ABSTRACT
Accurate investigations on the role played by an alpha-stereogenic center in controlling the reduction of various classes of beta-hydroxy ketones allowed us to set up a general and highly diastereoselective protocol for the synthesis of 2-alkyl-1,3-diols with 1,2-syn relationship. This methodology is based on the conversion of a beta-hydroxy ketone into the corresponding titanium alcoholate that permits us to organize the substrate in a stable and rigid structure, which stereofacially favors attacking hydride ions. The use of THF as solvent makes available a variety of hydride donors that cover a large spectrum of steric demand: the choice of the more appropriate one depends on the conformational stability of the cyclic intermediate. Excellent results are obtained also in the presence of an additional stereogenic center in the beta-position, even if it exerts a concordant or an opposite steric effect with respect to the alpha-substituent.
ABSTRACT
[reaction: see text] 1,3-Diketones can be reduced in high yields and with excellent diastereoselectivity to the corresponding syn-1,3-diols by carrying out the reaction with BH3-pyridine complex in CH2Cl2 at -78 degrees C in the presence of an equivalent of TiCl4 and 0.1 equiv of pyridine. This protocol shows a general character: excellent results are obtained when the groups bound to the carbonylic functions are linear or branched carbon chains and aromatic or benzylic frameworks as well.
