Synthesis of 3,4,5-trisubstituted isoxazoles in water via a [3 + 2]-cycloaddition of nitrile oxides and 1,3-diketones, β-ketoesters, or β-ketoamides

Herein we report a method for the synthesis of 3,4,5-trisubstituted isoxazoles in water under mild basic conditions at room temperature via a [3 + 2]-cycloaddition of nitrile oxides and 1,3-diketones, β-ketoesters, or β-ketoamides. We optimized the reaction conditions to control the selectivity of the production of isoxazoles and circumvent other competing reactions, such as O-imidoylation or hetero [3 + 2]-cycloaddition. The reaction happens fast in water and completes within 1–2 hours, which provides an environmentally friendly access to 3,4,5-trisubstituted isoxazoles, an important class of structures found in numerous bioactive natural products and pharmaceuticals. Additionally, we optimized the reaction conditions to produce trifluoromethyl-substituted isoxazoles, a prevalent scaffold in biomedical research and drug discovery programs. We also proposed a plausible mechanism for the selectivity of the [3 + 2]-cycloaddition reaction to produce 3,4,5-trisubstituted isoxazoles. Not to be overlooked are our optimized reaction conditions for the dimerization of hydroximoyl chlorides to form furoxans also known as 1,2,5-oxadiazole 2-oxides, a class of structures with important biological activities due to their unique electronic nature and coordination ability.

Facile synthesis of O-acylhydroxamates via reaction of oxime chlorides with carboxylic acids

A simple and efficient method for the synthesis of O-acylhydroxamate derivatives from oxime chlorides and carboxylic acids was developed. The reaction affords clean and facile access to diverse O-acylhydroxamates in high yields (up to 85%). The chemical structure of a typical product was confirmed using single-crystal X-ray structure analysis.

A simple and practical method for synthesizing a broad range of functionalized O-acylhydroxamates in high yields from oxime chlorides and carboxylic acids has been achieved.

A three-component approach to isoxazolines and isoxazoles under metal-free conditions

A 1,3-dipolar cycloaddition of 2-methylquinoline, tert-butyl nitrite (TBN) and alkynes or alkenes for the synthesis of biheteroaryls containing both isoxazoline/isoxazole and quinoline motifs has been developed. In this protocol, TBN serves as a convenient N-O source to convert 2-methylquinoline into intermediate nitrile oxides in situ.

Direct access to spirobiisoxazoline via the double 1,3-dipolar cycloaddition of nitrile oxide with allenoate

The double [3 + 2]-cycloadditions between nitrile oxides and allenoates have been achieved. In the presence of DABCO combined with Et₃N, 2-substituted buta-2,3-dienoates reacted with oxime chlorides to afford spirobiisoxazolines in 55-90% yields via the double 1,3-dipolar cycloaddition. Notably, the construction of double isoxazoline moieties and two chiral centers including a spiro carbon center was achieved.

A NHC-catalyzed 1,3-dipolar cycloaddition reaction of allyl ketones with azides: direct access to 1,4,5-trisubstituted 1,2,3-triazoles

A NHC-catalyzed 1,3-dipolar cycloaddition reaction of allyl ketones with azides has been developed. This strategy could generate 1,4,5-trisubstituted 1,2,3-triazoles in high yields and regioselectivities in the presence of a 20 mol% N-heterocyclic carbene catalyst. A broad substrate scope of this process is also presented.

Cycloadduct formation of α,β-unsaturated esters with azides catalyzed by NHC systems

NHC-catalyzed cycloadduct formation of α,β-unsaturated esters with azides has been developed. This strategy could generate 1,2,3-triazoles and dihydropyrazoles with high yields and regioselectivities. The broad substrate scope and mechanistic survey of this process are also presented.