(2R,2′R, 5R,5′R)-1,1′-(1,2-Ethanediyl)bis[2,5-dimethylphospholane]; (2R,2′R, 5R,5′R)-1,1′-(1,2-ethanediyl)bis[2,5-diethylphospholane]; (2S,2′S, 5S,5′S)-1,1′-(1,2-ethanediyl)bis[2,5-di(2-propyl)phospholane] and (2S,2′S, 5S,5′S)-1,1′-(1,2-ethanediyl)bis[2,5