Crystal Structures– AManifesto for the Superiority of the Valine-Derived 5,5-Diphenyloxazolidinone as an Auxiliary in Enantioselective Organic Synthesis

The crystal structures of 32 derivatives of 4-isopropyl-5,5-diphenyl-1,3-oxazolidin-2-one (A and 1–31) are presented (Fig. 2 and Tables 1–3). In all but four structures, the Me2CH group is in a disposition that mimick a Me3C group (Figs. 3–5). The five-membered ring shows conformations from an envelope form with the Ph2C group out of the plane containing the other four atoms to the twist form with the twofold axis through the CO group (Fig. 6, and Table 2). In the entire series, the Me2CH and the neighboring trans Ph group are approximately antiperiplanar (average torsion angle 155°). The structural features are used to interpret the previously observed reactivity behavior of the diphenyl-oxazolidinone derivatives. The practical advantages of the title compound over classical Evans auxiliaries are outlined (Figs. 1 and 7, and Scheme 2): high crystallinity of all derivatives, steric protection of the CO group in the ring, excellent stereoselectivities in reactions of its derivatives, and safe preparation and easy recovery of the auxiliary.