[3102-33-8] · C5H8O · trans-3-Penten-2-one · (MW 84.13)
Physical Data: bp 121.5-124 °C; d
Solubility: sol water and all organic solvents.
Form Supplied in: liquid; commercial material is often contaminated with 3-penten-2-ol which is difficult to remove by distillation.
Analysis of Reagent Purity: IR, NMR, and GC.13
Preparative Methods: an excellent method14 involves the Wittig reaction of Acetylmethylenetriphenylphosphorane and Acetaldehyde (2.5 mol equiv) in methylene chloride (reflux, 6 h, 76% yield). Another good method13 proceeds via the Aluminum Chloride catalyzed addition of Acetyl Chloride to propene followed by dehydrochlorination.
Handling, Storage, and Precautions: best stored in refrigerator; moderately toxic;15 lachrymator. This reagent should only be handled in a fume hood.
Enolates of esters,1 amides,2 ketones (eq 1),1 and organocuprates4 participate in conjugate addition reactions with trans-3-penten-2-one.
Basic conditions can be avoided by using the Lewis acid-catalyzed Michael reaction of enol silyl ethers with the reagent (eq 2).3
An important application of trans-3-penten-2-one is in the Robinson annulation reaction,5-11 which has been widely used for the synthesis of natural products such as eremophilane sesquiterpenes.5-7 Annulations involving this enone have been involved as key steps (eqs 3 and 4) in the total synthesis of isonootkatone5 and calarene.6
The steric outcome of these reactions is very dependent on the reaction conditions.5-9 In eq 4 the use of benzene as solvent gave a 10:1 ratio of the trans- and cis-dimethyl products while formamide gave a 1:1 ratio. Likewise, reaction of 2-methylcyclohexanone and trans-3-penten-2-one using Sodium Hydride in dioxane gave the cis-dimethyl product (eq 5), but DMSO gave the trans product.8 However, other workers reexamined this work and observed somewhat different yields and stereoselectivities.9
A variation of the Robinson annulation involves the Lewis acid-catalyzed reaction of a silyl enol ether with the enone (eq 6).11
James E. Shaw
Phillips Petroleum, Bartlesville, OK, USA