TiCl+-catalyzed Diels-Alder additions of the acrylate of commercial D-pantolactone to cyclopentadiene, isoprene and butadiene proceed with very high diastereofacial selectivity. Practical and mechanistic aspects of these reactions are discussed. Metal coordination is known to be important for a variety of useful asymmetric syntheses. Chelate formation, in particular, can amplify inherently weak diastereoface-discrimination to theextent that high levels of stereoselection may be achieved with very simple chiral auxiliaries. Following this general guideline, we have uncovered that acrylic esters of lactates are very effective dienophiles in Lewis acid promoted Diels-Alder reactions*. In an effort to clarify puzzling effects of Lewis acids associated with these reagents we have recently succeeded in growing single crystals of a I:1 acrylate-TiCl,, complex3. The structure of this complex has provided a basis for rationalization of our observations and, moreover, has given an essential hint for identifying the title compound to be a most useful chiral auxiliary. The facts which have induced the work presented here are comprised in Scheme 1 which is representative of a number of similar reactions. The reaction of acrylate 1 with cyclopentadiene preferentially yields endo adduct 22 with TiCl,, as catalyst. This result can be understood by assuming that a single species of structure 4, corresponding to the crystal structure of the l-TiCll, complex, reacts and that i is attacked at the less shielded face (2Si) of its enoate group. However, this simple rationale does not explain the response of diastereoselectivity to variations of reaction conditions (Fig. 1). In particular, steep decrease of diastereoselectivity is found with more than ca. 1 equivalent of TiC14. This indicates that besides i other reactive species are present. According to the modified Walborsky model 4, likely such species are the non-chelate Scheme 1 (LA Lewis acid) B' = EtAICI, TiCI,