Doing it Twice: Asymmetric Deprotonation/Alkylation of Weiss Diketone Derivatives as Key Steps in the Functionalization of Bicyclo[3.3.0]octanes

The silyl-protected allylhydropentalenone derivative 9, derived from the Weiss diketone, was functionalized by enantioselective deprotonation in the presence of lithium bis(1-phenylethyl)amide/LiCl as the chiral base, which was generated in situ from bis(1-phenylethyl)ammonium chloride [(R,R)- or (S,S)-14] and BuLi, and trapping of the resulting enolate with alkyl halide electrophiles to give pseudo-C2 or -Cs-symmetrical bicyclo[3.3.0]octanones 10 and 11. The influence of the chiral base and electrophiles on the regioselectivity and double stereodifferentiation was investigated. Taking the double stereocontrol into account, a matched selectivity for the pseudo-C2 pair (1R,4R)-10/(1S,4R)-10 and a mismatched selectivity for the pseudo-Cs pair (1S,6R)-11/(1R,6R)-11 were observed in the presence of (R,R)-14. The use of (S,S)-14, however, produced a mismatched selectivity for (1R,4R)-10/(1S,4R)-10 and a matched selectivity for (1S,6R)-11/(1R,6R)-11 with complementary diastereoselectivity depending on the electrophile. Furthermore, the hydropentalenone derivative 10a provided an alternative route to the bicyclo[3.3.0]octene core of the macrocyclic tetramic acid lactam, cylindramide (5).