N4-methylation changes the conformation of (3S,6S)-3-alkyl-6-benzylpiperazine-2,5-diones from folded to extended

Article abstract of DOI:10.1016/j.molstruc.2016.03.019

N4-methylation of (3S,6S)-3-alkyl-6-benzylpiperazine-2,5-diones (S,S)-1a-c was found to change their folded conformation to an extended conformation. Conformational aspects of N1- and/or N4-methylated (S,S)-1a-c were revealed by single crystal X-ray crystallography and ¹H NMR spectroscopy.

Full text of DOI:10.1016/j.molstruc.2016.03.019

Accepted Manuscript
N4-methylation changes the conformation of (3S,6S)-3-alkyl-6-benzylpiperazine-2,5-
diones from folded to extended
Michiyasu Nakao, Yuta Hiroyama, Shintaro Fukayama, Shigeki Sano
PII:
S0022-2860(16)30216-2
10.1016/j.molstruc.2016.03.019
MOLSTR 22330
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 24 November 2015
Revised Date: 7 March 2016
Accepted Date: 7 March 2016
Please cite this article as: M. Nakao, Y. Hiroyama, S. Fukayama, S. Sano, N4-methylation changes
the conformation of (3S,6S)-3-alkyl-6-benzylpiperazine-2,5-diones from folded to extended, Journal of
Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.03.019.
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ACCEPTED MANUSCRIPT
Title:
N4-Methylation Changes the Conformation of (3S,6S)-3-Alkyl-6-benzylpiperazine-2,5-diones from
Folded to Extended
Author names:
Michiyasu Nakao, Yuta Hiroyama, Shintaro Fukayama, Shigeki Sano*
Affiliation:
Graduate School of Pharmaceutical Sciences, Tokushima University, Sho-machi, Tokushima 770-8505,
Japan
Corresponding author:
E-mail address: ssano@tokushima-u.ac.jp (S. Sano)
Abstract:
N4-Methylation of (3S,6S)-3-alkyl-6-benzylpiperazine-2,5-diones (S,S)-1a–c was found to change their
folded conformation to an extended conformation. Conformational aspects of N1- and/or N4-methylated
(S,S)-1a–c were revealed by single crystal X-ray crystallography and 1H NMR spectroscopy.
Keywords:
Piperazine-2,5-dione
N-methylation
Folded conformation
Extended conformation
CH/π interaction
X-ray crystal structure

ACCEPTED MANUSCRIPT
1. Introduction
Piperazine-2,5-diones (2,5-DKPs) are an important class of heterocyclic compounds because of their wide
range of biological activities [1–5]. In addition, 2,5-DKPs could be regarded as an attractive scaffold for
functional molecules including pharmaceuticals due to the structural diversity achieved by introducing
substituents on the 2,5-DKP ring [6–12]. Therefore, the regulation of their conformation by chemical
modification has recently attracted much attention. In general, C3- or C6-mono-benzylated 2,5-DKPs are
known to adopt a folded conformation, in which the benzyl moiety is folded over the 2,5-DKP ring [13–
21]. Previously, we reported that 2,5-DKPs derived from L- or D-phenylalanine and α-substituted L-serine
1
adopted a folded conformation based on their H NMR spectra [22]. In addition, our recent study has
demonstrated the importance of intramolecular CH/π interaction in the folded conformation of mono-
1
benzylated 2,5-DKPs due to the electronic effects of para-substituents on the benzyl group in the H
NMR spectra [23]. Intriguingly, similar folded conformations were also observed in chiral 5-
benzylimidazolidin-4-ones, which are known as effective organocatalysts [24–30]. As the next step in our
ongoing research on the conformation of 2,5-DKPs, we herein investigated the possibility of using N1-
and/or N4-methylation to change the conformation of (3S,6S)-3-alkyl-6-benzylpiperazine-2,5-diones
(S,S)-1a–c from a folded to an extended one (Figure 1). In this context, we note that N-methylation of
amide bonds is considered to be one of the important chemical modifications for control of the
conformation and the biological function of peptides or proteins [31].
