The Journal of Organic Chemistry
Article
α/γ-dipeptide acid 6 and α/γ-dipeptide amine HCl salt 7, both in
>95% yield. α/γ-Dipeptide acid 6 (1.0 mmol, 1.0 equiv) was dissolved
in DMF (2.5 mL). i-Pr2EtN (1.045 mL, 6.0 mmol, 6.0 equiv) was
added followed by HOBt (162 mg, 1.2 mmol, 1.2 equiv) and EDCI
(230 mg, 1.2 mmol, 1.2 equiv). The reaction mixture was stirred at
room temperature for 15 min, at which point a solution of α/γ-
dipeptide amine HCl salt 7 (1.0 mmol, 1.0 equiv) in DMF was added
(2.5 mL). The reaction flask was sealed with a septum, and the
reaction mixture was stirred at room temperature for 24 h. The
reaction mixture was diluted 10-fold with EtOAc and washed (25 mL
each wash) twice each with 1 M aqueous NaHSO4, then saturated (aq)
NaHCO3, then brine. Each aqueous layer was extracted twice with 25
mL of CH2Cl2. All organic layers were combined, dried over MgSO4,
filtered, and evaporated to afford a crude solid. The product was
purified via column chromatography eluting with a gradient of 3:1
hexanes/EtOAc to 1:3 hexanes/EtOAc. The product eluted when the
column was flushed with neat EtOAc. Rf (2.5% (v/v) MeOH in
CH2Cl2) = 0.20. Tetramer 8 was isolated as a white solid (530 mg,
88% yield). 1H NMR (300 MHz, CDCl3): δ 7.28 (m, 5H), 7.24
(broad, 1H), 7.17 (broad, 1H), 679 (broad, 1H), 5.13 (J = 7.2 Hz,
1H), 5.07 (ABq, JAB = 13.7 Hz, νAB = 9.9 Hz, 2H), 4.29 (quintet, J =
7.5 Hz, 1H), 4.06 (quintet, J = 6.9 Hz, 1H), 3.28 (broad, 3H), 3.11
(broad, 1H), 2.48 (q, J = 7.8 Hz, 1H), 2.30 (sextet, J = 8.1 Hz, 1H),
2.19 (broad, 2H), 1.96−1.52 (broad, 11H), 1.36 (s, 9H), 1.31 (m,
2H), 1.27 (d, J = 7.2 Hz, 3H), 1.23 (d, J = 7.5 Hz, 3H). 13C NMR
(75.4 MHz, CDCl3): δ 177.0, 176.3, 174.1, 173.9, 156.0, 136.2, 128.8,
128.4, 128.2, 80.3, 66.7, 50.7, 49.6, 49.0, 48. 6, 44.7, 44.1, 43.4, 43.0,
31.1, 30.8, 30.5, 28.5, 25.8, 25.0, 18.3, 18.1. Calcd [M + H+] for
C32H48N4O7H+, 601.3596; found, 601.3591.
of α/γ-tetrapeptide amine HCl salt 11 in DMF (2.5 mL) was added.
The reaction flask was sealed with a septum, and the reaction mixture
was stirred at room temperature for 72 h. The reaction mixture was
diluted 10-fold with EtOAc and washed twice each with 1 M aqueous
NaHSO4, then saturated (aq) NaHCO3, then brine. Each aqueous
layer was extracted twice with 25 mL of CH2Cl2. All organic layers
were combined, dried over MgSO4, filtered, and evaporated to afford a
crude solid. The product was purified via column chromatography,
eluting with a gradient of 1 (v/v) to 5% MeOH in CH2Cl2. Rf (5%
MeOH) = 0.14. Octamer 12 was isolated as a white solid (190 mg,
58% yield). 1H NMR (300 MHz, CDCl3): δ 8.74 (d, J = 6.3 Hz, 1H),
8.37 (d, J = 6.3 Hz, 1H), 7.96 (d, J = 4.8 Hz, 1H), 7.60 (broad, 2H),
7.45 (d, J = 6.0 Hz, 1H), 7.36 (m, 5H), 6.60 (broad, 1H), 5.13 (ABq,
JAB = 11.0 Hz, νAB = 9.1 Hz, 2H), 5.10 (d, J = 9.9 Hz, 1H), 4.24 (m,
1H), 4.27 (broad, 3H), 3.54 (broad, 4 H), 3.13 (broad, 4H), 2.54 (q, J
= 8.4 Hz, 1H), 2.32 (broad, 7H), 1.87 (broad, 15 H), 1.64 (broad,
13H), 1.43 (s, 9H), 1.33 (broad, 10H), 1.26 (d, J = 7.8 Hz, 3H), 1.21
(d, J = 7.5 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ 178.4, 178.2,
178.1, 176.2, 174.9, 174.7, 174.6, 173.8, 171.2, 155.8, 135.8, 128.7,
128.3, 128.1, 80.1, 66.7, 60.4, 50.4, 50.4, 49.5, 49.3, 48.8, 44.5, 44.4,
44.2, 44.1, 44.0, 43.7, 43.0, 31.7, 31.6, 31.6, 31.5, 31.5, 31.4, 31.2, 30.7,
30.2, 29.7, 28.3, 26.8, 26.7, 26.3, 24.8, 22.7, 21.1, 18.1, 17.2, 17.1, 16.6,
14.2, 14.1. Calcd [M + H+] for C52H80N8O11H+, 993.6020; found,
993.6027.
