Synthesis of Spiro annelated isochromanones by ring expansion of benzocyclobutenones in the presence of lithium diisopropylphosphide

spiro-Annelated isochromanones are prepared by treatment of benzo-cyclobutenone with lithium diisopropylphosphide - borane adduct (LDP-BH₃), which is easily accessible by metalation of the air stable diisopropylphos-phane-borane adduct. The reaction takes place at very mild reaction conditions and involves an oxyanion driven ring opening. As a substituted example the synthesis of the first trifluoromethyl-substituted isochromanone derivative is reported.