N1-methylation
a: R = i-Pr
R
(S)
H
O
H
N
b: R = i-Bu
c: R = Me
(S)
H
N
H
O
(S,S)-1a-c
N4-methylation
Fig. 1. N1- and N4-methylation of (3S,6S)-3-alkyl-6-benzylpiperazine-2,5-diones (S,S)-1a–c.
2. Experimental section
All reactions were monitored by TLC employing 0.25 mm silica gel plates (Merck 5715; 60 F254).
Column chromatography was carried out on silica gel [Kanto Chemical 60N (spherical, neutral); 63-210
mm]. Anhydrous CH2Cl2 and DMF were used as purchased from Kanto Chemical. Triethylamine was
distilled prior to use. All other reagents were used as purchased. All melting points were determined on a
Yanagimoto micro melting point apparatus and are uncorrected. IR spectra were obtained using a JASCO
1
13
FT/IR-6200 IR Fourier transform spectrometer. H NMR (500 MHz) and C NMR (125 MHz) spectra
were recorded on a Bruker AV500 spectrometer. Chemical shifts are given in δ values (parts per million)

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using tetramethylsilane (TMS) as an internal standard. Electron spray ionization mass spectra (ESIMS)
were recorded on a Waters LCT Premier spectrometer. Elemental combustion analyses were performed
using a Yanagimoto CHN CORDER MT-5 and a J-SCIENCE LAB JM10. The microwave-assisted
reaction was performed utilizing an automated single-mode microwave synthesizer (InitiatorTM 60;
Biotage AB). Single crystal X-ray diffraction experiments were performed on a Rigaku RAXIS-RAPID
diffractometer using graphite monochromated Mo-Kα (λ = 0.71075 Å, 50 kV, 40 mA) radiation. Data
were corrected for Lorentz and polarization effects. The structure was solved by direct methods [32] and
expanded using Fourier techniques. The non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were refined using the riding model. All calculations were performed using the CrystalStructure
[33] crystallographic software package.
2.1. General procedure for the preparation of dipeptides (S,S)-4a–c and (S,S)-6a–c
2.1.1. Methyl (S)-2-[(S)-2-(tert-Butoxycarbonylamino)-N,3-dimethylbutanamido]-3-phenylpropanoate
[(S,S)-4a]
To a solution of (S)-2a (172 mg, 0.793 mmol) and (S)-3d (200 mg, 0.872 mmol) in anhydrous CH2Cl2 (3
mL) was added HOBt (118 g, 0.872 mmol), EDC • HCl (228 g, 1.19 mmol) and triethylamine (122
ꢀL, 0.872 mmol) at 0 °C under argon. The reaction mixture was allowed to warm to room temperature
and stirred for 24 h. The reaction mixture was treated with 5% citric acid (5 mL) and then extracted with
CHCl3 (10 mL × 3). The extract was dried over anhydrous MgSO4, filtered, and concentrated in vacuo.
The oily residue was purified by silica gel column chromatography [Silica Gel 60N: n-hexane–AcOEt
29
(3:1)] to afford (S,S)-4a (192.1 mg, 62%). White solid; mp 87.0–88.0 °C; [α]D −69.9 (c 1.01, CHCl3);
1H NMR (500 MHz, CDCl3, mixture of rotamers) δ 0.36 (d, J = 6.7 Hz, 0.4H), 0.66 (d, J = 6.7 Hz, 0.4H),
0.89 (d, J = 6.8 Hz, 2.6H), 0.95 (d, J = 6.8 Hz, 2.6H), 1.41 (s, 9H), 1.87–1.98 (m, 1H), 2.92/2.94 (s × 2,
3H), 3.00 (dd, J = 10.1, 14.6 Hz, 1H), 3.39 (dd, J = 5.7, 14.6 Hz, 1H), 3.72/3.73 (s × 2, 3H), 4.00–4.05
(m, 0.2H), 4.34 (dd, J = 6.1, 9.4 Hz, 0.8H), 4.94 (d, J = 9.8 Hz, 0.2H), 4.99 (dd, J = 4.4, 10.3 Hz, 0.2H),
13
5.05 (d, J = 9.3 Hz, 0.8H), 5.34 (dd, J = 5.7, 10.1 Hz, 0.8H), 7.15–7.33 (m, 5H); C NMR (125 MHz,
CDCl3, mixture of rotamers) δ 17.20, 17.27, 18.9, 19.5, 28.29, 28.34, 29.7, 30.7, 31.2, 32.8, 34.5, 35.2,
52.3, 52.6, 54.9, 55.0, 58.4, 61.4, 79.3, 79.4, 126.8, 127.1, 128.5, 128.8, 128.9, 129.2, 136.75, 136.80,
155.4, 155.7, 170.5, 171.1, 172.9, 173.1; IR (KBr) 2971, 1734, 1710, 1642, 1516, 1364 cm−1; ESIMS m/z:
calcd for C21H32N2NaO5 [M+Na]+, 415.2209; found, 415.2206.
