The endocyclic double bond C(2),C( ) in 5,6-dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene (1) can be coordinated selectively on its exo-face before complexation of the exocyclic s-cis-butadiene moiety. Irradiation of Ru,(CO),, or Os,(CO),, in the presence of 1 gave tetracarbonyl[(lR,2R,3S,4S)-2,3-~-(5,6-dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene)]ruthenium (6) or -osmium (8). Similarly, irradiation of Cr(CO), or W(CO), in the presence of 1 gave pentacarbonyl[( 1 R,2R,3S,4S)-2,3-~ -(5,6-dimethylidene-7-oxabicyclo[2.2.l]hept-2-ene)]chromium (10) or -tungsten (11). Irradiation of complexes 6 and 11 in the presence of 1 led to further CO substitution giving bcd-tricarbonyl-aebis[( 1 R,2R, 3S,4S)-2,3-~ -( 5,6-dimethylidene-7-oxabicyclo[2.2.l]hept-2-ene)]ruthenium (7) and trans -tetracarbonyl[( lR,2R,3S,4S>-2,3-~ -(5,6-dimethylidene-7-oxabicyclo-[2.2.l]hept-2-ene)]tungsten (12), respectively. The diosmacyclobutane derivative cis-,u-[( 1 R,2R,3S,4S)-(5,6-dimethylidene-7-oxabicyclo[2.2. I] hepta-2,3-diyl)
]bis(tetracarbonylosmium) ( 0 s -0 s ) (9) was also obtained. The Diels-Alder reactivity of the exocyclic s-cis-butadiene moiety in complexes 7 and 8 was found to be significantly higher than that of the free triene 1.
Introduction. -The 5,6-dimethylidene-7-oxabicyclo[2.2.l]hept-2-ene (l), readily prepared in three steps from furan and maleic anhydride [l], is a valuable synthetic intermediate. This has been illustrated recently by our discovery of the unique property of 1 in forming the polycyclic derivative 2 when treated with Fe,(CO), in MeOH [2] [3]. Diene 2 is a potential precursor in the synthesis of antitumoral anthracyclinone such as demethoxydaunomycinone [4] [5]. A few years ago, we reported that the endocyclic double bond of 1 reacts faster than the exocyclic diene with iron carbonyls. This allowed the isolation of the relatively stable (q2-triene)Fe(CO), complex 3 in good yield, in which the Fe(CO), group sits on the exo-face of the ligand .