Crystal Structures – A Manifesto 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 Me<INF>2</INF>CH group is in a disposition that mimick a Me<INF>3</INF>C group (Figs. 3–5). The five-membered ring shows conformations from an envelope form with the Ph<INF>2</INF>C group out of the plane containing the other four atoms to the twist form with the twofold axis through the C&dbond;O group (Fig. 6, and Table 2). In the entire series, the Me<INF>2</INF>CH 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 C&dbond;O group in the ring, excellent stereoselectivities in reactions of its derivatives, and safe preparation and easy recovery of the auxiliary.