A novel aldol condensation alternative: a,b-unsaturated aldehydes from 3-hydroxy-1-alkynes via dihydrodioxepins

The controlled aldol condensation between an aliph. ketone and an acetaldehyde equiv. remains a challenge. One soln. to this evergreen problem consists of the nucleophilic addn. of acetylene to the ketone and the subsequent isomerization of the resulting 3-hydroxy-1-alkyne to the corresponding 2-alkenal. So far, however, the latter step could only be executed with acid-insensitive substrates. The authors now present a milder, three-step method which extends the scope of the procedure considerably. In the first step, the 3-hydroxy-1-alkynes RCMeOHC.tplbond.CH (R = Me, Ph, Me2CH:CHCH2CH2, Q, CH2:CH, HC.tplbond.C) are converted into 2-propynyl ethylene glycol monoethers RCMe(OCH2CH2OH)C.tplbond.CH; these then undergo base-catalyzed cyclization to give the dihydro-1,4-dioxepins (I and II; R = same as above), which are hydrolyzed in acidic medium in the third and final step to give ketones RCMeOHCOMe (R = same as above) and a,b-unsatd. aldehydes RCMe:CHCHO (R = same as above), resp. Thus, RCMe(OCH2CH2OH)C.tplbond.CH (R = Q) (prepn. given) was treated with Me3COK in Me3COH for 12 h at 85 Deg to give a 4:96 mixt. of I (R = Q) and II (R = Q) in 95% yield. II (R = Q) was hydrolyzed by 6 M HCl in THF under reflux for 5 h to give 90% RCMe:CHCHO (R = Q). [on SciFinder (R)]