HOBt and HOAt-derived immonium salts: New and highly efficient coupling reagents for peptide synthesis

Article abstract of DOI:10.1016/S0040-4039(99)02125-5

Novel HOBt- and HOAt-derived immonium type coupling reagents BDMP, BPMP and AOMP were shown to be very efficient for peptide synthesis with high reactivity, low racemization and good yields. X-Ray analysis indicates BOMI exists as the N-substituted form r

Full text of DOI:10.1016/S0040-4039(99)02125-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 721–724
HOBt and HOAt-derived immonium salts: new and highly
efficient coupling reagents for peptide synthesis
Peng Li and Jie-Cheng Xu ∗
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
Received 29 September 1999; accepted 10 November 1999
Abstract
Novel HOBt- and HOAt-derived immonium type coupling reagents BDMP, BPMP and AOMP were shown to be
very efficient for peptide synthesis with high reactivity, low racemization and good yields. X-Ray analysis indicates
BOMI exists as the N-substituted form rather than as the O-substituted form assigned previously. A mechanism
for amide bond formation mediated by these reagents was proposed and studied. © 2000 Elsevier Science Ltd. All
rights reserved.
Subsequent to the first HOBt-derived phosphonium coupling reagent BOP, designed by Castro et
al.,1 many HOBt-based uronium and phosphonium type coupling reagents, such as PyBOP,2 HBTU,3
HBPyU,4 HBPipU5 and HBMDU,6 have been developed and employed in peptide synthesis both
in solution and solid phase. The predominance of carbodiimide and active ester techniques have
been gradually replaced with onium salts based upon 1-hydroxybenzotriazole (HOBt),7 but avoiding
racemization is still a challenge even with these reagents.4a,8 To suppress the level of epimerization
and enhance the coupling efficiency, the HOAt-derived phosphonium and uronium reagents, such as
HATU, HAPyU, AOP, PyAOP9 were suggested and found to perform well because of the anchimeric
assistance of HOAt.10 Recently, we have synthesized a series of HOBt- and HOAt-derived immonium
type coupling reagents, which were more reactive than the uronium reagents because the electron density
of their central carbon is lower than that of the corresponding uronium salts. In previous studies,11 we
reported the synthesis of the HOBt-based immonium salt BOMI by replacing one of the dimethylamino
groups of HBTU with hydrogen, and demonstrated its high efficiency in peptide synthesis. We report here
the further design and synthesis of three immonium type coupling reagents BDMP, BPMP and AOMP
based upon the modification of the structure of the uronium reagents HBMDU, HBPyU and HAMDU
(Fig. 1).
These reagents can be readily prepared from condensation of the corresponding amides with
bis(trichloromethyl) carbonate to yield the immonium chlorides, which were stabilized with SbCl5 and
subsequently reacted with the potassium salt of 1-hydroxybenzotriazole or 1-hydroxy-7-azabenzotriazole
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02125-5
tetl 16053

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Fig. 1. Structures of HOBt- and HOAt-based immonium type coupling reagents
in the presence of a tertiary amine, such as NEt3 to afford the desired compounds as crystalline and shelf-
stable solids (Scheme 1).
Scheme 1. Synthesis of HOBt- and HOAt-derived immonium type coupling reagents
Recently, X-ray analysis indicated that BOMI is the N-substituted form I12 shown in Fig. 1, rather than
the isomeric O-substituted form II. Unfortunately, the X-ray data for the analogous BDMP, BPMP and
AOMP have not yet been obtained, therefore the structural representations are arbitrarily assigned as the
O-substituted forms in this article.
The efficiency of these immonium reagents in peptide synthesis was evaluated by HPLC monitoring of
the racemization prone 2+1 peptide segment condensation during the formation of the model compound
Z-Gly-Phe-Val-OCH3 which was first adopted by Van der Auwera et al.13 It was observed that the
immonium reagents exhibited extremely high reactivity, very low racemization and good yields for
couplings. From the results shown in Table 1, it is obvious that, both in terms of reactivity and
racemization, the HOBt-based immonium salts are superior to the uronium salt HBPyU which is one
of the most efficient among the HOBt-based phosphonium and uronium salts,4a,14 even better than that
of the HOAt-derived reagent HAPyU, which is widely regarded as the most effective reagent so far
developed.15 These data also indicate that BDMP is the most efficient reagent among these immonium
salts with respect to the reaction speed, epimerization and coupling yield. However, the reagent AOMP
is shown to be less efficient, the most likely reason being that the extremely high reactivity of AOMP
causes its decomposition before achieving the activation of the N-protected amino acid in the presence
of base, such as 2,6-lutidine.
Because the amino component of the adopted model reaction is a primary amine without obvious
hindrance (R2=H, Fig. 2), the reaction rate is probably determined by the activation step, the formation
of the HOBt or HOAt active ester. To confirm this consideration, we synthesized the active ester of the
model reaction Z-G-F-OBt, Z-G-F-OAt and subjected these active esters to the coupling reaction (Z-
G-F-OBt/Z-G-F-OAt+Val-OCH3·HCl+2,6-lutidine→Z-G-F-V-OCH3). We found they coupled with the
amine quickly with a yield of above 90% after 10 min under the same reaction conditions as used in
Table 1. This indicated that the relatively low reactivity of HBPyU and HAPyU is mainly due to the

