Versatile and regioselective synthesis of polysubstituted and highly oxygenated phenols via Dötz reaction of α-alkoxyvinyl(ethoxy)carbene complexes with alkynes

A comparative study of reactivity and selectivity of α-alkoxyvinyl(ethoxy) Fischer carbene complexes (CO)₅M = C[C(=CHR¹)(OR)]OCH₂CH₃ [7a–d: M = Cr; 12a–c: M = Mo; 13a–d: M = W; a: R¹ = H, R = CH₂CH₃; b: R¹ = H, R = CH₂CH₂CH₃; c: R¹ = H, R = CH₂CH₂CH₂CH₃; d: R¹ = CH_3, R = Et (E and Z)] with terminal and internal alkynes (14a–g) in benzannulation reactions is described. The complexes 7 and 12–13 display a remarkable reactivity toward different alkynes. The [3 + 2+1] cycloaddition or benzannulation reactions between the latter carbene complexes and alkynes 14 were found to be highly regio- and chemoselective only giving those products with the largest substituent of the acetylenic component located at the ortho position to the hydroxyl group, and favoring the formation of new and highly oxygenated and polysubstituted phenols containing only electron-donating groups. The regiochemistry assignment of the cycloadducts was supported by NOE measurements. Phenols 15c, 16e, and 18b were further characterized by single-crystal X-ray diffraction.