2.1.2. Methyl (S)-2-[(S)-2-(tert-Butoxycarbonylamino)-N,4-dimethylpentanamido]-3-phenylpropanoate
[(S,S)-4b]

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29
1
White solid, mp 127–129 °C; [α]D −79.4 (c 1.20, CHCl3); H NMR (500 MHz, CDCl3, mixture of
rotamers) δ 0.69/0.71 (d × 2, J = 6.6 Hz, 1H), 0.92/0.96 (d × 2, J = 6.7, 6.5 Hz, 5H), 1.30–1.45 (m, 2H),
1.40/1.41 (s × 2, 9H), 1.65–1.75 (m, 1H), 2.90/2.93 (s × 2, 3H), 3.05 (dd, J = 10.5, 14.5 Hz, 1H), 3.38
(dd, J = 5.3, 14.5 Hz, 1H), 3.73/3.75 (s × 2, 3H), 4.31 (dt, J = 3.8, 10.1 Hz, 0.2H), 4.53 (dt, J = 4.0, 9.6
Hz, 0.8H), 4.88 (d, J = 9.8 Hz, 0.2H), 4.96 (dd, J = 4.1, 10.8 Hz, 0.2H), 5.02 (d, J = 9.2 Hz, 0.8H), 5.20
13
(dd, J = 5.5, 10.4 Hz, 0.8H), 7.16–7.53 (m, 5H); C NMR (125 MHz, CDCl3, mixture of rotamers) δ
21.5, 21.8, 23.2, 23.3, 24.2, 24.6, 28.28, 28.33, 29.9, 32.9, 34.5, 35.2, 41.1, 42.4, 47.8, 48.8, 52.2, 52.6,
58.9, 61.3, 79.3, 79.4, 126.8, 127.1, 128.5, 128.8, 129.0, 129.1, 136.76, 136.84, 155.2, 155.5, 170.5,
171.0, 173.5, 173.7; IR (KBr) 3385, 2978, 1731, 1705, 1645, 1517, 1363 cm−1; ESIMS m/z: calcd for
C22H34N2NaO5 [M+Na]+, 429.2365; found, 429.2343.
2.1.3. Methyl (S)-2-[(S)-2-(tert-Butoxycarbonylamino)-N-methylpropanamido]-3-phenylpropanoate
[(S,S)-4c]
19
1
Colorless needles (CH2Cl2–n-hexane), mp 88–89 °C; [α]D −64.9 (c 1.00, CHCl3); H NMR (500 MHz,
CDCl3, mixture of rotamers) δ 0.72 (d, J = 6.6 Hz, 0.5H), 1.25 (d, J = 6.8 Hz, 2.5H), 1.40/1.42 (s × 2,
9H), 2.86 (s, 2.5H), 2.92 (s, 0.5H), 2.95–3.02 (m, 0.2H), 3.08 (dd, J = 10.6, 14.5 Hz, 0.8H), 3.38 (dd, J =
5.2, 14.5 Hz, 1H), 3.73/3.74 (s × 2, 3H), 4.26–4.33 (m, 0.2H), 4.50 (brquint, 0.8H), 4.94 (brdd, 0.2H),
5.05–5.15 (m, 1H), 5.26 (brd, 0.8H), 7.15-7.31 (m, 5H); 13C NMR (125 MHz, CDCl3, mixture of
rotamers) δ 18.5, 18.6, 28.3, 28.4, 29.9, 32.4, 33.4, 34.4, 34.7, 35.2, 45.2, 46.5, 52.3, 52.6, 58.1, 59.5,
61.4, 79.4, 79.5, 126.8, 126.9, 127.2, 128.5, 128.77, 128.83, 128.9, 129.1, 136.6, 136.9, 154.8, 155.0,
170.4, 170.9, 173.2, 173.6; IR (KBr) 3387, 2980, 2946, 1737, 1710, 1644, 1513, 1360 cm−1; ESIMS m/z:
calcd for C19H28N2NaO5 [M+Na]+, 387.1896; found, 387.1916.
