Titanocene(III)-Mediated 5-exo-trig Radical Cyclization: En Route to Spirooxindole-Based Tetrahydrofuran and Bicyclic Lactone

Cp2TiCl-Promoted Endo-Trig Radical Cyclization to Six- and Seven-Membered Carbocycles: Synthesis of (±)-Isoclavukerin A

A systematic study is undertaken to investigate the less explored endo-trig radical cyclization in activated olefin-appended epoxides using Cp2TiCl. The radical generated by the Ti(III)-promoted reductive opening of the epoxy ring promptly underwent endo-trig cyclization, giving access to differently 1,3-disubstituted six- and seven-membered carbocycles in good yields and diastereoselectivity. This protocol was successfully employed in the construction of 5,7- and 6,7-fused bicyclic frameworks entailing a de novo synthesis of (±)-isoclavukerin A belonging to tri-nor-guaiane class of sesquiterpene natural products in eight simple steps from commercially available starting materials. Besides the Ti(III)-mediated reaction serving as a key step in the synthesis, a sequential [2,3]-sigmatropic rearrangement/syn-elimination of an allyl sulfenate intermediate successfully rendered the highly constrained diene moiety in the hydroazulene core of the target molecule.

Diversified Stitching of Ynones with Oxindole-3-oxy acrylates: One-Flask Spiro-annulation Protocol toward Assorted 3H/5H-Spiro[furan-2,3'-indolin]-2'-ones

Spiroannulation of oxindole-3-oxy acrylates with ynones involving two overlapping, base differentiated cascades has been observed. Initial exposure of ynones and oxindole 3-oxy acrylates to K2CO3 triggered a tandem Michael-Michael cascade to deliver a pair of spiroannulated diastereomers. Further exposure to LiHMDS led to deep restructuring through a second multistep cascade involving stereoselective recreation of the C3 quaternary center to furnish 3H-spiro[furan-2,3'-indolin]-2'-ones with functional amplification and scrambling. This new scaffold can be directly accessed in a one-flask operation from ynones and oxindole-3-oxy acrylates.

Strategies for the synthesis of furo-pyranones and their application in the total synthesis of related natural products

The furo-pyranone framework is widely present in the molecular structure of various biologically potent natural products and un-natural small molecules, and it represents a valuable target in synthetic organic chemistry and medicinal chemistry.

Stereoselective synthesis and applications of spirocyclic oxindoles

This review summaries recent synthetic developments towards spirocyclic oxindoles and applications as valuable medicinal and synthetic targets.