Treatment of 1-diazo-3-(2-thienyl)-2-propanone with catalytic rhodium (II) acetate results in cyclopropanation followed by acid-catalyzed ring opening and tautomerization to yield 5, 6- dihydro-4H-cyclopenta [b] thiopen-5-one. Under the same conditions, however, the isomeric 1-diazo-3-(3-thienyl)-2-propanone generates a cyclopropane intermediate which undergoes [4+ 2] cycloreversion, isomerization and Diels-Alder dimerization to give a complex spiro- ...