2.1.4.
Methyl
(S)-2-[(S)-2-(tert-Butoxycarbonylamino)-N-methyl-3-phenylpropanamido]-3-
methylbutanoate [(S,S)-6a]
Colorless oil; [α]D28 −68.9 (c 0.93, CHCl3); 1H NMR (500 MHz, CDCl3, mixture of rotamers) δ 0.55 (d, J
= 6.7 Hz, 0.3H), 0.80 (d, J = 6.7 Hz, 2.7H), 0.91 (d, J = 6.5 Hz, 0.3H), 0.95 (d, J = 6.5 Hz, 2.7H),
1.38/1.40 (s × 2, 9H), 2.07–2.18 (m, 1H), 2.72 (s, 2.7H), 2.86 (s, 0.3H), 2.93 (dd, J = 5.7, 13.1 Hz, 1H),
3.01 (dd, J = 8.6, 13.1 Hz, 1H), 3.66/3.69 (s × 2, 3H), 4.02 (d, J = 10.4 Hz, 0.1H), 4.79–4.86 (m, 0.9H),
4.91 (d, J = 10.7 Hz, 0.9H), 4.95–5.01 (m, 0.1H), 5.13 (brd, 0.1H), 5.28 (brd, 0.9H), 7.16–7.29 (m, 5H);
13C NMR (125 MHz, CDCl3, mixture of rotamers) δ 18.5, 19.6, 27.2, 28.3, 30.7, 39.4, 51.70, 51.75, 61.3,
79.8, 126.79, 126.84, 128.40, 128.46, 129.5, 129.6, 136.2, 155.3, 170.9, 173.0; IR (neat) 3315, 3029,
2972, 2876, 1740, 1711, 1659, 1642, 1513, 1494, 1453, 1411, 1391, 1367, 1294, 1249, 1170 cm−1;
ESIMS m/z: calcd for C21H32N2NaO5 [M+Na]+, 415.2209; found, 415.2210.

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2.1.5.
Methyl
(S)-2-[(S)-2-(tert-Butoxycarbonylamino)-N-methyl-3-phenylpropanamido]-4-
methylpentanoate [(S,S)-6b]
Colorless oil; [α]D29 −21.8 (c 1.34, CHCl3); 1H NMR (500 MHz, CDCl3, mixture of rotamers) δ 0.80/0.83
(d × 2, J = 6.4, 7.0 Hz, 0.5H), 0.89/0.91 (d × 2, J = 6.5, 6.7 Hz, 5.5H), 1.37/1.39 (s × 2, 9H), 1.42–1.49
(m, 1H), 1.60–1.74 (m, 2H), 2.79/2.81 (s × 2, 3H), 2.90 (dd, J = 6.2, 13.5 Hz, 1H), 3.08 (dd, J = 7.5, 13.5
Hz, 0.9H), 3.15 (dd, J = 7.7, 12.9 Hz, 0.1H), 3.67 (s, 3H), 4.55–4.60 (m, 0.1H), 4.84 (brq, 0.9H), 5.11
13
(brd, 0.1H), 5.26 (brd, 0.9H), 5.30 (dd, J = 5.2, 10.5 Hz, 1H), 7.17–7.30 (m, 5H); C NMR (125 MHz,
CDCl3, mixture of rotamers) δ 21.4, 22.2, 22.7, 23.2, 24.66, 24.72, 28.1, 28.3, 31.0, 37.2, 38.2, 39.0, 39.7,
51.3, 51.7, 52.1, 52.3, 54.5, 57.8, 79.7, 126.7, 126.8, 128.37, 128.41, 129.5, 129.6, 136.3, 136.7, 155.2,
171.2, 171.9, 172.7; IR (neat) 3319, 3029, 2956, 2871, 1741, 1711, 1643, 1495, 1454, 1411, 1391, 1366,
1249, 1172 cm−1; ESIMS m/z: calcd for C22H34N2NaO5 [M+Na]+, 429.2365; found, 429.2375.
