[95061-46-4] · C20H18O2 · 1,1,2-Triphenyl-1,2-ethanediol · (MW 290.38)
(derived chiral monoesters undergo stereoselective aldol additions; formation of O-silyl orthoesters and cyclic phosphonates)
Physical Data: mp 126 °C. (R): [a]
Solubility: sol dichloromethane, chloroform, THF, ethanol; insol hexane.
Form Supplied in: white solid; the (R)-form is commercially available.
Preparative Methods: (R)-1,1,2-triphenylethane-1,2-diol [(R)-(1)] is easily available from commercial (R)-Mandelic Acid, which is first esterified to give methyl mandelate and then treated with Phenylmagnesium Bromide (3.5 equiv). In an analogous way, (S)-(1) is accessible from (S)-mandelic acid, which is also commercially available (eq 1).2
2-Trimethylsilyloxy-1,2,2-triphenylethyl propionate, which is prepared from (R)-(1) by esterification with propionyl chloride and subsequent silylation of the tertiary hydroxy group, reacts in a highly stereoselective manner upon deprotonation, transmetalation with Dichlorobis(cyclopentadienyl)zirconium, and addition to 2-methylpropanal. The diastereoselectivity is 96:4, which is the ratio of the major product to the sum of all other diastereomers. Subsequent reduction with Lithium Aluminum Hydride affords (2S,3R)-2,4-dimethyl-1,3-pentanediol in 95% ee (eq 2).3 anti-Selective aldol additions which deliver chiral nonracemic products have been a longstanding problem of asymmetric synthesis.4 Doubly deprotonated 2-hydroxy-1,2,2-triphenylethyl propionate has been applied in a total synthesis of dolastatin.5
When 1,1,2-triphenylethane-1,2-diol-derived esters are submitted to a monodeprotonation and subsequently treated with Chlorotrimethylsilane, the formation of 2-trimethylsilyloxy-1,3-dioxolanes results. The orthoester moiety thus obtained serves as a protecting group for carboxylic acids (eq 3); it is stable towards alkyllithium reagents and can be cleaved under nonacidic conditions by alkaline hydrolysis.6
Methanephosphonyl dichloride reacts with (R)-(1) to give 2-methyl-4,4,5-triphenyl-2-oxo-1,3,2-dioxaphospholane (eq 4); the (RP,RC) diastereomer forms predominantly (9:1).7
A series of enantiomerically pure 1,1-diaryl-2-phenylethane-1,2-diols is available from methyl mandelate by addition of the corresponding substituted arylmagnesium bromides or aryllithium reagents.2b,8
Heinrich-Heine-Universität, Düsseldorf, Germany