Organic Letters
Letter
Plecanatide is primarily obtained by natural extraction
technology, which is severely affected by the limited existing
resources and is accompanied by a complicated extraction
procedure. Also, it is hard to pinpoint the process of natural
disulfide bond cyclization, and it is challenging to obtain high-
purity and large-scale target production. To date, the
automatable chemical synthetic strategy, solid-phase peptide
synthesis (SPPS),11 has been developed toward the synthesis
of plecanatide12 and relevant analogs. Still, inevitably, the large-
scale production of plecanatide by SPPS technology is severely
restricted by the expensive peptide synthetic resins, which are
also considered as refractory pollutants after use. Exclusive
resins with a lower loading value (0.3 to 0.8 mmol/g) also
adversely impact the massive peptide production. Moreover,
excessive solvents used for swelling and washing the resins give
rise to an incremental process mass intensity (PMI, the ratio of
the entire mass of the raw material over the mass of the
purified product).13 Hydrophobic peptide syntheses also pose
additional challenges due to the gradual aggregation of
prolonged peptide chains.14 In conventional liquid-phase
peptide synthesis (LPPS),15 it is likewise challenging to
integrate the hydrophilic peptide, which mainly contributes
to the worse solubility and lengthy reaction procedures.
The boundedness of SPPS and LPPS has challenged the
atom-economical and scaled-up synthesis of plecanatide and
related bioactive peptides, imperatively necessitating the
exploration of simplified greener synthetic protocols for
plecanatide and long-chain bioactive peptides.
Then, the resin-free total synthesis of the target plecanatide
was performed.
A high-yield and convenient two-step synthetic scheme for
DDK derivatives was implemented and began from the
homologous diphenylketone derivates 1a−c (Scheme 1).
a
Scheme 1. Synthesis of DDK Derivatives as Supports
a
Reaction conditions: i: Ph2POCl, Et3N in THF, 0 °C to rt, 1 h; ii:
To realize peptide synthesis in the homogeneous phase,
soluble polyethylene-glycol (PEG)-modified supports were
initially applied to the liquid-phase peptide synthesis.16
However, the loading value of PEG supports (0.1 to 0.5
mmol/g) was imperfect because each PEG support contained
only one coupling site. The soluble polymer-based anchors
were then introduced, such as the poly(norbornene)-derived
anchor providing multiattachment sites with a loading value of
0.6 to 1.1 mmol/g17 and the soluble globular multisite polymer
anchor for the ingenious membrane-enhanced peptide syn-
thesis.18 In contrast, these soluble polymer-based supports still
set limits on the large-scale, long-chain peptide synthesis.
Hence, soluble small-molecule-based supports were developed
to achieve an equivalent conversion synthesis, such as the
soluble OH−Bzl[Bzl(OC18)3]3 anchor,19 the hydrophobic
benzyl anchors,20 and the diphenylmethyl-derived anchors.21
Nevertheless, the preparation of anchors themselves required
complicated multistep schemes, which were extremely adverse
to the massive long-chain peptide synthesis. Because of their
nonrecyclable nature, it is hard to achieve a green and atom-
economical peptide synthesis.
Typical organophosphorus reagents have exerted a signifi-
cant impact on the peptide synthesis field, such as in the self-
activation of the phosphoryl transfer methodology to obtain
oligopeptides,22 and as reagents for efficient amide coupling.23
Also, on the basis of the unique precipitation-prone properties
of diphenyl phosphinate24,25 and our previous work on TBP
supports mediating resin-free peptide synthesis,26 we con-
tinued to exploit diphenyl phosphinate for long-chain peptide
synthesis, which was the design of high-yielding diphenylphos-
phonyloxyl diphenyl ketone (DDK) derivatives as the C-
terminal protecting groups of amino acids. We then utilized the
optimal DDK derivative with excellent performance to assist in
the peptide synthesis without the use of chromatography.
2a−c, NaBH4 in MeOH, NH4Cl quenching, 0 °C, 1 h; iii: 2a−c,
NH2OH·HCl in EtOH/pyridine (v/v 10:1), reflux, 4 h; iv: 3a−c,
SOCl2 in DCM, 0 °C to rt, 4 h.
First, 1a−c were processed with Ph2POCl to obtain the
DDK-derived intermediates 2a−c in 95−99% yield. Second,
2a−c were reduced by NaBH4 to attain the DDK−OH
supports 3a−c in 90−97% yield. Then, 3a−c continued to
react with SOCl2 to derive DDK−Cl supports 5a−c in 97 to
98% yield. Finally, 2a−c were reacted with NH2OH·HCl,
affording the DDKN−OH support 4a−c in 88−95% yield.
The DDK supports, with high solubility in media such as
chloroform and ethyl acetate, could thus be readily precipitated
in the mixed EA/PE system.
Initial studies were devoted to the coupling between Fmoc/
Boc-AA-OH and DDK supports by using the EDC·HCl/
DMAP reagent system to verify the loading capacity of DDK
supports. de-Fmoc and de-Boc processes were then carried out
to confirm the stability of Fmoc/Boc-AA−DDK by using 25%
DEA/MeCN and 25% TFA/DCM, respectively. Finally,
DDK−OH-attached Fmoc/Boc-AA-OH products were
shown to be merely stable in the 25% DEA/MeCN system,
and we obtained the de-Fmoc products H-AA-O−DDK.
DDKN−OH-attached Fmoc/Boc-AA-OH products demon-
strated stability only in the 25% TFA/DCM system, and we
acquired the de-Boc products H−AA−O−NDDK. The
unstable de-Fmoc/Boc products were proved to undergo
incomplete ester hydrolysis. All of the coupling and de-Fmoc/
Boc products were purified in good yield by rapid precipitation
in the EA/PE system with the aid of DDK supports (Table 1).
Next, a typical TFA/thioanisole/phenol/EDT/H2O (v/v
87.5/5/2.5/2.5/2.5) shearing reagent system was utilized to
detach amino acids from the DDK−OH supports to give the
bare amino acids as well as the DDK residues 4-diphenyl
B
Org. Lett. XXXX, XXX, XXX−XXX