Petasis Reaction and Total Synthesis
Beau and coworkers assembled the core dihydropyran framework of zanamivir congeners via a combination of PBM reaction and Iron(III)-promoted deprotection-cyclization sequence. A stereochemically-defined α-hydroxyaldehyde 2, diallylamine and a dimethylketal-protected boronic acid 1 is coupled to form the acyclic, stereochemically-defined amino-alcohol 3, which then undergoes an Iron(III)-promoted cyclization to form a bicyclic dihydropyran 4. Selective opening of the oxazoline portion of the dihydropyran intermediate 4 with water or timethylsilyl azide then furnish downstream products that have structures resembling the Zanamivir family members.
Wong and coworkers prepared N-acetylneuraminic acid with a PBM coupling, followed by nitrone- cycloaddition. Vinylboronic acid is first coupled with L-arabinose 1 and Bis(4-methoxyphenyl)methanamine 2 to form an stereochemically-defined allyl amine 3. Afterwards, the sequence of dipolar cycloaddition, base-mediated N-O bond breakage and hydrolysis then complete the synthesis of N-acetylneuraminic acid.
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