(+/-)-Illudin M (1) was synthesized in six steps starting from 1-acety
l-1-(diazoacetyl)cyclopropane (4) and 4-bromo-5,5-dimethyl-2-cyclopent
enone (5). The key step of the synthesis featured a carbonyl ylide 1,3
-dipolar cycloaddition reaction that was mediated by the formation of
a rhodium(II) carbenoid from 4 and catalytic rhodium(II) diacetate.