Preparation of 3-Alkenals and 3-Alkynals by Hydrolysis of the Corresponding Acetals

Chemistry of 3-Alkynals. A Review

“…22 One laboratory reported that 3-octynal and 3-nonynal polymerized after one week at room temperature. 16 In our hands solutions of 3-alkynals in CDCl 3 were stable for weeks in a freezer. 22 One reported attempt to purify 3-decynal using chromatography let to extensive decomposition.…”

“…14 Hydrolysis of 5 (R 2 = n-butyl, n-pentyl) in pentane/HCO 2 H for 1 h at 20 • C gave 3-octynal and 3-nonynal in yields of 50% and 57%, respectively, with less than 10% of their corresponding 2,3-dienals. 16 Interestingly, Sayre et al report that their attempts to hydrolyze 4,4-diethoxy-1-butyne to 3-butynal with different acids either failed due to insufficient acid strength or generated 3-butynal with significant amounts of 2,3-butadienal. 17…”

“…Although it is clear from the literature that 3-alkynals should not be heated to high temperatures or stored for long periods at room temperature, they can be distilled under vacuum 16,18 and heated to 60 • C in CDCl 3 solution for hours without significant rearrangement. 22 Heating a solution of 1 in CDCl 3 , though, in a sealed NMR tube at 100 • C for 4 hours resulted in almost complete rearrangement to 2.…”

“…This was undoubtedly the reason early attempts to prepare these compounds by hydrolysis of enol ethers and acetals using dilute solutions of strong acids resulted in considerable rearrangement. Biphasic acidic mixtures (e. g., formic acid/pentane 16 and aqueous TFA-CHCl 3 14 ) do hydrolyze acetals to 3-alkynals at room temperature, but 5-10% of their 2,3-alkadienals are also obtained. Care should be taken to neutralize reaction mixtures before workup to prevent rearrangement (see, for example, Reference 19).…”

“…Reaction of 3-octynal and 3-nonynal with allylmagnesium bromide led to 4-hydroxy-1-undecen-6-yne and 4-hydroxy-1-dodecen-6-yne in yields of 50 and 57%, respectively. 16 4-Octyn-2-ol and 4-undecyn-2-ol were obtained in overall yields of 35-40% starting with their anthraquinones (Scheme 6, R = C 3 H 7 and C 6 H 13 ). 21 Given that the photochemical step occurs in yields of 35-45%, the addition of methylmagnesium bromide to 3-heptynal and 3-decynal occurs in yields of at least 70%.…”