723
Table 1
Comparison of reactivity and racemization of immonium type reagents with uronium salts using
HPLCa,b and Young’s testc
lowered reactivity of the uronium/aminium carbon skeleton towards the attack of the carboxylate anion.
The two substituted amino groups of the uronium/aminium salts provide two equal resonance structures
to stabilize the molecule, and the lone electron pairs of the two nitrogen atoms are delocalized within the
bonds N–C–N, which causes the carbocation to share a higher electron density than the immonium salts;
consequently, the nucleophilic attack of the carboxylate anion was impeded. It is assumed that the higher
reactivity of BDMP over other immonium salts is probably due to the intra-annular imide bond. The
electronic effect also makes BDMP more reactive than its analogues due to the relatively low electron
density of the central carbon of BDMP.
Fig. 2. The two steps of coupling reactions mediated by HOBt- and HOAt-based reagents
When the C-protected amino acid ester is an N-alkyl or an α,α-dialkyl amino acid ester, such as N-
Me-Val-OCH3·HCl and Aib-OCH3·HCl, the coupling step becomes the rate-determining step in peptide
bond formation (ka>kc). In this case, HOBt-based coupling reagents including these novel immonium
salts prove to be inefficient, the major isolated products being the HOBt active esters when the amino
component is an N-alkylated amino acid ester.3b,11b However, the HOAt-based aminium salt HAPyU
displays very high efficiency for the construction of hindered amide bonds.15b
To verify the performance of these novel immonium salts, we synthesized a series of oligopeptides and
bioactive peptides both in solution and solid phase with good yields and low racemization.11b We also
synthesized the immunosuppressive cyclic undecapeptide Cyclosporin O using these reagents combined
with other reagents.16 These reagents can also be used for ester formation with satisfactory yields,
especially for the preparation of active esters such as benzotriazolyl, pentafluorophenyl and succinimidyl
esters which are useful for the synthesis of lactones and lactams.
In conclusion, several novel HOBt- and HOAt-derived immonium coupling reagents BOMI,11 BDMP,
BPMP and AOMP were designed and synthesized. Their utility for peptide synthesis was demonstrated
by rapid reaction speed, low racemization and good yields. In the case of the coupling of normal coded

724
amino acids and peptide segments, HOBt-derived immonium salts, BOMI, BDMP and BPMP17 seem to
be more efficient than HOBt- or HOAt-derived uronium/aminium and phosphonium salts.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (9772045).
References
1. Castro, B.; Dromoy, J. R.; Evin, G.; Selve, C. Tetrahedron Lett. 1975, 1219–1222.
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12. Crystal data for C9H11N4OCl6Sb BOMI: M=525.68, triclinic, a=9.058(2), b=15.729(4), c=6.598(1) Å, α=94.21(2),
β=98.99(2), γ=73.38(2)°, V=889.4(4) Å3, T=293 K, space group P1(#2), Z=2, µ=24.50 cm−1, Dc=1.963 g cm−3, 2943
reflections measured, 2727 were unique (Rint=0.076). The final R (Rw) values were 0.045 (0.057) for 2247 reflections
[I>3.00σ(I)) and 190 variables.
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14. Carpino, L. A.; El-Faham, A.; Minor, C. A.; Albericio, F. J. Chem. Soc., Chem. Commun. 1994, 201–203.
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16. Li, P.; Xu, J. C., unpublished work.
17. AOMP, 5-(1H-7-azabenzotriazol-1-yloxy)-3,4-dihydro-1-methyl 2H-pyrrolium hexachloroantimonate; BOMI, N-(1H-
benzotriazol-1-ylmethylene)-N-methylmethanaminium hexachloroantimonate N-oxide; BDMP, 5-(1H-benzotriazol-1-
yloxy)-3,4-dihydro-1-methyl 2H-pyrrolium hexachloroantimonate; BOP, (1H-benzotriazol-1-yloxy)tris(dimethylamino)-
phosphonium hexafluorophosphate; BPMP, 1-(1H-benzotriazol-1-yloxy)phenylmethylene pyrrolidinium hexachloroantim-
onate; HAPyU, 1-(1-pyrrolidinyl-1H-1,2,3-triazolo[4,5-b]pyridin-1-ylmethylene)pyrrolidinium hexafluorophosphate N-
oxide; HBMDU, (1H-benzotriazol-1-yloxy)-1,3-dimethyl-1,3-dimethyleneuronium hexafluorophosphate; HBPipU, (1H-
benzotriazol-1-yloxy)-N,N,N0,N0-bis(pentamethylene)uronium hexafluorophosphate; HBPyU, O-(1H-benzotriazol-1-yl)-
N,N,N0,N0-bis(tetramethylene)uronium hexafluorophosphate; HBTU, N-[(1H-benzotriazol-1-yl)-(dimethylamino)methyl-
ene]-N-methylmethanaminium hexafluorophosphate N-oxide; HOAt, 1-hydroxy-7-azabenzotriazole; HOBt, 1-
hydroxybenzotriazole.