The absolute configuration of the epoxy alcohol can be predicted by the following mneumonic model in which the hydroxymethylene group is positioned at the lower right. The epoxidation takes place from the upper face of the allyl alcohol when (+)-( R,R )-DET is used and vice versa (Scheme 13).
These reaction conditions, Ti(Oi -Pr)4 , t -BuOOH and ( R,R ) or ( S,S )-diethyltartarate, could also be used for asymmetric oxidation of aryl alkyl sulfides (Scheme 14).
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Scheme 14 |
3.8.5.4 Olefination
Tebbe's reagent, (η5 -C5 H5 )2 TiCH2 ClAlMe2 , allows even less reactive carbonyl compounds to be transformed to the corresponding methylene compounds in the presence of pyridine. The reagent is prepared by the reaction of titanocene dichloride with trimethyl aluminium in toluene (Scheme 15).
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Scheme 15 |
In the presence of pyridine the reagent transforms into a Schrock carbene and reacts with the carbonyl group in a [2+2] cyclization which then undergoes ring opening metathesis to form the alkene (Scheme 16).
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Scheme 16 |