Cyclohexyl ether as a new hydroxy-protecting group for serine and threonine in peptide synthesis

Abstract

A new hydroxy-protecting group for Ser and Thr, cyclohexyl (Chx), has been developed, and its application to peptide synthesis is described. The Chx group is introduced to the hydroxy functions of Boc-Ser-OH and Boc-Thr-OH in two steps; Boc-Ser-OH and Boc-Thr-OH are treated with NaH and then allowed to react with 3-bromocyclohexene to afford N-Boc-O-(cyclohex-2-enyl)-Ser and N-Boc-O-(cyclohex-2-enyl)-Thr in satisfactory yields, which are hydrogenated in the presence of PtO₂ to give Boc-Ser(Chx)-OH and Boc-Thr(Chx)-OH in good yields. The O-Chx group is stable to various acidic and basic conditions including TFA and 20% piperidine in DMF. It is not removed with catalytic hydrogenation over Pd–charcoal. The Chx group is, however, removed quantitatively with 1 mol dm⁻³ trifluoromethanesulfonic acid–thioanisole in TFA over a short period. These results indicate that the Chx group is suitable for the hydroxy-protection of Ser and Thr in peptide synthesis based on Boc-chemistry and can be used also in combination with either N^α-Fmoc or N^α-Z-protection. The apparent rate constant for removal of the Chx group with 50% TFA in CH₂Cl₂ (k_Chx) is found to be less than a twentieth of that of the Bzl group (k_Bzl), confirming the substantial stability of the Chx group under common Boc-deprotection conditions. Simulations of solid-phase peptide syntheses using k_Chx and k_Bzl indicate that the Chx group would be adequate for solid-phase synthesis of large peptides and proteins. Solid-phase synthesis of a peptide and conventional solution synthesis of a protected peptide segment using Chx protection are also demonstrated.