Crystal structures of chiral 2-[bis­(2-chloro­eth­yl)amino]-1,3,2-oxaza­phospho­lidin-2-one derivatives for the absolute configuration at phospho­rus

‘Nitro­gen mustard’ bis­(2-chloro­eth­yl)amine derivatives (2R,4S,5R)- and (2S,4S,5R)-2-[bis­(2-chloro­eth­yl)amino]-3,4-dimethyl-5-phenyl-1,3,2-oxaza­phos­pho­lidin-2-one (2a and 2b, respectively), C(14)H(21)Cl(2)N(2)O(2)P, and (2R,4R)- and (2S,4R)-2-[bis­(2-chloro­eth­yl)amino]-4-isobutyl-1,3,2-oxaza­phospho­lidin-2-one (3a and 3b, respectively), C(10)H(21)Cl(2)N(2)O(2)P, were synthesized as a mixture of diastereomers through a 1:1 reaction of enanti­omerically pure chiral amino alcohols with bis­(2-chloro­eth­yl)phospho­ramidic dichloride. Flash column chromatography yielded diastereomerically pure products, as supported by (31)P NMR. The crystal structures of 2b and 3b were obtained to determine their absolute configuration at phospho­rus, and (31)P NMR chemical shift trends are proposed based on the spatial relationship of the bis­(2-chloro­eth­yl)amine moiety and the chiral substituent of the amino alcohol. Oxaza­phospho­lidinones were observed to have a more downfield (31)P NMR chemical shift when the aforementioned substituents are in a syn configuration and vice versa for when they are anti.