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We described a copper(II)‐catalyzed [3+2] cycloaddition of N‐aryl‐α,β‐unsaturated nitrones with disubstituted allenoates bifurcated to prepare various [1,3]oxazino[3,2‐a]indolines and dihydropyrido[1,2‐a]indolines in moderate to excellent yields. Mechanistic studies revealed that [1,3]oxazino[3,2‐a]indoline was a kinetically favored product and dihydropyrido[1,2‐a]indoline was thermodynamically favored product. Moreover, the reaction was easily performed at gram scales and chiral dihydropyrido[1,2‐a]indoline could be obtained by using chiral auxiliary. The present method highlights broad substrate scope, good functional group tolerance, and diversity of indoline scaffolds with high diastereoselectivity. [ABSTRACT FROM AUTHOR]

Various spirooxindole‐benzo[d]oxazoles and dihydrobenzofurans were prepared in good to excellent yields by [3+2] cycloaddition and selective rearrangement of N‐vinyl oxindole nitrones and arynes under transition metal‐free conditions. Experimental results showed that the substituent on the nitrone N‐vinyl group controlled the [1,3]‐ or [3,3]‐rearrangement of their cycloadduct owing to its steric effect. The present method features broad substrate scope, good functional group tolerance, controllable [1,3]‐ or [3,3]‐rearrangement, and diverse oxindole scaffolds. [ABSTRACT FROM AUTHOR]

A variety of chiral nine‐membered N‐heterocycles were prepared in moderate to good yields with high diastereoselectivity through a silver(I)‐catalyzed [3+2] cycloaddition and [3,3]‐rearrangement of N‐vinyl‐α, β‐unsaturated nitrones and chiral 3‐propioloyloxazolidin‐2‐ones. Experimental studies showed that silver catalyst promoted the cycloaddition and rearrangement process, and the stereochemistry of the nine‐membered N‐heterocycles was controlled via [3,3]‐rearrangement by chiral oxazolidinone‐auxiliary through a boat‐like transition state. Moreover, the obtained nine‐membered N‐heterocycle diastereomers were converted to chiral pyrrolizines with high diastereoselectivity and pyrrolizine carboxylate was obtained in 54% yield with 90% ee by the removal of chiral auxiliary. [ABSTRACT FROM AUTHOR]

A variety of chromeno[4,3‐c]isoxazolidines were prepared in good yields through visible light promoted Chan‐Lam reaction and [3+2] cycloaddition cascade reaction in one pot. Mechanistic studies showed that visible light promoted both Chan‐Lam reaction and cycloaddition. The obtained products were converted to various useful chromenone derivatives. Moreover, the reaction was easily performed at gram scales with the purification of products without column chromatography and used to efficiently synthesize estrone‐derived chromeno[4,3‐c]isoxazolidine. [ABSTRACT FROM AUTHOR]

We report a Yb(OTf)3‐catalyzed formal [7+2] cycloaddition for the preparation of nine‐membered N‐heterocycles bearing three stereocenters in good yields with high diastereoselectivity through a [3+2] cycloaddition/[3,3]‐rearrangement cascade strategy from N‐vinyl‐α,β‐unsaturated nitrones and methylenecyclopropanes. Preliminary studies show that the Yb(OTf)3 catalyst accelerates the reaction affording kinetically controlled cis‐diastereomers as the major products. The obtained nine‐membered N‐heterocycles are easily converted to useful derivatives for late‐stage modification. [ABSTRACT FROM AUTHOR]

A variety of spirofluorenyl‐β‐lactams have been prepared in moderate yields over three steps from N‐aryl fluorenone nitrones and methylenecyclopropanes through a [3+2] cycloaddition, reduction and subsequent acid‐catalyzed ring contraction cascade strategy under mild reaction conditions. Experimental studies showed that the stereochemistry remained intact after the ring contraction step. Furthermore, the obtained spirofluorenyl‐β‐lactam was easily converted to two novel spiroazetidine and spiroquinolinone scaffolds in good yields. [ABSTRACT FROM AUTHOR]

Various polycyclic furo[2,3‐b]indolines containing a cyclopropane ring have been prepared in moderate yields with high diastereoselectivity from N‐aryl isatin nitrones and methylenecyclopropanes using a copper‐catalyzed [3+2] cycloaddition and subsequent interrupted Fischer indolization strategy under mild reaction conditions. The obtained furo[2,3‐b]indolines were easily converted to piperidinone‐fused furo[2,3‐b]indolines by selective [1,3]‐rearrangement through N−O bond cleavage. Finally, two chiral polycyclic furo[2,3‐b]indolines were obtained by chiral auxiliary control. [ABSTRACT FROM AUTHOR]

A metal-free construction of highly diastereoselective nine-membered heterocycles is described via the cycloaddition and rearrangement of N-vinyl- α,β-unsaturated ketonitrones and isocyanates. Notably, the prepared nine-membered heterocycles afforded 2,3-dihydropyrrolizines under heating conditions. Mechanistic studies showed that the nine-membered rings might undergo decarboxylation, isomerization, aza-Michael addition, and aromatization to afford 2,3-dihydropyrrolizines in a one-pot reaction. [ABSTRACT FROM AUTHOR]