Synthesis of New Polycyclic Oxazol-2-one Derivatives by a Tandem [4 + 2] Cycloaddition/Cyclopentannulation/1,5-Sigmatropic Rearrangement Process of Fischer (Arylalkynyl)(alkoxy)carbenes and exo-2-Oxazolidinone Dienes.

An efficient and simple synthesis of highly substituted 9-ethoxy-3-phenyl-3 H-fluorene[3,2- d]oxazol-2-(4 H,9 H,10 H)-one derivatives 10a− fby a tandem [4 + 2] cycloaddition/cyclopentannulation process of Fischer (arylethynyl)(ethoxy)carbene complexes (CO) 5MC(CCC 6H 4-R)OCH 2CH 3( 1a, M = Cr, R = H; 1b, M = Cr, R = p-CH 3; 1c, M = Cr, R = p-OCH 3; 1d, M = W, R = H; 1e, M = W, R = p-CH 3; 1f, M = W, R = p-OCH 3) with exo-2-oxazolidinone dienes 7a− dis described. A study of reactivity as well as regio- and stereoselectivity in a tandem process of the Fischer carbene complexes 1with the exo-heterocyclic dienes 7was carried out. The cycloadditions were found to be highly regioselective, favoring the paracycloadducts, and highly stereoselective, giving the transdiastereoisomers. The stereochemical assignment of the cycloadducts was supported by NOE measurements, and the derivatives 10b, c, ewere further characterized by single crystal X-ray diffraction. A rationalization of the regioselectivity was carried out through an FMO analysis of the energies and coefficients for the most stable conformations of carbenes 1a, dand for diene 7a, showing a stronger interaction for the observed paracycloadduct than for the metaregioisomer. The aromatization of cycloadducts 10a− fwith HCl and DDQ is also reported. [ABSTRACT FROM AUTHOR]