Vinylmagnesium bromide, Grignard additions

The addition of vinylmagnesium bromide to methyl (S)-3-benzyloxy-4-oxobutanoate (5) in tetrahydrofuran proceeded with a slight preference for the nonchelation-controlled reaction product (40 60)5°. A reversal of the diastereoselectivity (80 20) could be observed when the Grignard reagent, as a solution in tetrahydrofuran, was added to a dichloromethane solution of the aldehyde which had been precomplexed with one equivalent of magnesium bromide. The almost exclusive formation of the chelation-controlled reaction product 6 was achieved when tetrahydrofuran was completely substituted by dichloromethane the presence of tetrahydrofuran interferes with the formation of the chelate complex, which is a prerequisite for high chelation-controlled diastereoselection. 

Posner has also described the stereocontrolled addition of vinyl Grignard reagents to enantiomerically pure cyclopentanone sulfoxides [43,47]. The addition of vinylmagnesium bromide to (70) pre-complexed with zinc bromide, and further reaction as described in Scheme in 4.36, yielded exclusively (5,5)-3-vinylcyclopentanone (71) in 30% overall yield. 

The addition of 2 moles of the Grignard reagent, R MgBr, to a-chloro acid chlorides, R R C(Cl)COCl results in the formation of alkenes R R C=CR 2 >n creditable yields, and excellent yields of Y,S-unsaturated ketones are obtained by the addition of vinylmagnesium bromide to imidoyl chlorides. Finally, unsymmetrical alcohols, R R CHOH are prepared in a convenient one-pot procedure by the stepwise addition of R MgX and R MgX to 2(iV-formyl-iV-methylamino)pyridine. ... 

Organolithiums react with / -benzoquinone to afford 1 1 adducts. Under more severe conditions, another equivalent binds to the second carbonyl, whereas Grignard reagents cause 1,4-addition instead. Methyllithium attacks 2,6-dimethyl-/7-benzoquin-one on the methyl-flanked carbonyl. Treatment of the adduct (215) with vinylmagnesium bromide creates a quaternary center diastereoselectively(Scheme 1-158). ... 

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