagents since they are costless and leave no residue in the reaction mixture. In many cases the solar radiation could be successfully used in place of toxic or expensive chemical reagents to overcome the activation energy in organic synthesis. In this perspective, the emerging trends on the use of solar light for green synthesis are summarized, highlighting the advantages of this photochemical method.

An Overview of Stereoselective Synthesis of α-Aminophosphonic Acids and Derivatives

An overview of all methodologies published during the last few years focused to the stereoselective (diastereoselective or enantioselective) synthesis of α-aminophosphonic acids and derivatives is reported. The procedures have been classified according a retrosynthetic strategy and taking into account the formation of each one of the bonds connected to the chiral centre.

Synthesis of Unnatural Amino Acids from Serine Derivatives by β-Fragmentation of Primary Alkoxyl Radicals

The fragmentation of primary alkoxyl radicals has been scarcely used in synthesis since other competing processes (such as oxidation or hydrogen abstraction) usually predominate. However, when serine derivatives were used as substrates, the scission took place in excellent yields. Tandem scission-allylation, -alkylation, or -arylation reactions were subsequently developed. This one-pot methodology was applied to the synthesis of unnatural amino acids, which are useful synthetic blocks or amino acid surrogates in peptidomimetics.