Hexenoic esters

Hexenoic esters 124 suitable for 1,3-cyclo elimination can be obtained, if the allyl alcohol component for making the vinyl-enolether system for Claisen rearrangements already contains the chlorine atoms, thus avoiding the introduction of carbon tetrachloride at a later stage [179] (Reaction scheme 79). 

Sorbic acid anhydride [13390-06-2] can be prepared by heating the polyester of 3-hydroxy-4-hexenoic acid with sorboyl chloride [2614-88-2] or by reaction of sorbic acid with oxalyl chloride (15,16). Preparation of the esters of sorbic acid must be controlled to prevent oxidation and polymerization. The lower sorbic acid esters have a pleasant odor. 

Other methods include ring opening of parasorbic acid [108-54-3] (5-lactone of 5-hydroxy-2-hexenoic acid) in hydrochloric acid or in alkaline solutions (43,44), the ring opening of y-vinyl- y-butyrolactone in various catalysts (45,46), or isomerization of 2,5-hexadienoic acid esters (47,48). Other methods are described in thehterature (6,49,50). 

CN ( )-6-(l,3-dihydro-4-hydroxy-6-methoxy-7-methyl-3-oxo-5-isobenzofuranyl)-4-methyl-4-hexenoic acid 2-(4-morpholinyl)ethyl ester... 

Figure 11.2 Py/silylation GC/MS chromatograms of aged linseed oil pyrolysed in the pre sence of HMDS, (a) Pyrogram obtained with a microfurnace pyrolyser pyrolysis temperature 600 °C furnace pressure 14 psi purge flow 0.5 ml min (b) Pyrogram obtained with a resistively heated filament pyrolyser pyrolyser interface I80°C transfer line 300°C valve oven 290°C. 1, Hexenoic acid, trimethylsilyl ester 2, hexanoic acid, trimethylsilyl ester 3, heptenoic acid, trimethylsilyl ester 4, heptanoic acid, trimethylsilyl ester 5, octenoic acid, trimethylsilyl ester 6, octanoic acid, trimethylsilyl ester 7, nonenoic acid, trimethylsilyl ester 8, nonanoic acid, trimethylsilyl ester 9, decanoic acid, trimethylsilyl ester 10, lauric acid, trimethylsilyl ester 11, suberic acid, trimethylsilyl diester 12, azelaic acid, trimethylsilyl diester 13, myristic acid, trimethylsilyl ester 14, sebacic acid, trimethylsilyl diester 15, palmitic acid, trimethylsilyl ester 16, stearic acid, trimethylsilyl ester...

Again, addition of trialkylphosphine was found to be effective for reducing the amount of products obtained by attaching a formyl group to a carbon other than that of the original carbon-carbon double bond (73). Results obtained with hexenoic acid esters are outlined in Table XXIV. 

Table 14.3 Hydrogenation of methyl sorbate to 3-hexenoic acid methyl ester with [Cr(CO)3(arene)] (45) catalysts.

SYNTHESIS OF CHIRAL (E)-CROTYLSILANES [3R- AND 3S-]-(4E)-NIETHYL 3-(DIMETHYLPHENYLSILYL)-4-HEXENOATE (4-Hexenoic acid, 3-(dimethylphenylsilyl)-, methyl ester,... 

Selective protection of the primary alcohol gave 138 (P=TBDMS), which was then esterified with ( )-3-hexenoic acid to produce the key intermediate 139 for cyclization. Ireland ester-enolate Claisen rearrangement and hydrolysis produced a protected hydroxyacid, which, after reduction of the acid and deprotection of the alcohol, yielded meso diol 128 more quickly and efficiently than in the previous synthesis. The meso diol was then converted to the racemate of the lactol pheromone 130 as previously described. 

Thus, the (R)-glycidol (R)-897 was transformed to ethyl (S)-6-benzyloxy-3-methyl-4(E)-hexenoate (S)-899 via addition of acetylide followed by spontaneous isomerization, stereoselective reduction, and Claisen-Johnson rearrangement. The chiral ester (S)-899 was converted to (R)-4-methyl-6-phenylthiohexanol (R)-902. The primary alcohol (R)-902 was then transformed to the terminal acetylene (R)-904, a common intermediate for the synthesis of carbazoquinocins A (272) and D (275). Chain elongation of (R)-904 by two carbon atoms led to (R)-905, the chiral precursor for carbazoquinocin D (275) (639) (Scheme 5.116). 

