Temperature-controlled regioselectivity in the reductive cleavage of p-methoxybenzylidene acetals.

ABSTRACT: The regioselective ring opening of pyranosidic 4,6-p-methoxybenzylidene acetals with BH(3)/Bu(2)BOTf in THF can be tuned by adjusting the reaction temperature and reagent concentrations. Reductive cleavage at 0 degrees C resulted in the exclusive formation of 4-O-p-methoxybenzyl (PMB) ethers, whereas reaction at -78 degrees C produced 6-O-PMB ethers in high yields. The latter condition was observed to be compatible with a variety of acid-sensitive functional groups, including allyl and enol ethers. The presence of water does not interfere with reductive ring opening and may contribute toward in situ generation of H(+) as a catalyst for 6-O-PMB ether formation. Reductive cleavage under rigorously aprotic conditions is greatly decelerated, and yields only the 4-O-PMB ether. The temperature-d

SUBMITTER: Hernandez-Torres JM 

PROVIDER: S-EPMC1851890 | biostudies-literature | 2004 Oct

REPOSITORIES: biostudies-literature