Synthesis and hydrogenolysis of the methylene, ethylidene, isopropylidene, and diastereoisomeric 1-phenylethylidene acetals of β-l

Add time:08/12/2019    Source:sciencedirect.com

Both diastereoisomers of 1-phenylethylidene acetals (acetophenone acetals) of methyl and benzyl β-l-arabinopyranoside and α-l-rhamnopyranoside were prepared. Acetal-exchange reactions gave only the endo-phenyl isomers; their 2-O- and 4-O-acetyl derivatives were isomerised into the exo-phenyl compounds. 1H-N.m.r. data were used to determine the absolute configuration at the acetal carbon atom in these compounds. The protons of the methyl group of the exo-phenyl isomers resonate at lower field than those of the endo-phenyl isomers. Hydrogenolysis of various methylene, ethylidene, and isopropylidene derivatives gave axial ethers. The endo-phenyl isomers of the acetophenone derivatives also gave axial 1-phenylethyl ethers in two diastereoisomeric forms. The exo-phenyl isomers of the arabinosides were stable towards the reagent (LiAlH4AlCl3), whereas the corresponding rhamnopyranosides gave the 2-(1-phenylethyl) ethers, but cleavage required prolonged reaction time and higher temperature.

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