Conjugated ketones epimerization

In a similar fashion 4-methylbicyclo[4.4.0]dec-2-en-10-one 1 is epimerized to epi-l on short treatment with acid or base or during flash chromatography. Only long exposure to acid transforms epi-l into the conjugated ketone 294. 

Because of the action of Et3N on the activated alcohol, some side reactions—beginning with a deprotonation—can happen in sensitive substrates. For example, a-epimerization of sensitive aldehydes and ketones,260 and migration of alkenes into conjugation with carbonyl groups261 are occasionally found. 

The same two ketones featured again in a synthesis of ( )-lasubine II (910) by Pilli et al. (Scheme 119) (368,369). In this case, reaction of A -Boc-2-ethoxypiperidine (925) with enone 926 in the presence of trimethylsilyl triflate sparked off a remarkably efficient (90%) one-pot synthesis involving condensation (via an A -acyliminium ion to give the intermediate 927), deprotection, and intramolecular conjugate addition. The familiar products 921 and 922 were obtained as a 3 2 mixture. Base-induced epimerization of the mixture enriched the latter component, which was converted into ( )-910 by reduction with LS-Selectride (lithium trisiamylborohydride) according to the procedure developed by Comins (vide infra). 

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