Functional group modifications are made to produce a cyclobutane which is elaborated through a [ 2+2 ] photocycloaddition with a cyclopentenone to produce a second cyclobutane ring .
ADJ NOUN NOUN AUX VERB PART VERB DET NOUN DET AUX VERB ADP DET [ NUM ] NOUN ADP DET NOUN PART VERB DET ADJ NOUN NOUN .