An efficient and simple synthesis of highly substituted 9-ethoxy-3-phenyl-3H-fluorene[3,2-d]oxazol-2-(4H,9H,10H)-one derivatives 10a−f by a tandem [4 + 2] cycloaddition/cyclopentannulation process of Fischer (arylethynyl)(ethoxy)carbene complexes (CO)5MC(CCC6H4-R)OCH2CH3 (1a, M = Cr, R = H; 1b, M = Cr, R = p-CH3; 1c, M = Cr, R = p-OCH3; 1d, M = W, R = H; 1e, M = W, R = p-CH3; 1f, M = W, R = p-OCH3) with exo-2-oxazolidinone dienes 7a−d is described. A study of reactivity as well as regio- and stereoselectivity in a tandem process of the Fischer carbene complexes 1 with the exo-heterocyclic dienes 7 was carried out. The cycloadditions were found to be highly regioselective, favoring the para cycloadducts, and highly stereoselective, giving the trans diastereoisomers. The stereochemical assignment of the cycloadducts was supported by NOE measurements, and the derivatives 10b,c,e were 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,d and for diene 7a, showing a stronger interaction for the observed para cycloadduct than for the meta regioisomer. The aromatization of cycloadducts 10a−f with HCl and DDQ is also reported.

Dienes. -Both chromium and tungsten carbene complexes give the desired polycyclic oxadiazolone compounds (III) in a highly regio-and stereoselective manner, favoring the para cycloadducts and the trans-diastereomers. The aromatization of these compounds by treatment with either DDQ or silica gel in the presence of a catalytic amount of HCl affords compounds (IV) and (V). The formation of carbene adduct (IX) corroborates the high para-selectivity in the cycloaddition reactions. -(REYES, L.; MENDOZA, H.; VAZQUEZ, M. A.; ORTEGA-JIMENEZ, F.; FUENTES-BENITES, A.; FLORES-CONDE, M. I.; JIMENEZ-VAZQUEZ, H.; MIRANDA, R.; TAMARIZ, J.; DELGADO