title: Enantioselective synthesis of spiro γ-butyrolactones by N-heterocyclic carbene (NHC)-catalyzed formal [3 + 2] annulation of enals with 3-hydroxy oxindoles.
abstract: The N-heterocyclic carbene (NHC)-catalyzed enantioselective formal [3 + 2] annulation of α,β-unsaturated aldehydes with 3-hydroxy oxindoles is presented. Under oxidative conditions using the bisquinone oxidant, the reaction resulted in the synthesis of spiro γ-butyrolactones in moderate to good yields, enantioselectivity and diastereoselectivity. The reaction likely proceeds via the generation of the NHC-bound α,β-unsaturated acylazolium intermediate from enals, which was intercepted by the dioxindoles in a formal [3 + 2] pathway to form the spirocyclic compounds. However, a deeper mechanistic investigation revealed that the reaction can also proceed via the homoenolate intermediate. In this case, the dioxindole was oxidized to the corresponding isatin derivative using traces of air under basic conditions, and was intercepted with the NHC-bound homoenolate intermediate in a formal [3 + 2] pathway to afford the spiro compound.
authors.affiliation: Organic Chemistry Division, CSIR-National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India. y.santhivardhana@ncl.res.in at.biju@ncl.res.in.
authors.forename: Subrata
authors.initials: S
authors.lastname: Mukherjee
authors.affiliation: Organic Chemistry Division, CSIR-National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India. y.santhivardhana@ncl.res.in at.biju@ncl.res.in.
authors.forename: Sumi
authors.initials: S
authors.lastname: Joseph
authors.affiliation: Organic Chemistry Division, CSIR-National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India. y.santhivardhana@ncl.res.in at.biju@ncl.res.in.
authors.forename: Anup
authors.initials: A
authors.lastname: Bhunia
authors.affiliation: Centre for Materials Characterization, CSIR-National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India.
authors.forename: Rajesh G
authors.identifier: http://orcid.org/0000-0002-2841-0197
authors.initials: RG
authors.lastname: Gonnade
authors.affiliation: Organic Chemistry Division, CSIR-National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India. y.santhivardhana@ncl.res.in at.biju@ncl.res.in.
authors.forename: Santhivardhana Reddy
authors.initials: SR
authors.lastname: Yetra
authors.affiliation: Organic Chemistry Division, CSIR-National Chemical Laboratory (CSIR-NCL), Dr. Homi Bhabha Road, Pune-411008, India. y.santhivardhana@ncl.res.in at.biju@ncl.res.in.
authors.forename: Akkattu T
authors.identifier: http://orcid.org/0000-0002-0645-8261
authors.initials: AT
authors.lastname: Biju