Preparation of orthogonally protected (2S,3R)-2-amino-3-methyl-4-phosphonobutyric acid (Pmab) as a phosphatase-stable phosphothreonine mimetic and its use in the synthesis of polo-box domain-binding peptides

Abstract

Reported herein is the first stereoselective synthesis of (2S,3R)-4-[bis-(tert-butyloxy)phosphinyl]-2-[(9H-fluoren-9-ylmethoxy)carbonyl]amino-3-methylbutanoic acid [(N-Fmoc, O,O-(bis-(tert-butyl))-Pmab, 4] as a hydrolytically-stable phosphothreonine mimetic bearing orthogonal protection compatible with standard solid-phase protocols. The synthetic approach used employs Evans’ oxazolidinone for chiral induction. Also presented is the application of 4 in the solid-phase synthesis of polo-like kinase 1 (Plk1) polo box domain (PBD)-binding peptides. These Pmab-containing peptides retain PBD binding efficacy similar to a parent pThr containing peptide. Reagent 4 should be a highly useful reagent for the preparation of signal transduction-directed peptides.

Phosphatase‐Stable Phosphoamino Acid Mimetics That Enhance Binding Affinities with the Polo‐Box Domain of Polo‐like Kinase 1

“…This synthesis utilized a stereospecific alkylation to introduce the key 3 R -methyl group and required a minimum of 15 steps (10–12% overall yield). 21 Envisioning a more direct approach to this stereochemistry, we took note of progress that has been made in the application of asymmetric aminocatalytic Mannich reactions for the synthesis of amino acid intermediates having chiral centers in their side chains. 22,23 Gianelli and co-workers reported the first examples of a synthetic proline catalyst to effect anti -selective Mannich reactions, which proceede in high diastero- and enantioselectivity using N -Cbz or N -Boc protected imines, generated in situ from α-amido sulfones.…”

“…A Pudovik reaction on 10a using di- tert -butyl phosphite gave 11a (83% yield), with subsequent Barton-McCombie deoxygenation provided the globally protected 12a (61% yield over two steps). 21 Finally, saponification and exchange of the amine-protecting group gave the desired final product 13a on gram-scale in 8 steps and 22–25% overall yield (Scheme 1, upper panel).…”

Peptoid–Peptide Hybrid Ligands Targeting the Polo Box Domain of Polo‐Like Kinase 1

“…We determined the effects of replacing the pThr residue with the phosphatase-stable pThr mimetic (2 S ,3 R )-2-amino-3-methyl-4-phosphonobutyric acid (Pmab). [13] Peptides containing Pmab could be potentially advantageous over their pThr-containing counterparts in cellular studies, where cytoplasmic phosphatases can cleave phosphoryl ester bonds. [2] When the pThr residue in 6q was replaced with Pmab to give the corresponding peptide 13 (Scheme 3) a moderate reduction in affinity was observed ( 13 , IC 50 = 71 ± 17 nM).…”

“…Incorporation of (2 S ,3 R )-2-amino-3-methyl-4-phosphonobutyric acid (Pmab) in place of the pThr residue was accomplished as previously reported. [13] For peptide pull-down assays, a cysteine residue was attached to the amino-terminus by means of a 6-aminohexanoylamide (Ahex) spacer. Completed resins were sequentially washed with DMF, MeOH, CH 2 Cl 2 and ether and then dried under vacuum (overnight).…”

Investigation of Unanticipated Alkylation at the N(π) Position of a Histidyl Residue Under Mitsunobu Conditions and Synthesis of Orthogonally Protected Histidine Analogues

“…A more useful approach that would allow the generation of multiple peptides containing unique N(π)-substituents, could be realized through on-resin modification of the histidyl residue in post-peptide synthesis fashion. Scheme 3 shows the application of such a protocol to the synthesis of peptide Peptide 13 is a variant of 3 having the labile pThr residue replaced by (2S,3R)-2-amino-3-methyl-4-phosphonobutyric acid (Pmab), which is a phosphatase-stable pThr mimetic 32 that retains the PBD-binding affinity of parent pThr-containing peptides. 15,16 A key feature of the route outlined in Scheme 3 is its use of commercially-available N(α)-Fmoc-[N(τ)-Trt]-His for the incorporation of an N(τ)-Trt-protected histidine residue.…”