Using your knowledge of Felkin-Anh control, rationalize why compound (4) has a syn stereochemical relationship between the stereogenic centres labelled 3 and 4. 

 Using 3D representations, invoke a Zimmerman structure to account for why compound (4) has the anti stereochemical relationship between the stereogenic centres labelled 2 and 3. 

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LDA (2) EtS (1) он recall LDA= Li (3) он он EtS (4) LDA (2) EtS (1) он recall LDA= Li (3) он он EtS (4)