Sometimes the acetylenic ester rearranges to the corresponding allenic ester. For example, when the triethylamine salt of 3-chloro-2-ethoxycarbonyl-4-phenyl-2-hexenoic acid is refluxed in toluene, the allenic ester and acetylenic ester are obtained in a ratio of 3 7 (total yield 70%). There are alternative routes to cyclopropylpropiolic acids and esters, such as adding butyllithium to corresponding acetylenes and treating the product with carbon dioxide or methyl chloroformate. ... 

Fig. 8.2 Esters as character-impact compounds 4 isobutyl acetate in passion frml 5 isopentyl acetate in banana 6 ethyl butanoate in cupuacu 7 ethyl (32)-hexenoate and 8 ethyl-3-(methylthio)propanoate in pineapple...

Dehydrohalogenation of 6-halohexopyranosides.2 This reaction has been effected with DBU or AgF, but NaH in DMF at 0-50° is as efficient. Hydroxyl, azide, benzyl, and ester groups are stable under these conditions. Yields of 5,6-hexeno-pyranosides are 60-83%. 

The ethyl ester of diphenesenic acid residue is dissolved in ethanol. H20 and KOH are then added and the mixture is heated under reflux for 2 h. The alcohol is evaporated under vacuum. The residue is dissolved in H20. Aqueous solution is washed with petroleum ether. The solution is then decolorized with active carbon and acidified slowly with HCI 10% till pH 2.1. After stirring at room temperature for 4 h the solid product is filtered, washed with H20 till washing liquid is neutral. The solution is then dried under vacuum at 50°C. So the 2-[4-biphenylyl]-4-hexenoic acid (diphenesenic acid), melting point 118-119°C (crystallized from cyclohexane), boiling point 182-183°C is obtained. 

Figure 1 shows part of a reconstructed ion chromatogram of a pineapple aroma extract isolated after incubation of pineapple slices with 3-hydroxyhexanoic acid-3-di. GC-MS detection of deuterated compounds showed that the following pathways are active (a) esterification leading to methyl and ethyl esters, (b) dehydration to (E)-2-and (E)-3-hexenoates, and (c) chain elongation to methyl 5-hydroxy octanoate followed by acetylation (methyl 5-acetoxyoctanoate) and cyclization ( 6-octalactone). 

The cyclization of ethyl (5)-( ,)-6-tert-butyldiphenylsilyloxy-5-hydroxy-2-hexenoate (1) has been performed under thermodynamic conditions. Treatment of 1 with sodium ethoxide in ethanol, first at 20 °C, then at reflux for 4 hours, establishes an equilibrium and the silyl group migrates to the oxygen at C-5. Subsequent Michael addition of the primary hydroxy group to the a,/ -unsaturated ester gives the corresponding tetrahydro-2-furanacetate in 87% yield as a 67 33 (trans/cis) diastereomeric mixture, as determined by HNMR at 250 MHz. This result shows that the 1,3-asymmetric induction is poor compared to the iodocyclization reaction48- 4S>. 

More highly functionalized products can be obtained, often in better yield, by the ene reaction of a-substituted acrylate esters. EtAlCb is a more effective catalyst than AlCb for these reactions because it is a Brpnsted base as well as a Lewis acid. The EtAlCb catdyzed reactions of methyl a-bromo- and a-chloro-acrylate with trans-1,2-disubstituted and trisubstituted alkenes are both regio- and stereo-specific. - The major product (13a 85-95%) is formed via transition state (12a) in which the methoxycar-bonyl group is endo. The stereochemistry of the major adduct was established by the stereospecific conversion of (13a) to both diastereomers of ( )-2-amino-4-methyl-5-hexenoic acid and confirmed by X-ray crystallographic analysis in a related system. ... 

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