Into a 250-ml flask was added 1,3-dihydronaphtho [2,3-c] thiophene 2-oxide (10, 1.0 g, 4.95 mmol), JV-phenylmaleimide (0.86 g, 4.96 mmol), and 20 ml of acetic anhydride under dry nitrogen. The mixture was refluxed for 20 min. The yellow solution was cooled slightly and the excess acetic anhydride removed under reduced pressure to leave 1.80 g (101%) of a yellow crystalline solid. The separation of the isomeric adducts is detailed as follows.Exo Adduct.-The yellow solid was recrystallized from acetonitrile to give 0.60 g (34%) of white solid. An analytical sample was obtained by recrystallization from chloroform-hexane: mp 281.5-282.5°; ir (KBr) 1765, 1700 (broad, imido C=0), 790 (m), 745 (s), 740 (sh), 690 cm-1 (m); nmr (DMSO-de) r 2.2-3.05 (m, 11, aromatic), 4.89 (s, 2, bridgehead), 6.50 (s, 2, endo H); mass spectrum (70 eV; source 250°, probe 150°) m/e (rel intensity) 357 (9.7), 325 (3.1), 186 (5.9), 185 (13.8), 184 (100), 178 (10).