Scheme 4. Reactions of benzocyclobutene derivatives. Conditions: (a) 10-camphorsulfonic acid, MeOH–CH₂Cl₂ (1 : 1), rt, 80%. (b) MnO₂ (excess), CH₂Cl₂, rt, 72%. (c) Dess–Martin periodinane, CH₂Cl₂, rt, 82%. (d) TMSCHN₂, BF₃·Et₂O, CH₂Cl₂, –20 °C, 55%. (e) TBAF, THF, rt, 86%. (f) NaH, THF, 50 °C, 81%.