Concise Synthesis of α‑Substituted 2‑Benzofuranmethamines and Other 2‑Subsituted Benzofurans via α‑Substituted 2‑Benzofuranmethyl Carbocation Intermediates

Propargyl amines 4, where R3 is
aryl, undergo
5-exo-dig cyclization reactions
under relatively mild conditions (AgNO3, DMF, 60 °C,
1 h) to give 3-amino-2,3-dihydro-2-arylmethyl­idene­benzo­furans 5 (R3 = aryl). In contrast, substrates where R3 is alkyl undergo competing 6-endo-dig and 5-exo-dig cyclization
processes. The hydroxymethyl substrate 4 (R3 = CH2OH), 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 (AgNO3, 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 AgNO3 in DMF at 100 °C for 18 h or BF3·Et2O at rt. 2-(3-Butenyl)­benzofurans 7 (Nu = allyl) can
be prepared by treatment of 5 with BF3·Et2O and allyltributylstannane. Furan and MeOH could also be
employed as external nucleophiles in these BF3·Et2O-promoted reactions.