Stereoselective Titanium-Mediated Aldol Reactions of Menthyl Acetate Enolates, and Allylations With Benzaldehyde
1993
The enolates of (+)- and (-)- menthyl acetate, formed by transmetallation from lithium with chloro ( cyclopentadienyl )bis(1,2:5,6-di-O-isopropylidene-α-D-glucofuranosato-O3)titanium [ CpTi (DAGO)2Cl] (1), react with benzaldehyde from the Re-face with high stereoselectivities (95 and 92% diastereomeric excess) when 12-crown-4 is used during the transmetallation . Use of the chiral cyclopentadienyltitanium-α,α,αα,αα-tetraaryl-2,2-dimethyl-1,3-dioxolan-4,5-dimethanol (TADDOL) complex (3) gives lower diastereoselectivities (58 and 40%) but the Si -face of benzaldehyde is attacked preferentially. Transmetallation of the 2-methylprop-2-enyl moiety to CpTi (DAGO)2Cl gives an allyltitanium reagent (2) which reacts with benzaldehyde from the Re-face with 88% enantiomeric excess.
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