SYNTHESIS OF FUNCTIONALIZED AZOMETHINE YLIDES VIA THE RH(II)-CATALYZED CYCLIZATION OF ALPHA-DIAZO CARBONYLS ONTO IMINO PI-BONDS

alpha-Diazo carbonyl compounds containing an imino group in the gamma-position have been found to undergo a rhodium(II) acetate induced cyclization reaction to generate cyclic azomethine ylides. The reactive dipole undergoes a subsequent 1,3-dipolar cycloaddition with added dipolarophiles. The imino/oxime nitrogen lone pair of electrons must be properly oriented so as to interact with the rhodium carbenoid. Thus, acyclic oxime ethers which exist in the proper E-configuration readily undergo the tandem cyclization-cycloaddition reaction. In sharp contrast, the isomeric Z-oximino diazo carbonyl system does not cyclize to an azomethine ylide but rather intramolecularly inserts into the oximino C-H bond. Addition of a catalytic amount of rhodium(II) acetate to (E)-2-(diazoacetyl)benzaldehyde O-methyloxime in the presence of DMAD or N-phenylmaleimide affords a dipolar cycloadduct in high yield. When p-quinone was used as the dipolarophile, the initially formed cycloadduct was treated with acetic anhydride to give a compound containing the basic core dibenzo[a,d]cyclohepten-5,10-imine skeleton found in MK-801. Cyclic imines such as isoxazolines were particularly effective substrates for azomethine ylide formation. The rhodium(II) catalyzed reaction of 3-(4-diazo-3-oxobutyl)-5-phenyl-Delta(2)-isoxazoline with DMAD produced a 4:1 mixture of diastereomeric cycloadducts in good yield. Cyclization of the rhodium carbenoid did not occur with the aromatic isoxazole system and this is presumably due to the low basicity of the nitrogen lone pair of electrons.