Butenyl Grignard reagent, addition

Chelation control does not operate in the addition of 2-butenyl Grignard reagents to a-oxyalkanals, since with (racemic) 2-benzyloxy-, 2-(benzyloxymethoxy)- and 2-(t< rt-butyldi-methylsilyoxy)propanal similar ratios of isomers are formed28. Several cations were investigated, but the best choices, e.g., allylboronates or allylsilanes, were not included in this study. 

It is of interest to note that the preformed butenyl Grignard reagent does not show this addition reaction to the double bond. 

Trimethyl silane does not react under these conditions. However, under similar conditions, heptamethylcyclotetrasiloxane reacts with isoprene by 1,2-addition (78). Thiophene reacts with isoprene in the presence of phosphoric acid to give mainly 2-(3-methyl-2-butenyl)thiophene and some higher boiling compounds (79). Reactions of isoprene with Grignard reagents have been described (80,81). 

Finally, route C affords chiral aUylic cyclopentenols 55 via the 1,2-addition of butenyl magnesium bromide to enone 49, followed by debrominatiOTi and metathesis reaction. However, the moderate enantioselectivities (44-52% ee) inherent to the use of Unear Grignard reagents are a drawback of this last route. 

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