3‐Triethylsilyloxypentadienyllithium, a Versatile 1,3‐Diene‐ or Vinyl Ketone‐Building Block

Deprotonation of the 3‐trialkylsilyloxy‐1,4‐diene 3a and subsequent electrophilic substitution of the non‐isolated 3‐trialkylsilyoxypentadienyllithium 4 gives the α‐ and γ‐products 8 and/or 6 in good yields. Whereas alkylation of 4 proceeds with variable regioselectivity (Table 1) aldehydes and ketones attack preferentially the γ‐position of 4 (Table 2) . The desired γ‐products 6 may be directly subjected to inter‐ and intramolecular [4 + 2]‐additions as demonstrated by the reactions 5a ( 6d ) → 7 and 6h → 19 (Schemes 4 and 12 ). Alternatively, smooth fluoride‐promoted silylether‐cleavage 6 → 11 (Scheme 8) provides a convenient approach to substituted vinyl ketones such as to the natural products 11f (Table 3) . The stereoselective conversion 6k → 23 (Scheme 13) implies an endo ‐selective intramolecular Diels‐Alder addition ( 26 → 23 ) and exemplifies the use of 4 as an equivalent of the hypothetical anion IV . Furthermore, some electrophilic substitutions of the hexadienyllithium 15 have been studied (Scheme 10) .