Butenylmagnesium bromides, methyl

Ketone 13 possesses the requisite structural features for an a-chelation-controlled carbonyl addition reaction.9-11 Treatment of 13 with 3-methyl-3-butenylmagnesium bromide leads, through the intermediacy of a five-membered chelate, to the formation of intermediate 12 together with a small amount of the C-12 epimer. The degree of stereoselectivity (ca. 50 1 in favor of the desired compound 12) exhibited in this substrate-stereocontrolled addition reaction is exceptional. It is instructive to note that sequential treatment of lactone 14 with 3-methyl-3-butenylmagnesium bromide and tert-butyldimethylsilyl chloride, followed by exposure of the resultant ketone to methylmagnesium bromide, produces the C-12 epimer of intermediate 12 with the same 50 1 stereoselectivity. 

The central fragment (281) of monensin is prepared from 275 as outlined in Scheme 38 [62]. The key step in the sequence is addition of 3-methyl-3-butenylmagnesium bromide to 279. Since 279 contains the necessary features for chelation-controlled addition of the Grignard reagent to the carbonyl group, product 280a is formed with high diastereoselectivity (50 1). 

Methyl-2-butenylmagnesium chloride allowed to react 2 hrs. at 0° with 0.5 equivalent n-heptyl bromide in tetrahydrofuran in the presence of Cul 2-... 

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