Compound (S1): Boc-HN-(DPhe-AMCP)-OBn. See the Supporting
Information for the synthesis scheme. Benzyl ester 4 (Boc-HN-
AMCP-OBn; 1.09 g, 3.3 mmol, 1.0 equiv) was Boc-deprotected
according to general procedure B in >95% yield to give H2N-AMCP-
OBn·HCl, which was carried forward without further purification. Boc-
HN-DPhe-OH (1.041 g, 3.9 mmol, 1.2 equiv) was dissolved in 10 mL
of distilled CH2Cl2. i-Pr2EtN (3.5 mL, 19.6 mmol, 6.0 equiv) was
added followed by HOBt (530 mg, 3.9 mmol, 1.2 equiv) and EDCI
(751 mg, 3.9 mmol, 1.2 equiv). The solution was stirred for 10 min.
H2N-AMCP-OBn·HCl was then added, and the reaction mixture was
stirred for 24 h at room temperature. The reaction mixture was diluted
4-fold with EtOAc and washed twice each (25 mL each wash) with 1
M aqueous NaHSO4, then saturated (aq) NaHCO3, then brine. Each
aqueous layer was extracted twice with 25 mL of CH2Cl2. All organic
layers were combined, dried over MgSO4, filtered, and evaporated to
afford a crude solid. The product was purified via column
chromatography, eluting with a gradient from 1:5 (v/v) to 1:1
EtOAc/hexanes to give dipeptide S1 as an amorphous solid (1.027 g
isolated, 65% yield). Rf (20:1 CH2Cl2/MeOH) = 0.49. 1H NMR (300
MHz, CDCl3): δ 7.32 (broad m, 10H), 6.13 (broad, 1H), 5.09 (ABq,
JAB = 12.0 Hz, νAB = 16.8 Hz, 2H), 4.96, (broad, 1H), 4.27 (q, J = 6.9
Hz, 1H), 3.11 (superimposed ABX patterns, 4H), 2.38 (q, J = 8.4 Hz,
1H), 2.19 (sextet, J = 7.8 Hz, 1H), 1.85 (m, 3H), 1.62 (broad, 2H),
1.40 (s, 9H), 1.22 (m 1H). 13C NMR (75.4 MHz, CDCl3): δ 176.0,
171.4, 137.1, 136.2, 129.5, 128.8, 128.5, 128.3, 127.1, 66.6, 56.2, 48.4,
43.9, 43.1, 38.9, 30.7, 30.3, 28.5, 24.9, 19.0. Calcd [M + Na+] for
C28H36N2O5Na+, 503.2517; found, 503.2528.
