Crimmins Thiazolidinethione Aldol
A more recent version of the Evans' auxiliary is the Crimmins thiazolidinethione. The yields, diastereoselectivities, and enantioselectivities of the reaction are, in general, high, although not as high as in comparable Evans cases. Unlike the Evans auxiliary, however, the thiazoldinethione can perform acetate aldol reactions (ref: Crimmins, Org. Lett. 2007, 9(1), 149–152.) and can produce the "Evans syn" or "non-Evans syn" adducts by simply varying the amount of (−)-sparteine. The reaction is believed to proceed via six-membered, titanium-bound transition states, analogous to the proposed transition states for the Evans auxiliary. NOTE: the structure of sparteine shown below is missing a N atom.