A fluoroisoprenylation sequence employing 2-fluoroalkenal derived azomethines as key intermediates: a stereocontrolled synthesis of 2-fluorogeraniol and 2-fluorofarnesol

Azomethines (Schiff's bases) derived from a,b-unsatd. a-fluoroaldehydes can be deprotonated with lithium diisopropylamide. The resulting "anions" or, correctly, 3-fluoro-1-azapentadienyl lithium compds. are conformationally mobile. While eight different coplanar structures are possible, one of them, a zigzag shaped (W) conformation must be largely favored. Depending on their nature, electrophiles attack the 1-azapentadienyl intermediates at either of the three nodal points: the nitrogen atom, the fluorine bearing a-position or the terminal g-position. Allyl type alkylating reagents produce a mixt. of a- and g-regioisomers from which the pure components can be sepd. Consecutive hydrolysis and redn. provides a-fluoroalkenals and a-fluoroallyl alcs. [on SciFinder (R)]