Cyclization Reactions of Oxyallyl Cation. A Method for Cyclopentane Ring Formation

Abstract

Unsaturated oxyallyl cations with a suitably positioned alkene bond undergo 5-exo-cyclization with the formation of vinylcyclopentane derivatives. Alkyne analogues provide allenes. The reaction proceeds with a moderate to excellent level of stereoselectivity and allows for asymmetric induction in the reaction with chiral substrate.

Construction of Cyclopentanes Consisting of Five Stereocenters via NHC-Catalyzed Cascade Reactions of Enals with Oxindole-Dienones

“… Given the pivotal role played by chiral cyclopentanes, an array of synthetic methodologies has been devised to access this motif (Scheme A) . These strategies include [3 + 2] cycloaddition, radical cycloaddition, cascade reaction, intramolecular cyclization, ring contraction, and more. Asymmetric [3 + 2] cycloadditions of “donor–acceptor” cyclopropanes, such as vinylcyclopropanes (VCPs), with alkenes offer an efficient route to enantioenriched cyclopentanes with one to three stereocenters (Scheme B) .…”

A Nazarov‐Ene Tandem Reaction for the Stereoselective Construction of Spiro Compounds

“…We report the discovery of an unpreceded Nazarov/ene cyclization tandem reaction for the stereoselective construction of spirocyclic molecules, which extends the recently published ene-type reactions of oxyallyl cations with tethered olefins by the Saicic group (Scheme 2). [12] In order to explore the general reaction conditions, we started our investigations with simplified 3, reduced to the essential functional groups that the envisioned cyclization required (Scheme 3).…”

“…Additionally, the very distinct relative stereochemistry at the αposition of the cyclization products (8, 11, 13 and 15) led to the suggestion that the reaction between the initially formed oxyallyl cation C and the tethered alkene proceeds through an ene-type mechanism (Scheme 7, I.). [12,21] In order to evaluate this hypothesis we designed cyclization precursor 17 with a perdeuterated (E)-methyl group. Exposure to our catalytic cyclization conditions afforded deuterated spirocycle 18 as major product with selective deuterium incorporation at position 2.…”

“…We report the discovery of an unpreceded Nazarov/ene cyclization tandem reaction for the stereoselective construction of spirocyclic molecules, which extends the recently published ene‐type reactions of oxyallyl cations with tethered olefins by the Saicic group (Scheme 2 ). [12] …”

“…Additionally, the very distinct relative stereochemistry at the α‐position of the cyclization products ( 8 , 11 , 13 and 15 ) led to the suggestion that the reaction between the initially formed oxyallyl cation C and the tethered alkene proceeds through an ene‐type mechanism (Scheme 7 , I.). [ 12 , 21 ]…”