Diastereoselective Aldol Reactions of (Z)-N-[Bis(methylthio)methylene]-α,β-didehydroglutamates

Abstract

In the reaction of the enolates of (Z)-N-[bis(methylthio)methylene]-α,β-didehydroglutamates with aldehydes, the retroaldolization process is faster than the aldol reaction, thus inhibiting the obtainment of the desired targets 6. However, in the presence of TMS−Cl as trapping agent, the equilibrium can be deplaced and the aldols 6 are obtained in the form of their TMS-derivatives 8. In the presence of BF3 or TBAF, either the syn or the anti adducts can be selectively obtained with good yields and diastereoselectivities.

New Chiral Didehydroamino Acid Derivatives from a Cyclic Glycine Template with 3,6-Dihydro-2H-1,4-oxazin-2-one Structure: Applications to the Asymmetric Synthesis of Nonproteinogenic α-Amino Acids

“…α,β-Didehydro-α-amino acids (DDAAs) constitute a family of conformationally constrained amino acids found in many biologically active natural peptides . Their presence in peptide chains enhances resistance to enzymatic and chemical degradation .…”

Diastereodifferentiating Z-E Photoisomerization of 3-Benzoyloxycyclooctene: Diastereoselectivity Switching Controlled by Substrate Concentration through Competitive Intra- vs Intermolecular Photosensitization Processes

“…However, the direct photolysis of ( Z )-3-benzoyloxycyclooctene ( 1Z ) gave unexpected results, showing that the intramolecular photosensitization was not efficient in this case and, interestingly, that the major diastereomer obtained is switched by changing the substrate concentration. Although diastereoselectivity of the product is known to be altered by employing different additives , or protective groups, to the best of our knowledge, this is the first example in which such a switch is caused by a change in the substrate concentration. We now wish to report the mechanistic details of this unprecedented concentration-dependent diastereodifferentiating photoisomerization of 1Z and further to propose a potentially general method to control the diastereoselectivity of the product through competing intra- and intermolecular photosensitization processes.…”