Mechanism of Acylative Oxidation–Reduction–Condensation Reactions Using Benzoisothiazolones as Oxidant and Triethylphosphite as Stoichiometric Reductant

Synthesis of Mesylated and Tosylated α-Hydroxy-Benzylphosphonates; Their Reactivity and Cytostatic Activity

α-Hydroxyphosphonates and their acylated and phosphorylated derivatives may be of significant biological activity including cytotoxic effects. To extend the pool of the potentially bioactive species, new methane- and arenesulfonyloxyphosphonates were synthesized by the sulfonylation of differently substituted α-hydroxy-benzylphosphonates using methanesulfonyl chloride or p-toluenesulfonyl chloride at 25 °C in the presence of triethylamine in toluene. The new sulfonyl derivatives were obtained in 54-80% yields. In the case of the 4- or 2-methoxy substituent in the aromatic ring, surprisingly the corresponding α-chlorophosphonates were the exclusive products, whose formation was explained assuming a quinoid intermediate and supported by theoretical calculations. With a 3-methoxyphenyl substituent, the expected mesylation of the hydroxy group took place. Attempted alcoholyses of the diethyl α-methanesulfonyloxy-benzylphosphonates with different substituents in the benzyl ring at ∼140 °C in the presence of triethylamine under microwave irradiation left the P-function intact under the conditions applied, instead, the mesyloxy group was substituted by an alkoxy unit in a selective new reaction. The α-alkoxy-benzylphosphonates were isolated in 60-77% yields. While α-chloro- or α-bromo-benzylphosphonates proved to be rather inefficient in the Michaelis-Arbuzov reaction with triethyl phosphite, according to a new possibility, the α-methansulfonyloxy-benzylphosphonates underwent an efficient Arbuzov fission using the phosphite in excess at 135 °C. The arylmethylenebisphosphonates were obtained in yields of 76-81%. Bioactivity studies with the members of the phosphonate library revealed pronounced in vitro cytostatic effect of the α-hydroxy- and α-mesyloxy-3,5-di-tert-butylbenzylphosphonates on human breast carcinoma cell culture with IC50 values of 16.4 and 28.0 μM, respectively. The mesyloxy species was also cytostatic on melanoma cells (IC50 = 34.9).

Recent Advances in the Application of P(III)-Nucleophiles to Create New P-C Bonds through Michaelis-Arbuzov-Type Rearrangement

Organophosphorus compounds have long been considered valuable in both organic synthesis and life science. P(III)-nucleophiles, such as phosphites, phosphonites, and diaryl/alkyl phosphines, are particularly noteworthy as phosphorylation reagents for their ability to form new P-C bonds, producing more stable, ecofriendly, and cost-effective organophosphorus compounds. These nucleophiles follow similar phosphorylation routes as in the functionalization of P-H bonds and P-OH bonds. Activation can occur through photocatalytic, electrocatalytic, or thermo-driven reactions, often in coordination with a Michaelis-Arbuzov-trpe rearrangement process, to produce the desired products. As such, this review offers a thorough overview of the phosphorylated transformation and potential mechanisms of P(III)-nucleophiles, specifically focusing on developments since 2010. Notably, this review may provide researchers with valuable insights into designing and synthesizing functionalized organophosphorus compounds from P(III)-nucleophiles, guiding future advancements in both research and practical applications.

Rational Design of an Organocatalyst for Peptide Bond Formation

Amide bonds are ubiquitous in peptides, proteins, pharmaceuticals, and polymers. The formation of amide bonds is a straightforward process: amide bonds can be synthesized with relative ease because of the availability of efficient coupling agents. However, there is a substantive need for methods that do not require excess reagents. A catalyst that condenses amino acids could have an important impact by reducing the significant waste generated during peptide synthesis. We describe the rational design of a biomimetic catalyst that can efficiently couple amino acids featuring standard protecting groups. The catalyst design combines lessons learned from enzymes, peptide biosynthesis, and organocatalysts. Under optimized conditions, 5 mol % catalyst efficiently couples Fmoc amino acids without notable racemization. Importantly, we demonstrate that the catalyst is functional for the synthesis of oligopeptides on solid phase. This result is significant because it illustrates the potential of the catalyst to function on a substrate with a multitude of amide bonds, which may be expected to inhibit a hydrogen-bonding catalyst.

Esterification by Redox Dehydration Using Diselenides as Catalytic Organooxidants

Ortho-functionalized aryl diselenides are catalytic (5.0 mol %) oxidants for the construction of esters from carboxylic acids and alcohols in the presence of stoichiometric triethyl phosphite and dioxygen in air as the terminal redox reagents (redox dehydration conditions). The reaction proceeds through the intermediacy of the anhydride and requires the presence of 10% DMAP to drive the esterification.

Reinvestigation of the iodine-mediated phosphoramidation reaction of amines and P(OR)3 and its synthetic applications

A systematic study on the iodine-mediated phosphoramidation reaction of amines and trialkyl phosphites was conducted, which not only disclosed the factors affecting the reaction but also revealed that it could proceed smoothly in CH2Cl2 at room temperature in open air. Using this method, various phosphoramidates with different aliphatic amines and aromatic amines were synthesized in good to excellent yields. Our present investigation shows that this underused method is actually a mild, practical and general way to synthesize phosphoramidates and will have wide applications.

Aerobic, Diselenide-Catalyzed Redox Dehydration: Amides and Peptides

At 2.5 mol % loadings using reaction temperatures between 30-55 °C, ortho-functionalized diaryl diselenides are highly effective organocatalytic oxidants for aerobic redox dehydrative amidic and peptidic bond formation using triethyl phosphite as a simple terminal reductant. This simple-to-perform organocatalytic reaction relies on the ability of selenols to react directly with dioxygen in air without recourse to metal catalysts. It represents an important step toward the development of a general, economical, and benign catalytic redox dehydration protocol.