This example demonstrates the advantage of ring closure by Wittig reaction: under the usual conditions 2-acetonyl-1,3-cyclohexanedione does not cyclize to (6) but undergoes fragmentati~n[~l.
Cyclopentannelation with (bromoacetylmethy1ene)triphenylphosphorane ( I ) is not restricted to stabilized enolates. Reaction of the anion (7a), generated by treatment of cyclohexanone with lithium diisopropylamide in tetrahydrofuran, with one equivalent of (1) at -20 "C affords, after 2 hours' stimng at 0 "C, 14 hours' boiling under reflux, and aqueous workup with pentane, the annelated cyclopentenone (8a)l6I (b.p. 50-52 OC/O.O2 torr; yield 29%). (a), R' = H, R~ = H (h), R' = H, R2 = CH, (c), R' = CH,, R~ = H ( I ) is sufficiently reactive towards the enolates (76) (generated by reaction of 2-methyl-I-cyclohexenyl acetate with methyllithium['I) and (7c) (obtained by reaction of 2-methylcyclohexanone with lithium diisopropylamide in tetrahydrofuran[']) to form regioselectively (Sb)[*I (yield 25%) and ( 8 ~) ~~~ (yield 22%; mixture of diastereomers), respectively.
The method is also applicable to potassium enolates of aldehydes, If the aldehyde (9)r9l is deprotonated with potassium hydride in tetrahydrofuran["' and the enolate is allowed to react with (l), the spirocyclic compound can be isolated (b. p. 54-56 "C/O.I torr; yield 14%).