2.1.6. Methyl (S)-2-[(S)-2-(tert-Butoxycarbonylamino)-N-methyl-3-phenylpropanamido]propanoate
[(S,S)-6c]
Colorless oil; [α]D29 −19.1 (c 0.60, CHCl3); 1H NMR (500 MHz, CDCl3, mixture of rotamers) δ 0.91 (d, J
= 7.0 Hz, 0.5H), 1.36 (d, J = 7.3 Hz, 2.5H), 1.40/1.41 (s × 2, 9H), 2.73 (s, 0.5H), 2.81 (s, 2.5H), 2.91 (dd,
J = 6.3, 13.6 Hz, 0.8H), 2.99 (d, J = 7.2 Hz, 0.4H), 3.06 (dd, J = 7.1, 13.6 Hz, 0.8H), 3.67 (s, 0.5H), 3.70
(s, 2.5H), 4.40 (q, J = 7.0 Hz, 0.2H), 4.80–4.88 (m, 1H), 5.14 (q, J = 7.3 Hz, 0.8H), 5.30 (brd, 0.8H), 5.37
13
(brd, 0.2H), 7.17–7.31 (m, 5H); C NMR (125 MHz, CDCl3, mixture of rotamers) δ 14.2, 14.7, 28.3,
28.8, 31.4, 39.3, 40.5, 51.4, 51.6, 52.2, 52.5, 52.6, 54.9, 79.6, 126.8, 126.9, 128.4, 128.6, 129.4, 129.6,
136.2, 136.5, 154.9, 155.1, 171.2, 171.8, 172.0, 172.1; IR (neat) 3318, 2979, 1743, 1711, 1659, 1642,
1513, 1494, 1484, 1462, 1452, 1410, 1366, 1248, 1171 cm−1; ESIMS m/z: calcd for C19H28N2NaO5
[M+Na]+, 387.1896; found, 387.1862.
2.2. General procedure for the preparation of piperazine-2,5-diones (S,S)-5a–c and (S,S)-7a–c
2.2.1. (3S,6S)-6-Benzyl-3-isopropyl-1-methylpiperazine-2,5-dione [(S,S)-5a]
A suspension of dipeptide (S,S)-4a (80 mg, 0.204 mmol) in a mixed solvent of H2O (1.5 mL) with MeOH
(0.5 mL) was irradiated at 170 °C for 10 min using an automated single-mode microwave synthesizer.
The reaction mixture was treated with H2O (2 mL) and then extracted with CHCl3 (5 mL × 2). The extract
was dried over anhydrous MgSO4, filtered, and concentrated in vacuo. The residue was purified by silica
gel column chromatography [Silica Gel 60N: CHCl3–MeOH (50:1)] to afford (S,S)-5a (47.3 mg, 89%
yield). Colorless columns (CH2Cl2–AcOEt); mp 173–175 °C; [α]D27 −53.8 (c 1.00, CHCl3); 1H NMR (500
MHz, CDCl3) δ 0.49 (d, J = 6.8 Hz, 3H), 0.85 (d, J = 7.0 Hz, 3H), 1.35–1.46 (m, 1H), 2.96 (s, 3H), 3.26
(dd, J = 4.7, 14.2 Hz, 1H), 3.30 (dd, J = 5.0, 14.2 Hz, 1H), 3.60 (dd, J = 2.7, 5.9 Hz, 1H), 4.18 (brt, 1H),

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13
6.34 (brs, 1H), 7.14–7.19 (m, 2H), 7.21–7.32 (m, 3H); C NMR (125 MHz, CDCl3) δ 17.3, 19.3, 32.5,
33.6, 38.1, 60.8, 63.5, 127.4, 128.8, 129.9, 135.7, 165.5, 167.3; IR (KBr) 3247, 2972, 1687, 1661, 1456,
1301 cm−1; ESIMS m/z: calcd for C15H20N2NaO2 [M+Na]+, 283.1422; found, 283.1401. Anal. Calcd for
C15H20N2O2: C, 69.20; H, 7.74; N, 10.76. Found: C, 69.20; H, 7.88; N, 10.55%.
