Zinc‐Catalyzed Tandem Diels–Alder Reactions of Enynals with Alkenes: Generation and Trapping of Cyclic o‐Quinodimethanes (o‐QDMs)

Abstract

A systematic investigation of ZnCl2‐catalyzed reactions of enynals with alkenes has been undertaken. Structurally unique propeller‐like products could be obtained under mild conditions. Cyclic o‐quinodimethanes (o‐QDMs) are generated through [4+2] cycloaddition between enynals and alkenes. Both electron‐poor and electron‐rich dienophiles could be used to trap the active intermediate through [4+2] cycloaddition. But [1,5]‐H shift products could also be observed when electron‐rich alkenes were used as dienophile. DFT calculations were performed to understand the reaction mechanism. A competition between the [4+2] cycloaddition and [1,5]‐H shift was proposed for the transformation of cyclic o‐QDMs. The selectivity could be affected by the properties of the substrates.magnified image

Styrene as 4π-Component in Zn(II)-Catalyzed Intermolecular Diels–Alder/Ene Tandem Reaction

“…Biju et al has taken advantage of benzyne as an effective dienophile to react with styrene under mild conditions (Scheme b) . As our continuing efforts to develop tandem reactions based on alkynes, we desire to develop a more general intermolecular Diels–Alder/ene-tandem reaction between styrene as a 4π-component and alkynes as a 2π-component (Schemec).…”

Isolation, structure elucidation, and KDD study of (−)-Celosine, a new skeleton with potent anti-atherosclerosis activity

“…Therefore, the total synthesis of Celosine was carried out to determine the absolute conguration of Celosine. Based on the skeleton of Celosine, Diels-Alder cycloaddition [8][9][10][11][12][13][14][15] would be the best choice to complete the total synthesis (Scheme 1). Diene compound 3 (or compound 4) and dienophile compound 5, 16 which was also isolated from Celosia cristata L. and other CTMs, 17,18 are the two key intermediates to construct Celosine 1 (or Celosine 2) through asymmetric endo Diels-Alder cycloaddition.…”

NHC–AuCl/Selectfluor: An Efficient Catalytic System for π-Bond Activation

“…Recently, we have reported a ZnCl 2 -catalyzed reaction of enynals with alkenes. 31 Although these reactions gave results different to those with the NHC-AuCl/Selectfluor catalytic system, o-QDM was proposed as the intermediate as well. Therefore, the real mechanism of this unique reaction system still remains hidden.…”