Optically active 1,3-O-disubstituted glycerols were obtained by enantioselective homogeneous hydrogenation of the corresponding 1,3-0 -disubstituted 1,3-dihydroxypropan-2-0nes with different Ru'l-binap complexes. The highest enantioselectivities were obtained with dimeric chloro-ruthenium complexes on the substrates bearing bulky trityloxy RrouDs. Introduction. -Optically active 1,3-glycerol derivatives are important and versatile key building blocks for the preparation of a large variety of compounds that are biologically important or are of interest in biochemical research such as phospholipids [I], PAF (platelet activating factor) [2], p-blockers [3], prostaglandins [4], and other compounds [5].
These compounds are usually prepared from the chiral pool substances such as D-mannitol, L-arabinose, L-ascorbic acid, D-or L-tartaric acid, L-serine. New approaches to synthesizing chiral glycerol derivatives are based on enzymatic differentiation of enantiotopic hydroxymethyl groups of glycerol [6] or enantioselective microbial reduction of monoesters of 1,3-dihydroxypropanone [7]. Prompted by the reports on the impressive enantioselective activity of Ru"-binap (binap : atropisomeric chiral diphosphines (+)-(R)-and (-)-(S)-bis(diphenylphosphin0)-1,l'-binaphthalene) catalysts in the reduction of a large variety of prochiral ketones [S] [9], we have investigated the possibility of the enantioselective reduction of some prochiral diethers of 1,3-dihydroxypropanone with different RuII-binap complexes. Part of this work concerning some preliminary results in the catalytic reduction of 4a and 4b has been object of a short communication [lo].
Results and Discussion. ~ The substrates investigated together with the synthesis sequence are reported in the Scheme; they are prepared in good yields from racemic 1,2-0 -isopropylideneglycerol (1). The unprotected primary OH group is functionalized with a benzyl or an octadecyl group followed by acetonide removal to give the diols 2a and 2b.
Tritylation of 2a and 2b affords the diethers 3a and 3b; the reaction of 2a with octadecyl para -toluenesulfonate gives the diether 3c. The secondary alcohols are oxidized to the corresponding ketones 4a, 4b, and 4c with pyridinium chlorochromate in dry CH,Cl,.