Synthesis of unsaturated aldehydes by sequential Claisen and Cope rearrangements

Abstract

Condensation of hexa-1,5-dien-3-ol (Ia) with 1,1,3-triethoxy-2-methylbutane, catalysed by o-nitrobenzoic acid, proceeds with elimination of three moles of ethanol to form an intermediate dienol ether that undergoes Claisen rearrangement to 2-methyl-2-vinylocta-4,7-dienal (Va). At higher temperatures (ca. 160°) this aldehyde (Va) rearranges to 2-methyl-5-vinylocta-2,7-dienal (VIa), which at still higher temperatures (ca. 190°) equilibrates through a second Cope rearrangement with the more stable 2-methyldeca-2,5,9-trienal (VIIa). The same sequence of reactions was carried through with two methyl derivatives of the hexadienol (Ia).

Reduction of the aldehyde (VIa) formed the allyl alcohol (X), which rearranged to the same dimethylated straight-chain alcohol (XI) as was produced by reduction of the isomer (VIIa). Condensation with 1,1,3-triethoxy-2-methylbutane converted (X) through Claisen rearrangement and two successive Cope rearrangements into the ‘linear’ 2,6-dimethyltetradeca-2,6,9,13-tetraenal (XV), also formed in the same way from the rearranged isomer (XI) through Claisen rearrangement and one Cope rearrangement.