title: Concise synthesis of α-substituted 2-benzofuranmethamines and other 2-subsituted benzofurans via α-substituted 2-benzofuranmethyl carbocation intermediates.
abstract: Propargyl amines 4, where R(3) is aryl, undergo 5-exo-dig cyclization reactions under relatively mild conditions (AgNO(3), DMF, 60 °C, 1 h) to give 3-amino-2,3-dihydro-2-arylmethylidenebenzofurans 5 (R(3) = aryl). In contrast, substrates where R(3) is alkyl undergo competing 6-endo-dig and 5-exo-dig cyclization processes. The hydroxymethyl substrate 4 (R(3) = CH(2)OH), however, was smoothly converted to its corresponding 5-exo-dig cyclization product 5, likely due to the assistance of the primary hydroxyl group in the 5-exo-dig cyclization process by silver cation coordination. Under more enforcing conditions (AgNO(3), DMF, 100 °C, 18 h), the initially formed products 5 undergo a 1,3-allylic rearrangement to their corresponding 2-substituted benzofuran derivatives 6. This rearrangement can also be effected by treating 5 with AgNO(3) in DMF at 100 °C for 18 h or BF(3)·Et(2)O at rt. 2-(3-Butenyl)benzofurans 7 (Nu = allyl) can be prepared by treatment of 5 with BF(3)·Et(2)O and allyltributylstannane. Furan and MeOH could also be employed as external nucleophiles in these BF(3)·Et(2)O-promoted reactions.
authors.affiliation: School of Chemistry, University of Wollongong, Wollongong, New South Wales, 2522, Australia.
authors.forename: Nikhom
authors.initials: N
authors.lastname: Wongsa
authors.forename: Ubonta
authors.initials: U
authors.lastname: Sommart
authors.forename: Thunwadee
authors.initials: T
authors.lastname: Ritthiwigrom
authors.forename: Arife
authors.initials: A
authors.lastname: Yazici
authors.forename: Somdej
authors.initials: S
authors.lastname: Kanokmedhakul
authors.forename: Kwanjai
authors.initials: K
authors.lastname: Kanokmedhakul
authors.forename: Anthony C
authors.initials: AC
authors.lastname: Willis
authors.forename: Stephen G
authors.initials: SG
authors.lastname: Pyne
mesh_terms: D001572:Benzofurans / Q000138:chemical synthesis* / Q000737:chemistry*; D002412:Cations / Q000737:chemistry*; D003500:Cyclization; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D009025:Morpholines / Q000737:chemistry*
chemical_list: D001572:Benzofurans; D002412:Cations; D009025:Morpholines