2.2.2. (3S,6S)-6-Benzyl-3-isobutyl-1-methylpiperazine-2,5-dione [(S,S)-5b]
Colorless columns (CH2Cl2–AcOEt); mp 170–172 °C; [α]D26 −11.8 (c 1.01, CHCl3); 1H NMR (500 MHz,
CDCl3) δ −0.22 (ddd, J = 4.5, 11.0, 13.7 Hz, 1H), 0.70 (d, J = 6.6 Hz, 3H), 0.75 (d, J = 6.6 Hz, 3H), 1.03
(ddd, J = 3.9, 10.3, 13.7 Hz, 1H), 1.28–1.40 (m, 1H), 3.07 (s, 3H), 3.16 (dd, J = 4.6, 14.0 Hz, 1H), 3.33
(dd, J = 3.7, 14.0 Hz, 1H), 3.74 (brdt, 1H), 4.20 (brt, 1H), 6.27 (brs, 1H), 7.09–7.15 (m, 2H), 7.25–7.34
(m, 3H); 13C NMR (125 MHz, CDCl3) δ 20.4, 22.9, 23.8, 33.0, 36.6, 43.6, 53.3, 63.1, 127.7, 128.8, 130.2,
134.9, 166.7, 166.8; IR (KBr) 3609, 3520, 3328, 2956, 1681, 1649, 1468 cm−1; ESIMS m/z: calcd for
C16H22N2NaO2 [M+Na]+, 297.1579; found, 297.1552. Anal. Calcd for C16H22N2O2: C, 70.04; H, 8.08; N,
10.21. Found: C, 69.97; H, 8.11; N, 10.24%.
2.2.3. (3S,6S)-6-Benzyl-1,3-dimethylpiperazine-2,5-dione [(S,S)-5c]
28
1
Colorless plates (CH2Cl2–AcOEt), mp 143.5–145.5 °C; [α]D +30.2 (c 1.02, CHCl3); H NMR (500
MHz, CDCl3) δ 0.50 (d, J = 7.1 Hz, 3H), 3.08 (s, 3H), 3.18 (dd, J = 4.6, 14.1 Hz, 1H), 3.33 (dd, J = 3.7,
14.1 Hz, 1H), 3.87 (dq, J = 2.6, 7.1 Hz, 1H), 4.20 (brt, 1H), 6.27 (brs, 1H), 7.10–7.14 (m, 2H), 7.24–7.33
13
(m, 3H); C NMR (125 MHz, CDCl3) δ 20.7, 33.0, 36.7, 51.1, 63.2, 127.6, 128.9, 130.2, 134.8, 166.5,
166.8; IR (KBr) 3181, 3137, 1681, 1632, 1477 cm−1; ESIMS m/z: calcd for C13H16N2NaO2 [M+Na]+,
255.1109; found, 255.1099. Anal. Calcd for C13H16N2O2: C, 67.22; H, 6.94; N, 12.06. Found: C, 67.21; H,
6.86; N, 11.88%.
2.2.4. (3S,6S)-3-Benzyl-6-isopropyl-1-methylpiperazine-2,5-dione [(S,S)-7a]
Colorless prisms (CH2Cl2–AcOEt); mp 133–134 °C; [α]D29 −155.0 (c 1.10, CHCl3); 1H NMR (500 MHz,
CDCl3) δ 1.06 (d, J = 7.0 Hz, 3H), 1.19 (d, J = 7.0 Hz, 3H), 2.25 (dsept, J = 4.2, 7.0 Hz, 1H), 2.80 (dd, J
= 11.4, 13.5 Hz, 1H), 3.03 (s, 3H), 3.54 (dd, J = 3.2, 13.5 Hz, 1H), 3.75 (brd, 1H), 4.17 (brt, 1H), 5.67
13
(brs, 1H), 7.19–7.24 (m, 2H), 7.26–7.31 (m, 1H), 7.32–7.38 (m, 2H); C NMR (125 MHz, CDCl3) δ
18.1, 19.7, 31.6, 34.6, 41.5, 57.0, 67.7, 127.4, 129.1, 129.4, 136.1, 165.6, 165.8; IR (KBr) 3207, 3133,
2984, 2972, 1690, 1636, 1456, 1412, 1336 cm−1; ESIMS m/z: calcd for C15H20N2NaO2 [M+Na]+,
283.1422; found, 283.1438. Anal. Calcd for C15H20N2O2: C, 69.20; H, 7.74; N, 10.76. Found: C, 69.05;
H, 7.87; N, 10.71%.
