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.

Infrared irradiation promoted the Diels-Alder cycloadditions of exo-2-oxazolidinone dienes 1–3 with the Knoevenagel adducts 4–6, as dienophiles, leading to the synthesis of new 3,5-diphenyltetrahydrobenzo[d]oxazol-2-one derivatives (7, 9, 11 and 13–17), under solvent-free conditions. These cycloadditions were performed with good regio- and stereoselectivity, favoring the para-endo cycloadducts. We also evaluated the one-pot three-component reaction of active methylene compounds 20, benzaldehydes 21 and exo-2-oxazolidinone diene 2 under the same reaction conditions. A cascade Knoevenagel condensation/Diels-Alder cycloaddition reaction was observed, resulting in the final adducts 13–16 in similar yields. These procedures are environmentally benign, because no solvent and no catalyst were employed in these processes. The regioselectivity of these reactions was rationalized by Frontier Molecular Orbital (FMO) calculations.

The synthesis of novel and highly substituted ( 2Z ,3 Z )-3 -ethox y-1-(aryl/cyc lohexe nyl )-4 -aryl-2 -(phenylimino)but-3-en-1-one derivatives 24a−k and 27a−e by an unexpected and previously unreported [3 + 2] cycloaddition/rearrangement and ring-opening cascade process of the Fischer carbene complexes (CO) 5 MC(CC−Ar)-OCH 2 CH 3 1a−c (M = Cr) and 1d−f (M = W) and (CO) 5 MC(CC-cyclohexenyl)OCH 2 CH 3 23a (M = Cr) and 23b (M = W) with C,N-diaryl nitrones (7a−f) is described. It is likely that the unstable 2,3-dihydroisoxazole carbene complexes produced in the [3 + 2] cycloaddition undergo a rapid and new rearrangement, followed by a stereoselective electrocyclic ring-opening process at a low temperature, to give only the (Z,Z) diastereoisomers 24a−k and 27a−e as the isolated products. The stereochemical assignment of the products was supported by NOE measurements and by single-crystal X-ray diffraction.

A series of 2,4,6-trisubstituted phenols 7a-7p has been prepared in moderate to good yields (23-77 %) by a retroDiels-Alder reaction from hindered tricyclic alcohols 6a-6q, using mild acidic conditions. The tricyclic alcohols were obtained by reduction of highly functionalized cyclohexadienones 5a-5q, which in turn were regioselectively prepared under mild conditions in high yields by treatment of a series of stable chromium(0) and tungsten(0) Fischer dienyl carbenes 4a-4g with different terminal alkynes 3a-3l. Carbenes 4a-4g were pre-