Synthesis of 5-Phosphonoisoxazol­ineN-Oxides by SelectiveO-Alkylation of Nitronate Anions

Synthetic Methods for Azaheterocyclic Phosphonates and Their Biological Activity: An Update 2004-2024

The increasing importance of azaheterocyclic phosphonates in the agrochemical, synthetic, and medicinal field has provoked an intense search in the development of synthetic routes for obtaining novel members of this family of compounds. This updated review covers methodologies established since 2004, focusing on the synthesis of azaheterocyclic phosphonates, of which the phosphonate moiety is directly substituted onto to the azaheterocyclic structure. Emphasizing recent advances, this review classifies newly developed synthetic approaches according to the ring size and providing information on biological activities whenever available. Furthermore, this review summarizes information on various methods for the formation of C-P bonds, examining sustainable approaches such as the Michaelis-Arbuzov reaction, the Michaelis-Becker reaction, the Pudovik reaction, the Hirao coupling, and the Kabachnik-Fields reaction. After analyzing the biological activities and applications of azaheterocyclic phosphonates investigated in recent years, a predominant focus on the evaluation of these compounds as anticancer agents is evident. Furthermore, emerging applications underline the versatility and potential of these compounds, highlighting the need for continued research on synthetic methods to expand this interesting family.

Reductive Recyclization of sp3-Enriched Functionalized Isoxazolines into α-Hydroxy Lactams

An efficient synthesis (up to a 200 g scale) of 3-hydroxypyrrolidin-2-ones bearing alkyl substituents or functional groups at the C-5 position is described. The reaction sequence started from 1,3-dipolar cycloaddition of in situ generated nitrile oxides with (meth-)acrylates into 3-substituted isoxazoline-5-carboxylates. The catalytic hydrogenolysis of the isoxazoline N-O bond was optimal upon using H₂ (1 atm) at rt, with the following order of the catalyst activity: Pd-C > Pd(OH)₂-C > Pt-C. The reactions with Pt-C were more selective for the synthesis of pyrrolidones, while Pd-C provided the fastest conversion rates. The stirring efficiency had a positive impact on conversion rather than elevated temperatures (up to 40 °C) or pressure (up to 50 atm). The diastereoselectivity was governed mainly by steric factors, with a dr of 1:1 to 3:1 (cis- and trans-isomers could be separated). Higher homologues (isoxazolinylacetates and -propanoates) were suitable for the synthesis of 6- or 7-substituted 4-hydroxypiperidones and 5-hydroxyazepanones, respectively. The proposed methods are tolerant to functional groups, including CF₃ (but not CHF₂ or CH₂Cl), ester, and most N-Boc-protected amines. The utility of hydroxyl groups in lactams was shown by functional group transformations. Hydrogenolysis of C(5)-functionalized isoxazolines, bearing trimethylsilyl, phosphonate, or sulfone groups, was also studied to demonstrate limitations.

Synthesis of 2-Isoxazoline N-Oxides by Copper-Mediated Radical Annulation of Alkenes with α-Nitrobenzyl Bromides

Copper-mediated [3 + 2] annulation of alkenes with α-nitrobenzyl bromides has been developed. The reaction is promoted simply by a copper salt to produce the corresponding 2-isoxazoline N-oxides with perfect regioselectivity. The present method can be conducted under mild conditions, affording a diverse array of 2-isoxazoline N-oxides. The obtained products can readily be converted to the related heterocycles such as 2-isoxazoline and isoxazole. A radical-polar crossover pathway initiated by single-electron transfer from nitronate to a copper salt is proposed.

Regioselective synthesis of isoxazole and 1,2,4-oxadiazole-derived phosphonates via [3 + 2] cycloaddition

The regioselective synthesis of 3,4- and 3,5-disubstituted izoxazole- and 1,2,4-oxadiazole-derived phosphonates is described, including analogues of phosphohistidine.