[136849-75-7] · C12H16O5 · (MW 240.25)
(reagent used as a hyperacid sensitive linker in solid-phase synthesis, compatible with Fmoc/tBu solid-phase peptide synthesis)
Physical Data: mp 83-86°C.
Solubility: soluble in N-methyl pyrrolidone, DMF, other organic solvents.
Form Supplied in: commercially available as 4-(4-hydroxymethyl-3-methoxyphenoxy)butyric acid (1) or as prefunctionalized resins, in the form of 4-[4-(hydroxymethyl)-3-methoxyphenoxy] butanoic acid benzhydrylamine resin (HMPB-BHA) (2) and 4-[4-(hydroxymethyl)-3-methoxyphenoxy]butanoic acid 4-methyl benzhydrylamine resin (HMPB-MBHA) (3).
Handling, Storage, and Precautions: no special precautions. Keep cool and dry. Store at 8 °C.
Linker 1 is conveniently prepared from 4-hydroxy-2-methoxybenzaldehyde in three steps.1 The linker may be coupled to any aminomethylated resin using standard coupling methods.2,3 The linker has also been coupled to dendrimer3 and TentaGel4 resins. Fmoc amino acids can be esterified onto the resulting HMPB functionalized resin using 2,6-dichlorobenzoyl chloride5 or standard DIC/DMAP conditions.3 Alcohols can be immobilized onto the HMPB functionalized resin under Mitsunobu conditions.6
The inclusion of the additional two methylene units in the carboxylic acid chain renders the HMPB linker 1 to be approximately 30 times more acid labile than the analogous 4-hydroxymethyl-3-methoxyphenoxy acetic acid linker introduced by Sheppard and Williams.7,8 This offers the potential that peptides attached to the linker can be easily cleaved by treatment with mildly acidic solutions such that common, acid susceptible amino acid side-chain protecting groups are retained on the liberated peptide fragments,9 the only exception being His(Trt) that in practice often loses small amounts of the trityl protecting group (although reports of this vary). It has been reported that the trityl protecting group may be selectively cleaved by mild acid treatment in the presence of other acid-sensitive functionalities.10 Conditions (0.1 M HOAt, 0.12 M Me3SiCl in 2,2,2-trifluoroethanol) suitable for the quantitative removal of a Na trityl unit of a resin-bound peptide using the 3-(4-hydroxymethylphenoxy)propionic acid linker were found to be too harsh for the HMPB handle as evidenced by the partial release of the peptide from the solid support.10 A more general approach for alkoxybenzyl ethers was found to be 0.2% TFA, 1% H2O in CH2Cl2, resulting in the desired cleavage without premature release of the peptide. The HMPB linker 1 has been successfully exploited in the preparation of peptides using a fragment-coupling approach, which conveniently combines the solid-phase synthesis of fully protected peptide units with the solution-phase union of these fragments.9 This fragment condensation methodology has been fruitfully applied to the synthesis of human calcitonin-(1-33), human neuropeptide Y (NPY), and the 230-249 sequence of mitogen-activated 70K S6 kinase.9 This method also has been used in the preparation of repetitive protein domains, protected peptide fragments being synthesized on solid-phase using Fmoc peptide synthesis.11 Interestingly the acid sensitive trityl group was successfully used to protect the side chain of histidine, which survived the cleavage protocol (1% TFA in CH2Cl2) to release the protected peptide from HMPB-MBHA (3) resin. Peptide couplings were performed using HATU/HOAt/i-Pr2NEt with isolated overall yields in excess of 80%.
Head to tail cyclization of tyrosine-containing peptides on the solid-phase using Fmoc chemistry has been used for the preparation of cyclic peptides.6 The phenolic group of the tyrosine side chain was immobilized onto the solid support via the Mitsunobu reaction. Peptides corresponding to centrally truncated analogs of neuropeptide Y were formed. Cyclization was achieved prior to TFA cleavage from the resin. A further example in this category was the synthesis of cyclic thioether peptides12 on HMPB-MBHA (3) resin.
The HMPB linker 1 has been successfully utilized in the synthesis of C-terminally modified peptides by use of a dual linker system.13 The linker was coupled to the amino side chain of a resin-bound lysine residue and a peptide built up through the hydroxymethyl group of the aromatic chain of the linker via conventional Fmoc peptide synthesis.
Coupling of the free amino of the peptide to form 4 followed by concomitant palladium (0) mediated deprotection of the two allyl moieties afforded an intermediate that was subsequently cyclized to 5, containing a dual linker system (
A multiple release protocol was developed (
The HMPB linker has been attached to generation [3.0] polyamidoamino (PAMAM) dendrimers for the solid-phase synthesis of aryl ethers.3 Linker 1 has also been used to evaluate the synthetic potential of new resins. It has been used as a linker both on magnetite impregnated beads15 and on novel poly(styrene-oxyethylene) grafted copolymer resins.16
2-Chlorotrityl chloride resin;17
University of Southampton, Southampton, UK