Vinyl allyl ethers, isomerisation

Scheme 2.7 Transition-metal-catalysed isomerisation of allyl ethers to vinyl ethers, followed by cleavage.

Before tethering can be attempted, the 2-0-allyl group must first be iso-merised to a vinyl ether. Isomerisation was accomplished by treating glycosyl fluoride 74 with Wilkinson s catalyst and n-butyl lithium, which afforded vinyl ether 75 in an excellent yield of 96%. It is important to note that the Wilkinson s catalyst mediated isomersiation is very efiicient when compared to the previously used and capricious Tebbe methylenation methodology. Subsequent NIS-mediated tethering of secondary acceptor 76 to vinyl ether 75... 

Both IH- and 4//-chromenes are accessible from 2-allyIphenols. Isomerisation to the internal alkene and O-allylation followed by RCM using a Grubbs second generation catalyst affords the former benzopyrans. Conversion of the allylphenol to the vinyl ether prior to RCM gives the 4/f-[l]benzopyrans <05T9996>. 

On the other hand, Alexakis et al. have developed a sequential reaction exploiting the compatibility between a cationic iridium catalyst and chiral diphenylprolinol trimethylsilyl ether. The role of the iridium complex was proposed to isomerise the starting allylic primary alcohols into the corresponding aldehydes in the presence of H2, while the chiral amine catalyst promoted the a-functionalisation of these aldehydes through reaction with vinyl sulfone to give the corresponding Michael adducts as a mixture of syn-and (Z h -diastereomers obtained in moderate yields, moderate to excellent... 

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