Boc-HN-(DAla-AMCP)3-OBn (9). A 0.33 mmol portion of α/γ-
peptide dimer 5 was deprotected according to general procedure A (to
yield 6), and a 0.33 mmol portion of α/γ-peptide tetramer 8 was
deprotected according general procedure B. α/γ-Dipeptide acid 6
(0.33 mmol, 1.0 equiv) was dissolved in 2.5 mL of DMF. i-Pr2EtN
(348 μL, 2 mmol, 6.0 equiv) was added followed by HOBt (54 mg, 0.4
mmol, 1.2 equiv) and EDCI (77 mg, 0.4 mmol, 1.2 equiv). The
reaction mixture was stirred at room temperature for 15 min, at which
point a solution of N-deprotected 8 in 2.5 mL of DMF was added. The
reaction flask was sealed with a septum, and the reaction mixture was
stirred at room temperature for 48 h. The reaction mixture was diluted
10-fold with EtOAc and washed (25 mL each wash) twice each with 1
M aqueous NaHSO4, then saturated (aq) NaHCO3, then brine. Each
aqueous layer was extracted twice with 25 mL of CH2Cl2. All organic
layers were combined, dried over MgSO4, filtered, and evaporated to
afford a crude solid. The product was purified via column
chromatography eluting with a gradient of 1:1 hexanes/EtOAc to
neat EtOAc. Rf (neat EtOAc) = 0.32. Hexamer 9 was isolated as a
white solid (200 mg, 76% yield). 1H NMR (300 MHz, CDCl3): δ 8.09
(d, J = 6.9 Hz, 1H), 7.79 (d, J = 7.2 Hz, 1H), 7.54 (d, J = 6.6 Hz, 1H),
7.34 (broad, 1H), 7.29 (m, 5H), 6.56 (broad, 1H), 5.07 (ABq, JAB
=
13.1 Hz, νAB = 7.7 Hz, 2H), 5.02 (d, J = 3 Hz, 1H), 4.26 (quintet, J =
7.5 Hz, 1H), 4.21 (quintet, J = 7.5 Hz, 1H), 4.13 (quintet, J = 5.0 Hz,
1H), 3.55−3.29 (broad, 3H), 3.22−2.98 (broad, 3H), 2.48 (q, J = 8.1
Hz), 2.22 (broad, 5H), 1.80 (broad, 11H), 1.59 (broad, 7H), 1.36 (s,
9H), 1.32 (m, 2H), 1.28 (d, J = 9.9 Hz, 3H), 1.25 (d, J = 9.9 Hz, 3H),
1.16 (d, J = 9.0 Hz, 3H). 13C NMR (125 MHz, CDCl3): δ 178.0,
177.9, 176.2, 174.5, 174.5, 173.8, 155.8, 135.8, 128.7, 128.3, 128.1,
80.1, 66.7, 50.4, 50.3, 49.6, 49.2, 49.1, 48.7, 44.4, 44.3, 44.1, 44.0, 43.0,
43.03, 31.6, 31.4, 31.2, 31.1, 30.7, 30.3, 28.3, 26.6, 26.1, 24.8, 18.1,
17.3, 16.9. Calcd [M + Na+] for C42H64N6O9Na+, 819.4627; found,
819.4606.
Boc-HN-(DPhe-AMCP)2-OBn (13). See the Supporting Information
for the synthesis scheme. Two 199 mg (0.42 mmol, 1.0 equiv)
portions of α/γ-peptide dimer S1 (see the Supporting Information for
preparation of this dipeptide) were deprotected orthogonally
according to general procedure A and general procedure B to yield
a α/γ-dipeptide acid S2 and a α/γ-dipeptide amine HCl salt S3,
respectively, both in >95% yield. The α/γ-dipeptide acid S2 (0.42
mmol, 1.0 equiv) was dissolved in 5 mL of CH2Cl2. i-Pr2EtN (444 μL,
2.5 mmol, 6.0 equiv) was added followed by HOBt (67 mg, 0.5 mmol,
1.2 equiv) and EDCI (95.4 mg, 0.5 mmol, 1.2 equiv). The reaction
mixture was stirred at room temperature for 15 min, at which point a
solution of dipeptide amine HCl salt S3 in 5 mL of CH2Cl2 was added.
The reaction flask was sealed with a septum, and the reaction mixture
was stirred at room temperature for 48 h. The reaction mixture was
diluted 4-fold with EtOAc and washed twice each (25 mL each wash)
with 1 M aqueous NaHSO4, then saturated (aq) NaHCO3, then brine.
Each aqueous layer was extracted twice with 25 mL of CH2Cl2. All
organic layers were combined, dried over MgSO4, filtered, and
evaporated to afford a crude solid. The product was purified via
Boc-HN-(DAla-AMCP)4-OBn (12). Two 0.33 mmol portions of α/γ-
peptide tetramer 8 were deprotected orthogonally according to general
procedure A and general procedure B to yield α/γ-tetrapeptide acid 10
and α/γ-tetrapeptide amine HCl salt 11, respectively, with both
reactions proceeding in >95% yield. Tetrapeptide acid 10 (0.33 mmol,
1.0 equiv) was dissolved in 2.5 mL of DMF. i-Pr2EtN (348 μL, 2
mmol, 6.0 equiv) was added followed by HOBt (54 mg, 0.4 mmol, 1.2
equiv) and EDCI (77 mg, 0.4 mmol, 1.2 equiv). The reaction mixture
was stirred at room temperature for 15 min, at which point a solution
H
dx.doi.org/10.1021/jo401501g | J. Org. Chem. XXXX, XXX, XXX−XXX