An approach toward the illudin family of sesquiterpenes using the tandem cyclization-cycloaddition reaction of rhodium carbenoids

The Rh(II)-catalyzed reaction of 1-acetyl-1-(diazoacetyl)cyclopropane with 5,5-dimethylcyclopentenone afforded the product of a 1,3-dipolar cycloaddition in high yield. The reaction involves formation of a rhodium carbenoid and subsequent transannular cyclization of the electrophilic carbon onto the adjacent keto group to generate a five-membered cyclic carbonyl ylide which undergoes a subsequent 1,3-dipolar cycloaddition reaction. The regiochemical results encountered can be rationalized on the basis of FMO considerations. Treatment of the cycloadduct with p-toluene-sulfonic acid results in loss of water followed by a subsequent acid-catalyzed cyclopropyl ketone rearrangement to give dihydrobenzofuran 21. The product distribution derived from the SmI2-induced reduction of the dipolar cycloadduct was found to depend on the reaction conditions. Under kinetic conditions, the reduction resulted in opening of the cyclopropyl ring adjacent to the carbonyl group. However, under thermodynamic conditions, cleavage of the oxy bridge corresponded to the major pathway. The cycloaddition-reduction protocol provides a rapid assembly of the basic core unit of ptaquilosin having most of the functionality in place. Generation of a carbanion adjacent to the oxy bridge leads to opening of the oxabicyclic ring system in a highly regioselective manner. A short synthesis of (+/-)-illudin M and the closely related isodehydroilludin M is described in which the key step involves a dipolar cycloaddition using a carbonyl ylide.