Synthesis of 2,3- and 3,4-Methanoamino Acid Equivalents with Stereochemical Diversity and Their Conversion into the Tripeptide Proteasome Inhibitor Belactosin A and Its Highly Potent <i>Cis</i>-Cyclopropane Stereoisomer
A series of chiral 2,3- and 3,4-methanoamino acid equivalents of stereochernical diversity were designed and synthesized from our chiral cyclopropane units, using a diastereoselective Grignard addition with (R)- or (S)-t-butanesulfinyl imines as the key step. These equivalents were converted into the proteasome inhibitor belactosin A and its cis-cyclopropane stereoisomer. The unnatural cis-isomer was shown to be more than twice as potent as belactosin A as a proteasome inhibitor.