Novel Phosphate Derivatives as Scaffolds for the Preparation of Synthetic Phosphoserine-Based Peptides Using the Fmoc/t-Bu Solid-Phase Strategy

Abstract

Synthetic peptides incorporating analogues of phosphoserine are valuable tools for the study of protein kinases and phosphatases. In addition, derivatives of naturally occurring peptides incorporating phosphate groups may have interesting biological properties. Herein we describe a new Fmoc/t-Bu solid-phase peptide synthesis (SPPS) strategy for the convenient generation of phosphoserine-based peptides. A proof-of-concept synthesis that demonstrates the feasibility of this approach is presented.

phosphopeptide - serine - phosphorus - solid-phase synthesis - amino acids

References and Notes

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Fmoc/t-Bu SPPS using derivatives of this sort as starting materials is superior to that employing the analogous compounds containing a free phosphoric acid group (see ref. 7).

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When the test was negative the Fmoc group was removed with piperidine-DMF (1:4); otherwise the Fmoc-amino acid was re-coupled under the same conditions. See Supporting Information for full details.

<A NAME="RD57911ST-18">18</A>

t-Bu and Boc were chosen for either carboxylic (11) or amino (14) protection as they were stable to piperidine treatment, used to remove the Fmoc group, and to low concentrations of TFA, used to cleave the peptide from the 2-CTC resin (Scheme  [4] ).

<A NAME="RD57911ST-19">19</A>

It is noteworthy that compounds 2a and 2b, as well as the corresponding free carboxylic acids 9a and 9b, were obtained as diastereomeric mixtures. However, once the cyanoethyl group undergoes β-elimination during the basic treatment to remove the Fmoc group in SPPS (Scheme  [4] ), the phosphorus centre is no longer chiral leading to only one stereoisomer of the desired phosphopeptide.

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HPLC analysis of the crude material obtained after workup revealed that the peptide was approx. 70% pure.

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The cyclised analogue of 1 is of interest; comparison of the biological effects of the acyclic and cyclic variants would provide useful information on the importance of the macro-cyclic ring structure for the biological activity of these types of compounds. Preliminary attempts at cyclising 1
via intramolecular amide formation have provided some evidence for the formation of the desired product. Studies towards the isolation and biological testing of the cyclised analogue of 1 are ongoing and will be reported in due course.

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