Trans-Hexenal synthesis

In acyclic systems the 1,4-relative stereoselectivity was controlled by the stereochemistry of the diene. Thus, oxidation of (E,E)- and (E,Z)-2,4-hexadienes to their corresponding diacetates affords dl (>88% dl) and mesa (>95% me so) 2,5-diacetoxy-3-hexene, respectively. A mechanism involving a t vans-accto xy pal I adation of the conjugated diene to give an intermediate (rr-allyljpalladium complex, followed by either a cis or trans attack by acetate on the allyl group, has been suggested. The cis attack is explained by a cis migration from a (cr-allyl)palladium intermediate. The diacetoxylation reaction was applied to the preparation of a key intermediate for the synthesis of d/-shikimic acid, 3,... 

Acid-catalyzed liquid-phase fluorine addition with xenon difluoride to cis and trans 1-phenylpropene resulted in the formation of vicinal difluoride in high yield63. Photo-initiated reaction in methylene chloride at 0 °C also gave vicinal difluorides in a nonstereospecific manner, and a free-radical mechanism was suggested68. Unfortunately, this radical reaction does not appear to represent a general synthesis since reaction with 1-hexene, cyclohexene and 2,3-dimethyl-2-butene does not give vicinal difluoride products68. 

Decahydroquinoline Alkaloids.—Full details of an earlier briefly described synthesis of ( )-pumiliotoxin C have been published.28 Other total syntheses of the same compound have been documented, starting from trans-4-hexenal,29 and from ethyl tra s-buta-l,3-diene-l-carbamate.30 An enantioselective synthesis of natural (-)-pumiliotoxin-C hydrochloride (34) has also been described (Scheme 6),... 

For the synthesis of the trans-mmulsitQd 4a- p/-pumiliotoxin C 195 (Scheme 10.66), the imine prepared from galactosyl amine 150 and 5-hexenal was reacted with Danishefsky s diene to give the 2-substituted dehydropiperidinone derivative 196 with high diastereoselectivity. The subsequent addition of propyl cuprate promoted by borontrifluoride etherate furnished the 2,6-c/5 -disubstituted... 

Attempted Synthesis of Specific Hexenyl Acetate Isomers. From Hexanal. Hexanal (5 ml. of 40% technical material) and 25 ml. 2-propenyl acetate were mixed, and 750 mg. p-toluene sulfonic acid was added. This solution was heated at reflux for 12 hours. The solution was then poured into 100 ml. water and extracted with two 50-ml. portions of ether. The combined ether extracts were washed with water, dilute aqueous sodium bicarbonate, and water and dried over sodium sulfate. After evaporating the ether, a gas chromatogram showed two major components present in a 3 2 ratio (probably the trans- and cis-hex-l-en-l-ol acetates, respectively). These components had identical retention times in the gas chromatograms to two of the major components present in the hexene-1 vinylation reactions. 

The oldest syntheses of chrysanthemates are those starting from 2,5-dimethyl-2,4-hexadiene (238). There have been more papers on the use of rhodium or antimony to catalyze the addition of diazoacetate and chiral copper complexes to create asymmetry during the addition (see Vol. 4, p. 482, Refs. 219-222). The problem with this route is to avoid the use of diazo compounds. An old synthesis of Corey and Jautelat used the ylide addition of a sulfurane to a suitable precursor (in this case a C3 unit was added to methyl 5-methyl-2,4-hexadienoate, 239), and a recent paper gives details about the addition of ethyl dimethylsulfuranylideneacetate to 2,5-dimethyl-4-hexen-3-one (240). This led exclusively to the tran -isomer 241, from which ethyl trans-chrysanthemate (185, R = Et) was made. Other ylide additions are mentioned below. 

This type of stereocontrol is featured in a synthesis of the fungitoxin ( )-chokol-A (156) (Scheme 32). Alcohol (147) (easily prepared from 1-hexen-S-one in two steps) efficiently gave the allyl chloride (1 ) (CCWPBus). Tlie metallation/cyclization/oxidation step (148) - (149) - (150) - (151) (described in detail) furnished ds-cyclopentylmethanol (151) (64% from 148 together with 2% of its trans epimer and 6% of a positional isomer derived from 141) which was oxidized to ds-caiboxylic acid (152). lodolactonizationAeduction (152) (153) -4 (154) secured the desired cis disposition of the r-hy-... 

Chapter 9 covered a synthesis of alkynes by a double dehydrohalogenation of dihalides. A student tried to convert trans-2,5-dimethyl-3-hexene to 2,5-dimethyl-3-hexyne by adding bromine across the double bond, then doing a double elimination. The infrared and mass spectra of the major product are shown here. 

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