Internal Azomethine Ylide Cycloaddition Methodology for Access to the Substitution Pattern of Aziridinomitosene A

Highly substituted, tethered alkyne dipolarophiles participate in the internal 2 + 3 cycloaddition with
azomethine ylides generated by treatment of oxazolium salts with cyanide ion. Starting from oxazole 26,
a sequence of N-methylation, cyanide addition, and electrocyclic ring opening of a 4-oxazoline intermediate
affords the indoloquinone 31 in a one-pot process. A similar reaction from the protected alkynol derivative
25 affords the sensitive, but isolable, enone 32, and subsequent oxidation affords 31 and the deprotected
quinone alcohol 34. Related azomethine cycloaddition methodology via intramolecular oxazolium salt
formation from 43 or 46 is also demonstrated and allows the synthesis of quinone 45 and derived structures
having the substitution pattern of aziridinomitosene A. Removal of the N-trityl protecting group could
not be achieved without aziridine cleavage.