Acyclic stereocontrol using boron

The preparation of such dihydropyrans derivatives 21-23 modified at the C-6 position by a hydrophobic substituent (zsopentyl side chain) analogous to the oseltamivir side chain is summarized in Scheme 7. It uses the borono-Mannich Petasis reaction between an amine, ot-hydro)yaldehyde 25 and boronic acid 26. This reaction proceeds with remarkably high stereocontrol, producing 1,2-aminoalcohols with an a tf-configuration. The resulting acyclic aminoalcohol 24 is the key precursor to the functionalized cyclic dihydropyrans 21-23 with the proper functionality and stereochemistry at C4. 

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