Cycloadditions of 3,4-dihydro-2H-pyrrole 1-oxide to methylene-γ-butyrolactones

Acid-mediated coupling of γ-hydroxybutenolides and aldehydes: Synthesis of a new class of spirocyclic ketal-lactones

In the presence of trimethylsilyl trifluoromethanesulfonate (TMSOTf), γ-methyl-γ-hydroxybutenolide reacts with aromatic aldehydes to generate a new class of stereochemically rich spirocyclic ketal-lactones in good yields and with excellent stereoselectivities. We believe that this process takes place through the in situ generation of protoanemonin followed by a Prins reaction. Herein, we describe this discovery, along with substrate scope and preliminary mechanistic studies.

1′,3′,3′-Trimethyl-2,3-diphenyl-2,3-dihydroisoxazole-5(4H)-spiro-2′-indoline

Two diastereoisomers of the title compound, C₂₅H₂₆N₂O, have been prepared by cyclo­addition between 1,3,3-trimethyl-2-methyl­eneindoline and C-phenyl-N-phenyl­nitrone. The stereochemistry of the major diastereoisomer, viz. S,R/R,S, is confirmed by the X-ray analysis. The oxazole and the pyrole rings have envelope conformations. The packing is stabilized by weak C—H⋯π inter­actions involving the phenyl ring attached to the N atom of the oxazole and the phenyl ring of the indole fragment.

Spiroheterocycles from reaction of nitrones with methylene-gamma-butyrolactones and some of their rearrangements

Cycloaddition of nitrones 1 with methylene-gamma-butyrolactones 2, 3 and 4 afforded spiroadducts 5, 6 and 7 with high selectivity. Mixtures of diastereoisomers were usually obtained, whose structures were established by 1H and 13C NMR spectroscopies or X-ray crystallography. Treatment of spiroadducts in acidic and alkaline media led to different, unexpected and novel rearrangements.