Catalytic Asymmetric (3 + 3) Cycloaddition of Oxyallyl Zwitterions with α-Diazomethylphosphonates

Synthesis of Spirooxindole-1,2-oxazinan-5-ones through 2,2,2-Trifluoroethanol Promoted [3 + 3] Cycloaddition of N-Vinyl Oxindole Nitrones and Oxyallyl Cations

A variety of spirooxindole-1,2-oxazinan-5-one derivatives were prepared in moderate to excellent yields through 2,2,2-trifluoroethanol (TFE)-promoted [3 + 3] cycloaddition of N-vinyl oxindole nitrones with oxyallyl cations generated from α-tosyloxy ketones under mild reaction conditions. Mechanistic studies revealed that [3 + 3] cycloaddition might involve two possible reaction pathways, including direct [3 + 3] cycloaddition of N-vinyl oxindole ntirones with oxyallyl cations, or the addition of TFE to N-vinyl oxindole nitrones, sequential addition to oxyallyl cations, elimination, and cyclization. The present method features mild reaction conditions, broad substrate scope, good functional group tolerance, easy gram scalable preparation, and new applications of TFE.

Oxygen-Promoted 6-endo-trig Cyclization of β,γ-Unsaturated Hydrazones/Ketoximes with Diazonium Tetrafluoroborates for Pyridazin-4(1H)-ones/Oxazin-4(1H)-ones

An oxidative 6-endo-trig cyclization and [2 + 2] cycloaddition of β,γ-unsaturated hydrazones/ketoximes and diazonium tetrafluoroborates for a synthetic strategy to pyridazin-4(1H)-ones/oxazin-4(1H)-ones under metal-free conditions is presented in a one-pot procedure. This protocol features excellent functional group tolerance and remarkable regioselectivity. A mechanistic study has been verified via 18O labeling and the H218O labeling method, in which O2 acts as both a reaction component and an oxidant.

DBU-Promoted Tandem Cyclization of Ynones and Diazo Compounds: Direct Synthesis of Eight-Membered Cyclic Ethers

A novel DBU-promoted tandem cyclization reaction of ynones with diazo compounds as the N-terminal electrophiles has been developed. The reaction provides a simple and efficient method for the synthesis of fused eight-membered oxocino[2,3-c] pyrazoles from readily available acyclic starting materials in a single step. This reaction allows the formation of four new bonds and two rings in a highly regio- and diastereoselective manner, where two adjacent stereocenters are created simultaneously in an atom-economic manner.

Highly Enantioselective Synthesis of [1,2,4]Triazino[5,4-a]isoquinoline Derivatives via (3 + 3) Cycloaddition Reactions of Diazo Compounds and Isoquinolinium Methylides

An array of chiral [1,2,4]triazino[5,4-a]isoquinoline derivatives were obtained in excellent yields (up to 98%) and with excellent enantioselectivities (up to 99% ee) via a new highly asymmetric (3 + 3) cycloaddition reaction of diazo compounds and isoquinolinium methylides, with a bifunctional chiral phase-transfer catalyst (PTC). Density functional theory calculations show that PTC has a bridge role in the deprotonation/protonation process. The obtained products were transformed into densely functionalized polycyclic heterocompounds with multiple stereocenters.