Syntheses of chiral intermediates of 1-β-methylcarbapenems: (3 S ,4 R )-3-[1( R )- tert -butyldimethylsilyloxyethyl]-4-chloroazetidin-2-one and (3 S ,4 S )-3-[1( R )- tert -butyldimethylsilyloxyethyl]-4-[1( R )- tert -butylthiocarbonylethyl]azetidin-2-one