Lewis acid mediated diastereoselective and enantioselective cyclopropanation of Michael acceptors with sulfur ylides

Ethyl 4,4-dimethyl pyroglutamate (DMPG): a chiral auxiliary in cyclopropanation and carbonyl group activator

“…These results contrast with our previous studies where EDSA reacted with enones but showed a lack of reactivity towards α,β-unsaturated aldehydes, esters, nitriles and amides. 14 Furthermore, although the stereoselectivity in the reaction with EDSA is low, DMPG promotes cyclopropanation in excellent yields and with a good level of diastereoselection under nonchelation control conditions of the substrate 9,12 when Trost´s salt is used as the sulfonium ylide precursor. The origin of this diastereoselectivity presumably involves a dominant reactive rotamer for 2 with predominant attack of the ylide at the β-carbon from the opposite face to the pyroglutamate carboxylate group.…”

“…The residue was purified by chromatography NP-HPLC Kromasil 60 (10 µm, 8x18 cm)] using hexane/acetone 85:15 to give 450 mg (76%) of 2. A second purification by chiral NP-HPLC [Chiralcel OD (10 µm, 2x25 cm) 12 …”

“…10 Hanessian et al have reported an efficient asymmetric cyclopropanation using chloroallyl phosphonamides. 11 More recently an enantioselective cyclopropanation of N-enoyloxazolidinones 12 mediated by Lewis acids has been reported and Barluenga 13 has described the diastereo-and enantioselective cyclopropanantion of alkenyl oxazolines with chromium Fischer carbene complexes. Our contributions in the field include the diastereoselective cyclopropanation of cyclic and acyclic enones with ethyl dimethylsulfonium acetate bromide, 14 and the stereoselective cyclopropanation of enantiomerically pure dihydroxycyclopentenones for the enantioselective synthesis of 2-aminobicyclo[3.1.0]hexane-2,6-dicarboxylic acid.…”

Short Access to the Aromadendrane Family: Highly Efficient Stereocontrolled Total Synthesis of (±)‐Cyclocolorenone and (±)‐α‐Gurjunene

“…We reasoned that this failure was due to the ability of 7 to be readily enolized. By screening [17] various bases, solvents, temperature conditions, and Lewis acids, [18] we found that NaHMDS in THF at -110°C in the presence of Sc(OTf) 3 gave the best results. The desired cyclopropane product 8 was isolated in 40 % yield.…”

Evans Enolates: Solution Structures of Lithiated Oxazolidinone-Derived Enolates

“…2 What followed is now history: oxazolidinone-based chiral enolates—so-called Evans enolates—have appeared in more than 1600 patents and countless academic and industrial syntheses. 3 Variations of the auxiliaries 4 and extensions of oxazolidinones beyond the chemistry of enolates 5 attest to their importance. …”