conformational assistance, or both to enable an internal
hydroxyl or thiol nucleophile to undergo an intramolecular
NꢀO or NꢀS acyl shift reaction to form an ester or
thioester from an amide.9ꢀ14
Scheme 1. Preparation of starting Materials for Synthesis of
Peptide Thioesters
The NꢀO or NꢀS acyl shift is known to occur in the
early stage of protein splicing to afford an ester or
thioester.1b The acyl shift is proximity driven and confor-
mationally assisted with the scissile peptide bond in a cis
conformation. Among the naturally occurring peptide
bonds, only the tertiary Xaa-Pro bonds can favorably exist
in a cis conformation without enzymatic assistance. To
mimic the NꢀS acyl shift in protein splicing, many sought
to develop tertiary amide or proline-like surrogates with a
thiol handle to enable a ciscoid NꢀS acyl shift of a tertiary
amide bond to form a thioester. Tertiary amide surrogates
include structures such as N-alkylated Cys and its
mimetics,9 thiol-substituted proline,10 oxazolidinone,11
anilide,12 or modified benzyl13 groups. However, the re-
ported conformationally assisted NꢀS acyl methods are
often complicated by synthetic complexity in their pre-
paration, by side reactions at either the activation or the
conversion step, and in some cases, by the need of an
additional step for their removal.
Based on our previous ligation chemistry using thiazo-
lidoine and oxazolidoine as a proline surrogate,18 we
envisioned that a thiazolidine with a 2-thiomethyl group
could undergo a proximity-driven NꢀS acyl shift reaction
in a cis conformation, mimicking the NꢀS acyl shift of
protein splicing. Herein, we describe the use of 2-thio-
methylthiazolidine as an Fmoc-compatible proline surro-
gate to enable a tandem acid-catalyzed thiol switch to
afford a thioester, first through an NꢀS acyl-transfer
reaction and then a SꢀS (thiol thioester) exchange. To
obtain the desired 2-thiomethylthiazolidine, our scheme
began with2-mercaptoethanol 1, which was protected with
a trityl group upon treatment withtriphenylmethyl alcohol
in the presence of trifluoroacetic acid (TFA) in CHCl3
(Scheme 1a)16 to afford tritylthioethyl alcohol 2 in 85%
yield within 2 h. The purified alcohol 2 was then subjected to
oxidation by reacting it with pyridinium chlorochromate2
(PCC) under anhydrous conditions for 6 h to obtain the
corresponding tritylthioethyl aldehyde 3 in 70% yield.
The desired thiazolidine can be synthesized from alde-
hyde 3 by allowing it to undergo intermolecular cyclization
with a moiety containing a 1,2-aminothiol functionality as
previously reported by our group.18 The unprotected
cysteine anchored on an Fmoc-compatible resin support
would provide such a functionality. Keeping this in mind,
the synthesis of unprotected cysteine-anchored resins was
carried out, first by coupling Fmoc-Cys(S-t-Bu)-OH 4 to
the Wang or Rink-amide resin 5 using benzotriazole-1-yl-
oxytris(dimethylamino)phosphonium hexafluorophosphate
(BOP) and N,N-diisopropylethylamine (DIEA), followed
by the removal of Fmoc group (Scheme 1b). Treatment
with 2-mercaptoethanol (10%, v/v) in DMF for 12 h re-
sulted in removal of tert-butylsulfenyl protecting group,
thereby rendering the unprotected cysteine 6 with desired
1,2-aminothiolfunctionality. The presenceof freethiol was
confirmed by treating 6 with Ellman’s reagent [5,50-dithiobis-
(2-nitrobenzoic acid)], leading to the appearance of a red
solution.
To synthesize the thiazolidine ring with a highly acid-
sensitive trityl moiety both on the thiol side chain and resin
supports (e.g., Rink resin), the tritylthioethyl aldehyde 3
was reacted with resin-coupled unprotected cysteine 6
under neutral conditions for 24 h in DCM (Scheme 2).
Treatment of resin beads with Ellman’s reagent did not
produce red color, confirming the absence of free thiol
group. To further characterize the cyclized product, the
reaction was reproduced in the solution phase, confirming
the quantitativeformation of thiazolidine ring(Supporting
Information Figure 3). The intermolecular cyclization was
efficient and proceeded via an imine capture between the
free amino group of 6 and the aldehyde carbonyl of 3,
followed by ringꢀchain tautomerization by the free thiol
group leading to the formation of tritylthiomethylthiazo-
lidineꢀWang (TMTꢀW) or ꢀRink amide (TMTꢀR)
resin 7. Collectively termed as TMT resins, their main
advantage is that the suitably placed trityl protected thiol
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