Synthesis and <i>Diels</i>‐<i>Alder</i> Reactivity of 5,6,7,8‐Tetramethylidene‐2‐bicyclo[2.2.2]octanol and ‐octanone. Selective Oxidations of the Corresponding Bis(irontricarbonyl) Complexes

Abstract

Abstract Hydroboration of the syn , anti ‐[Fe(CO) 3 ] 2 double complex 24 of the readily available 5,6,7,8‐tetramethylidene‐2‐bicyclo[2.2.2]octene ( 22 ) gave the corresponding doubly complexed 2‐bicyclo[2.2.2]octanol 25. CrO 3 ‐oxidation furnished ketone 27 . The syn ‐Fe(CO) 3 ‐groups in 25 and 27 were oxidized selectively with trimethyl‐amine oxide and yielded the corresponding anti ‐Fe(CO 3 )‐monocomplexed tetraenes 26 and 28. The anti ‐Fe(CO) 3 ‐group in 28 could be removed, and 5,6,7,8‐tetramethylidene‐2‐bicyclo[2.2.2]octanone ( 11 ) was obtained. NaBH 4 ‐reduction of 11 afforded tetraenol 10. TCE‐cycloadditions to 10 and 11 ( k 1 ) were at least 10 times as fast as those ( k 2 ) to the corresponding monoadducts 35/36 and 34 , respectively. This Diels ‐ Alder reactivity difference vanishes ( k 1 ≈ k 2 ) with methyl propynoate. The latter dienophile added to the anti ‐Fe(CO) 3 ‐monocomplexed tetraenone 28 with ‘ para ’‐regioselectivity.