Dienals Wittig reaction

Lehnert s reagent was used in the synthesis of the extended 1,3-dithiole 224 from the aldehyde 223, which in turn was obtained in the Wittig reaction involving the phosphonium salt 218 and hexa-2,4-dienal 222 (Scheme 27) <1998JMC1173>. 

The Wittig reaction and its numerous derivations have undoubtedly proven to be one of the most useful and efficient methods for forming carbon-carbon double bonds . The reaction of an organophosphorus reagent with an aldehyde or ketone has also been frequently employed to extend simple dienals and dienones into more elaborate polyene systems. A key step in the convergent synthesis of the TB DMS-protected leukotriene A4 methyl ester, 19/ ,5 -Z-butyldimethylsiloxy-5S, 6S -epoxyeicosa-7 ,9 , 1IZ, 14Z-tetraenoate (43), was accomplished using a Wittig reaction between homoallylic phosphorus ylide 44 and Cl—Cll chiral epoxy dienal 45, derived from (—)-2-deoxy-D-ribose, shown in equation 29. ... 

The key aldehyde 1151 is prepared from the malic acid-derived tosylate 59 as illustrated in Scheme 168. Attempted displacement of the tosyl function of 1145 with carbon nucleophiles fails due to base sensitivity caused by the ester group. Consequently, the ester is instead converted to a diisopropyl acetal, which alleviates the problem. Nucleophilic displacement of the tosylate with cyanide proceeds cleanly in this case to give nitrile 1147. Conversion of the nitrile to aldehyde 1148 followed by Wittig reaction with 1149 produces the protected (Z,Z)-diene 1150 with >95% (Z)-stereoselectivity. Hydrolysis of the acetal gives the dienal 1151 in 21% overall yield from dimethyl (5)-malate. 

Allylation of the 2,3-Wittig products (167) leads to diallylic ethers (171) in which one of the allylic double bonds is part of a 1,5-diene system. These ethers undergo 3,3-oxy-Cope rearrangement to afford vinyl allyl ethers (170), which rearrange in situ by a 3,3-Claisen process to yield the ( )-dienals (169). Some typical results are summarized in Table 14. The cyclopentanols (175) were shown to arise from aldehydes (169) by an intramolecular thermal ene reaction. ... 

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