was treated with a mixture of 2 N NaOH and DMF (5:95)
at room temperature for 1 h to hydrolyze a small amount of
O-acylated phenol. Washing the resin with acetic acid/DMF,
MeOH, and CH2Cl2 provided polystyrene-bound aldehyde
(3) in quantitative yield based on the amount of amino
function on the aminomethylated polystyrene.5
Compounds synthesized using the AHB linker are listed
in Table 1. Scheme 1 shows a typical synthetic route to obtain
Table 1. Synthesis of C-Terminal Amidated Compounds Using
AHB Linkera.
entry
product
method
yield (%)
1
2
3
4
5
Bz-Gly-NHEt (4a )
Bz-Gly-NHEt (4a )
Bz-Val-NHEt (4b)
Bz-Gly-Leu-OH (4c)
Bz-Gly-NHNH2 (5)
A
B
A
A
C
97
74
85
78
64
Figure 1. Structure of the AHB linker-bound resin (1) and
illustration of switching of the acid stability of the linker.
a
Yields were calculated from HPLC integration using a standard curve
relative to the loading level of the aldehyde linker.
used in peptide chemistry: Boc and Fmoc. Thus the handle
is stable to piperidine treatments and after proper modifica-
tion, the acylated form is stable to TFA treatment. In addition,
the coupling reaction of an N-protected amino acid with the
resin-bound secondary amine moiety could be accelerated
by an OfN acyl migration mechanism similar to that
observed for the Hmb protecting group.2 In this communica-
tion, we report on the synthesis of the polystyrene-based
AHB linker (3) and synthesis of several small molecules to
demonstrate the utility of the new linker.
A representative AHB linker (3) was conveniently pre-
pared from an aminomethylated polystyrene resin3 suitable
for solid-phase synthesis and 4-(4-formyl-3-hydroxyphe-
noxy)butyric acid.4 Thus, 4 equiv of the acid was coupled
with the amino function on the resin using DIC in the
presence of an excess amount of HOBt to minimize undesired
O-acylation to the phenolic hydroxyl group. The crude resin
C-terminal modified amino acid derivatives and peptides
using the AHB linker. In method A, reductive amination of
resin 3 using ethylamine or leucine tert-butyl ester (10 equiv)
and NaBH(OAc)3 in DMF/TMOF (2:1) gave the correspond-
ing secondary amine 6.6 A test of the modified resin with
phenylhydrazine detected no residual aldehyde. Acylation
of the amino group with an N-benzoylated amino acid (Gly
or Val, 4 equiv) via the HBTU/HOBt/DIEA method, fol-
lowed by treatment with 20% piperidine/DMF to remove
the O-acyl group afforded the carboxamide resins 7a-c. The
removal of the O-acyl function with piperidine was com-
pleted within 30 min in the case of 7a. Cleavage of resins
7a-c with 95% TFA in H2O produced the corresponding
N-benzoylated amino acids 4a, 4b, or dipeptide 4c in high
yield (Table 1, entries 1, 3, and 4). Fairly good yields of
dipeptide 4c suggested utility of the AHB linker for the
backbone amide linker (BAL) strategy,7 in which the growing
peptide is anchored through a backbone nitrogen, with more
flexibility. Thus, tert-butyl-type protecting groups (N-Boc
and tert-butyl ester) can be used for C-terminal carboxyl and/
or side chain protection combined with other orthogonal
protecting groups such as allyl-type ones (N-Aloc and allyl
ester) in the BAL strategy. Method B employed an acid
treatment during the synthetic process to demonstrate the
versatility of the AHB linker and gave 4a in satisfactory yield
(Table 1, entry 2). Since dimeric peptides connected through
their C-terminus via hydrazine such as Biphalin8 have been
of great interest to us, synthesis of hydrazide 5 was examined
(method C). Boc-protected benzylhydrazine 10 was generated
(2) Johnston, T.; Quibell, M.; Owen, D.; Sheppard, R. C. J. Chem. Soc.,
Chem. Commun. 1993, 369.
(3) Resins used were purchased from Calbiochem-Novabiochem Corp.,
CA.
(4) Preparation of 4-(4-formyl-2-hydroxyphenoxy)butyric acid was ac-
complished as follows: A mixture of ethyl 4-bromobutyrate (23.4 g, 0.12
mol), 2,4-dihydroxybenzaladehyde (16.6 g, 0.12 mol), potassium carbonate
(16.6 g, 0.12 mol), and potassium iodide (2 g, 12 mmol) in DMF (60 mL)
was stirred at 70 °C for 9 h and then at room-temperature overnight. Cold
water (700 mL) and 6 N HCl (50 mL) were added to the reaction mixture.
The resulting precipitate was filtered off and dried in vacuo. Column
chromatography on silica gel, eluted with hexane/ethyl acetate (9:1), afforded
17.6 g (58%) of ethyl 4-(4-formyl-2-hydroxyphenoxy)butyrate as a white
powder: mp 52-53 °C. The ethyl ester thus obtained (17.2 g, 68 mmol)
was dissolved in MeOH (100 mL) and 2 N NaOH (100 mL) was added to
this solution. The mixture was stirred at room temperature for 1 h. The
solution was diluted with water (350 mL) and acidified with 6 N HCl (40
mL). The precipitated white powder was filtered, washed with water, and
1
dried in vacuo over P2O5. Yield, 15.2 g (100%), mp 145-6 °C. H NMR
(200 MHz, DMSO-d6) δ 12.17 (s, 1H), 10.98 (s, 1H), 9.99 (s, 1H), 7.60 (d,
J ) 8.7 Hz, 1H), 6.56 (dd, J ) 8.7, 2.3 Hz, 1H), 6.46 (d, J ) 2.3 Hz, 1H),
4.03 (t, J ) 6.4 Hz, 2H), 2.37 (t, J ) 7.2 Hz, 2H), 1.93 (m, 2H). ESI-MS
m/z for C11H12O5 calcd 225 (M + H+), obsd 225. Anal. Calcd for
C11H12O5: C, 58.93; H, 5.39. Found: C, 58.70; H, 5.47.
(5) The loading level of the aldehyde linker onto the aminomethylated
resin was determined from the quantitative HPLC analysis of Bz-Gly-NHEt
obtained by method A in Scheme 1 based on the substitution level of the
aminomethylated polystyrene used.
(6) In an attempt to prepare 6, several reaction conditions for the reductive
amination were examined. NaBH(OAc)3 afforded better results as a reducing
agent rather than NaBH3CN. As a solvent system, both DMF/TMOF and
CH2Cl2/TMOF generally gave good results.
(7) Jensen, K. J.; Alsina, J.; Songster, M. F.; Va´gner, J.; Albericio, F.;
Barany, G. J. Am. Chem. Soc. 1998, 120, 5441.
(8) Misicka, A.; Lipkowski, A. W.; Horvath, R.; Davis, P.; Porreca, F.;
Yamamura, H. I.; Hruby, V. J. Life Sci. 1997, 60, 1263.
1788
Org. Lett., Vol. 2, No. 13, 2000