Methodology for the construction of quaternary carbon centers

Construction of Quaternary Stereogenic Centers via [2 + 2] Cycloaddition Reactions. Synthesis of Homochiral 4,4-Disubstituted 2-Azetidinones and Imine Substituent Effects on beta-Lactam Formation

A study on the asymmetric construction of quaternary stereogenic centers via [2 + 2] cycloaddition reaction of ketenes with ketimines is described. Reaction of achiral ketenes and chiral alpha-alkoxy ketone-derived imines resulted in formation of new beta-lactams as single diastereomers. The cycloaddition was extended to pyruvate imines, aralkyl ketone-derived imines, and dialkyl ketimines. In these cases the asymmetric induction was satisfactorily achieved using beta-silylalkanoyl ketenes and the Evans-Sjögren ketene. C,C-Bis (trimethylsilyl)methylamine ketimines derived from enolizable dialkyl ketones cleanly led to the corresponding C(4) disubstituted beta-lactams without deprotonation. Therefore, a general methodology for a convergent asymmetric synthesis of beta-lactams in which C(4) exists as a quaternary carbon is provided.

Application of the Ester Enolate Claisen Rearrangement in the Synthesis of Amino Acids Containing Quaternary Carbon Centers

Ester enolate Claisen rearrangement of highly substituted amino acid allylic esters 4 allows for the synthesis of sterically demanding amino acids 5 with beta-quaternary carbon centers. Because of enolate fixation by chelation, the rearrangement occurs in a highly diastereoselective fashion. The methodology is suitable not only for glycine derivatives but also for allylic esters of various amino acids. In this case amino acids with two vicinal quaternary carbon centers are created. With unsymmetrically substituted allylic esters like 4k-n the rearrangement proceeds with a high degree of diastereoselectivity.