Methyl 2,3-Butadienoate1

[18913-35-4]  · C5H6O2  · Methyl 2,3-Butadienoate  · (MW 98.11)

(reactive electrophilic reagents for nucleophilic addition,2 Michael addition,3,4 [2 + 2],5,6 [2 + 3],7,8 and [2 + 4]9,10 cycloadditions, and hetero-Cope rearrangements4)

Physical Data: bp 40 °C/14 mmHg.

Solubility: sol common organic solvents.

Form Supplied in: colorless liquid; not commercially available.

Analysis of Reagent Purity: IR (film): 1965/1940, 1725 cm-1; 1H NMR (CCl4): 5.65-5.35 (H-C(2)), 5.25-5.00 (2H-C(4)), 3.68 ppm (OMe).

Preparative Methods: most conveniently obtained via a Wittig reaction of [E(R)CHPPh3]+X- or E(R)C=PPh3 and RR´CHCOCl (R, R´ = H, alkyl) in the presence of Triethylamine in CH2Cl2.1 Alternatively, the Wittig reaction can be performed with the corresponding ketenes.11

Purification: rapid distillation under reduced pressure in a short-path distillation apparatus or column chromatography on Alox N with hexane/Et2O (9:1). Hydroquinone may be added for distillation.

Handling, Storage, and Precautions: can undergo polymerization and so should be stored in the presence of hydroquinone. For best results, it should be freshly distilled. Use in a fume hood.

Preparation of Substituted 2,3-Butadienoates.

The method described above also works with a,b-unsaturated acid chlorides to yield 4-vinyl substituted 2,3-butadienoates12 and may likewise be applied as a ring closure procedure for the synthesis of endocyclic allenelactones.10 Alternative methods for the synthesis of 2,3-butadienoates and analogs include the Pd0-catalyzed carbonylation of propargylic carbonates13 and halides.14 In the presence of acrylates instead of CO, vinyl analogs of allenic esters are obtained.15 2-Substituted 2,3-butadienoates are available from the Wittig reaction of corresponding phosphoranes or phosphonates with ketenes11,16,17 or by the oxidative degradation of 3,4-substituted pyrazol-3(2H)-ones with TlIII salts9,18 or (Diacetoxyiodo)benzene.19 2-Phenyl-substituted 2,3-butadienoates have been prepared by a combined photolysis of trimethoxy-(1-methoxycarbonylbenzylidene)phosphorane and cyclohexanones in benzene.20 Dimethyl 2,3-pentadienedioate can be prepared in three steps from diethyl acetone-1,3-dicarboxylate.21

Addition of Nucleophiles.

Nucleophiles add to 2,3-butadienoates and their analogs at the C-3 position.22-24 The addition of primary or secondary amines leads to the formation of b-amino acrylates,2a,3,25,26 which, on hydrolysis, yield b-keto esters (eq 1).3 In a similar manner, b-azido crotonates can be synthesized which are photolytically transformed into 2H-azirine-3-carboxylates.27 Triphenylphosphine adds to 2,3-butadienoates under acid catalysis to yield the corresponding phosphonium salts which, on treatment with MeONa/MeOH, form (E)-4-methoxy-2-butenoates (eq 2).28,29 The methylation of ethyl 2,3-butadienoate at C-3 with Lithium Dimethylcuprate at -90 °C was utilized in the synthesis of lavandulol.30

Grignard reagents add in the same way to allenecarboxylic acids.31 Blechert showed that ethyl 2,3-butadienoate reacts with the Li salt of N-acetyl-N-phenylhydroxylamines to yield, via a hetero-Cope rearrangement, ethyl 4-(2-acetamidophenyl)-3-oxo-butanoates which undergo indolization upon treatment with Formic Acid (eq 3).4

Cycloadditions.

[2 + 2] Cycloadditions of 2,3-butadienoates with simple alkenes, promoted by Ethylaluminum Dichloride5 or Aluminum Chloride,6 take place at the C(3)=C(4) double bond without loss of the stereochemical integrity of the alkenes and lead to the formation of the corresponding cyclobutylideneacetates (eq 4). Conjugated imines seem to add to ethyl 2,3-hexadienoate in a similar manner when heated to reflux in benzene.32 The azetidines which are formed primarily rearrange to yield 2-cyclohexenones after hydrolysis.

[2 + 3] Cycloadditions of 2,3-butadienoates and analogs have been investigated with nitrilium betaines (nitrile ylides,7,33 nitrile imines,34 nitrile oxides34), diazonium betaines (diazoalkanes,35 azides34), and nitrones.8,36 All 1,3-dipolar cycloadditions occur at the C(2)=C(3) double bond, but often regioisomers are formed. The cycloadducts may isomerize (spontaneously or with base catalysis) to yield the corresponding heteroaromatics (eq 5). Nitrones react smoothly with methyl 2,3-butadienoate and its 2-methyl derivative to yield regioselectively 5-methyleneisoxazolidines which, on heating, may undergo a 1,3-H shift or a skeletal rearrangement to the corresponding 3-pyrrolidones (eq 6).8,37

[4 + 2] Cycloadditions with allenecarboxylates as dienophiles have also been studied.9,22,23 Thermal reaction with cyclopentadiene leads to the formation of endo- and exo-3-methylenebicyclo[2.2.1]hept-5-ene-2-carboxylates in a ratio of ca. 2:1.9 The addition of AlCl39 or M(fod)3 (M = Eu, Pr)38,39 allows the cycloaddition to be carried out at lower temperatures and with higher endo/exo ratios. The cycloaddition with furan as diene can also be catalyzed by M(fod)3 and leads also to a predominance of the endo product.40 Synthetically valuable [4 + 2] cycloadditions of 2,3-butadienoates and analogs with 1,1-Dimethoxy-3-trimethylsilyloxy-1,3-butadiene have been performed by Gerlach and co-workers10 and applied to the synthesis of (+)-(R)-lasiodiplodin (eq 7).


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Andreas J. Rippert & Hans-J. Hansen

University of Zurich, Switzerland



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