Date of Award
Doctor of Philosophy (PhD)
Robert P. Hammer
Phosphonate analogs of peptides are important in biochemical research and potentially as medicinal agents because of their ability to strongly inhibit both metallo and aspartyl proteases. Although a number of phosphorus-containing molecules were reported as inhibitors for carboxypeptidase A (CPA, there have been only a few thiophosphonopeptide inhibitors reported. There has been no investigation in which the Ki values of a series of thiophosphonate inhibitors was compared to the analogous phosphonates to probe the role the sulfur atom in inhibition. The goal of this work is to prepare phosphonopeptide inhibitors of CPA and their thio-analogs so that this comparison can be made. To accomplish this goal, we have utilized reduced phosphorus [P(III)] intermediates in an activation-coupling-oxidation protocol both on solid support and in solution. In the solid-phase synthesis, only moderate yields of the protected amino phosphonite coupling were achieved (40--70%). Also, no full-length products were obtained due to a stable 6-membered cyclic oxazaphospholine formed during the deblocking step of the resin-bound phosphonodipeptide. In solution-phase synthesis, activation in the presence of base provides phosphonochloridite, which produces oxaphosphazoles that are unreactive towards alcohols, which explains low yields. Activation in the absence of base produces phosphonodichlorite, which does provide phosphonopeptides, but lacking the ester protection. An alternate route to CPA inhibitors that utilizes H-phosphonate esters as key intermediates successful produced a phosphonopeptide CPA inhibitor and its thio-analog. Preliminary enzyme assays found that phosphonopeptide peptide analog has a Ki value of 10-14 M, and thio-analog has a Ki value of about 10-15 M. This indicates that thiophosphonate peptide analog is among the lowest values yet determined for protease inhibition. Several of desired targets could not be prepared because of difficulties with C-terminal ester cleavage. Cleavage of methyl ester with thiolate led to decomposition for peptides with a phosphonoalanine analog. Allyl ester cleavage with palladium was successful for phosphonates, but not in general for thiophosphonates. The thiophosphonate sulfur- may react with the palladium catalyst to form a thioallyl ester, which during the aqueous workup process is hydrolyzed to displace the sulfur moiety.
Fan, Hong, "Synthesis of Phosphonate and Thiophosphonate Peptide Analogs as Inhibitors of Carboxypeptidase A." (2000). LSU Historical Dissertations and Theses. 7151.