Synthesis of 3‐Acyl‐Isoxazoles via Radical 5‐ endo trig Cyclization of β,γ‐Unsaturated Ketones with NaNO 2

Recent advances of 5-endo-trig radical cyclization: promoting strategies and applications

5-endo-trig radical cyclization has long been recognized as one of the most straightforward ways for the construction of densely functionalized five-membered rings. Nevertheless, according to Baldwin's rules, the 5-endo-trig radical cyclization is kinetically disfavored due to stereoelectronic effects and thus usually proceeds via a slow rate, which renders its application a challenging task. In recent years, with the emergence of efficient radical generation methods and effective cyclization strategies, 5-endo-trig radical cyclization has been successfully accelerated to a synthetically useful rate and has been utilized in the access of diverse five-membered carbo- and heterocyclic compounds. This review comprehensively summarizes the methodologies involving the 5-endo-trig radical cyclization process, with particular emphasis on the elucidation of the promoting strategies, which include the polar effect, geometrical constraints, spin delocalization effect, and persistent radical effect. Each of these strategies is discussed in detail, with illustrative examples from recent literature studies to highlight their practical applications and effectiveness. It is anticipated that the in-depth understanding of the 5-endo-trig radical cyclization provided by this review would inspire further innovation of this privileged reaction mode and expand its applications. Moreover, the potent ring-closure-promoting strategies revealed herein would also contribute to achieving other challenges of cyclizations with particular significance for organic synthesis.

Access to Isoxazoles via Photo-oxygenation of Furan Tethered α-Azidoketones

Photocatalyst-free visible light-enabled direct oxygenation of furan-tethered α-azidoketones was studied. The reaction yielded various products depending on the substituents, with isoxazoles forming as the major products. The findings suggest that singlet oxygen was generated during the reaction and reacted with α-azidoketones in a [4 + 2] fashion to yield endoperoxides, which rearranged in multiple ways to generate isoxazoles. Some of the synthesized isoxazoles were evaluated as α-glucosidase inhibitors, and three of them 5bi, 5bj, and 5bl exhibited good activity with IC50 values of 454.57 ± 29.34, 147.84 ± 2.28, and 272.58 ± 42.06 μM, respectively, when compared with the standard drug acarbose (IC50 = 1224.33 ± 126.72 μM).

Tunable Key [3 + 2] and [2 + 1] Cycloaddition of Enaminones and α-Diazo Compounds for the Synthesis of Isomeric Isoxazoles: Metal-Controlled Selectivity

The three-component reactions of enaminones, α-diazo esters/ketones, and t-butyl nitrite (TBN) for the switchable synthesis of isomeric isoxazoles have been realized. The catalysis with Cu(II) salt provides 3,4-disubsituted isoxazoles via [3 + 2] cycloaddition. On the other hand, the catalysis of Ag(I) with identical substrates leads to isomeric isoxazoles with reversed C3 and C4 substitution based on a key [2 + 1] cycloaddition.