Phosphine-catalysed(4+1) Annulations Of β′-acetoxy Allenoate With β,γ-unsaturated Carbonyl Compounds

Phosphine-catalysed annulation plays important role in the field of organic synthesis because of its advantages such as metal-free,environmentally benign and mild reaction condition.Among most of the phosphine-catalysed reactions,the reaction of β’-acetoxy allenoate withβ,γ-unsaturated carbonyl compounds is rarely reported.In this context,we investigated the reactivity of β’-acetoxy allenoate by screening a series of β,γ-unsaturated carbonyl compounds as nucleophiles,allowing the construction of trisubstituted cyclopentene derivatives.Mechanistically,β,γ-unsaturated carbonyl compound serves as bisnucleophile to readily react with the 1,4-biselectrophilic diene-phosphonium intermediate,thereby furnishing the(4+1)annulation product of multifunctional cyclopentene.By catalyst screening,the nucleophilicity and steric effect of a series of phosphine catalysts were evaluated.When bulky phosphine was employed,β’-substitution was obtained as the main product,indicating the reaction pathway is likely to be that the attack of dienophile takes place at β’-position instead ofγ-position as the γ-position of intermediate becomes more congested due to the steric hindrance,thus resulting the product formation in high regioselectivity.Nonetheless,a different reaction process was observed when β,γ-unsaturated N-acylpyrazole was utilised as reaction substrate,with special addition product obtained.Unlike the steric effect,the electronics that dominated the reaction pathways might origin from the more acidic αH of N-acylpyrazoles that would allow rapid deprotonation by the zwitterion intermediate generated by the addition of phosphine to allenoate,generating vinyl phosphonium intermediate,eventually forming the special adduct via an umpolung γ-addition process.Synthetic applications attempts have been carried out.Attempts on complex spirocyclic compounds and tertiary alcohol derivatives synthesis were successful,and the corresponding products are of great practical values.In addition,challenging asymmetric induction was performed with the anticipation of the such method being more versatile for the construction of new chiral quaternary carbon center.In conclusion,this new access to the multi-substituted cyclopentene derivatives should be a complementary to the known vinyl γ addition andα addition,paving the way for realizing new reactivity of β’-acetoxy allenoate aided by Lewis base from both theoretical and practical perspectives.