Design and synthesis of (S)- and (R)-enantiomers of [4-(2-hydroxy-1-phenylethylimino)pent-2-ol]dimethyltin(IV) and 2,2-dimethyl-4-phenyl-1,3,2-oxazastannolidine: in vitro antitumor activity against human tumor cell lines and in vivo assay of (S)-enantiomers