 In the reaction the trimethylsilyl (TMS) group fulfill 2 roles in directing the stereoselectivity of the reaction.First, the bulky size of TMS helped the molecule to preferably adopt a conformation where the TMS is not close to the methyl group on the alkene.Second, the TMS group conferred a stereoelectronic effect on the molecule by adopting an anti conformation to the directing orbitals of the alkene.