2.2.5. (3S,6S)-3-Benzyl-6-isobutyl-1-methylpiperazine-2,5-dione [(S,S)-7b]

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Colorless columns (CH2Cl2–AcOEt); mp 105–109 °C; [α]D18 −82.5 (c 1.02, CHCl3); 1H NMR (500 MHz,
CDCl3) δ 0.88 (d, J = 6.7 Hz, 3H), 0.94 (d, J = 6.5 Hz, 3H), 1.15 (ddd, J = 5.2, 8.7, 14.1 Hz, 1H), 1.33
(ddd, J = 4.2, 8.8, 14.1 Hz, 1H), 1.77–1.88 (m, 1H), 2.95 (s, 3H), 2.99 (dd, J = 8.7, 13.6 Hz, 1H), 3.26
(dd, J = 3.7, 13.6 Hz, 1H), 3.73 (dd, J = 4.2, 8.7 Hz, 1H), 4.22 (brdt, 1H), 5.94 (brs, 1H), 7.20–7.22 (m,
13
2H), 7.26–7.30 (m, 1H), 7.31–7.35 (m, 2H); C NMR (125 MHz, CDCl3) δ 21.9, 23.1, 24.8, 33.0, 41.1,
42.4, 57.1, 60.2, 127.4, 128.9, 129.9, 135.8, 165.4, 168.0; IR (KBr) 3242, 2960, 1683, 1638, 1442, 1344
cm−1; ESIMS m/z: calcd for C16H22N2NaO2 [M+Na]+, 297.1579; found, 297.1578. Anal. Calcd for
C16H22N2O2: C, 70.04; H, 8.08; N, 10.21. Found: C, 69.92; H, 8.12; N, 10.15%.
2.2.6. (3S,6S)-3-Benzyl-1,6-dimethylpiperazine-2,5-dione [(S,S)-7c]
27
Colorless needles (CH2Cl2–AcOEt), mp 126–129 °C; [α]D −47.8 (c 1.04, CHCl3); 1H NMR (500 MHz,
CDCl3) δ 1.01 (d, J = 7.1 Hz, 3H), 2.92 (s, 3H), 3.09 (dd, J = 7.5, 13.7 Hz, 1H), 3.17 (dd, J = 3.9, 13.7
Hz, 1H), 3.78 (q, J = 7.1 Hz, 1H), 4.27 (brquint, 1H), 6.14 (brs, 1H), 7.18–7.23 (m, 2H), 7.25–7.30 (m,
1H), 7.31–7.36 (m, 2H); 13C NMR (125 MHz, CDCl3) δ 18.0, 32.2, 41.1, 56.8, 57.5, 127.4, 128.8, 130.1,
135.5, 164.8, 168.6; IR (KBr) 3242, 1687, 1643, 1455, 1340, 1322 cm−1; ESIMS m/z: calcd for
C13H16N2NaO2 [M+Na]+, 255.1109; found, 255.1119.
2.3. General procedure for the preparation of piperazine-2,5-diones (S,S)-8a–c
2.3.1. (3S,6S)-3-Benzyl-6-isopropyl-1,4-dimethylpiperazine-2,5-dione [(S,S)-8a]
NaH (50-72%, 6.9 mg, 0.144 mmol) was added to a solution of (S,S)-7a (25 mg, 0.096 mmol) in
anhydrous DMF (2 mL) and stirred at 0 °C for 15 min under argon. After adding MeI (8.96 ꢀL, 0.144
mmol), the mixture was stirred at 0 °C for 30 min under argon. The reaction mixture was treated with 1N
HCl (2 mL) and then extracted with AcOEt (5 mL × 3). The extract was washed with sat. Na2S2O3 (5 mL)
and H2O (5 mL). The organic layer was dried over anhydrous MgSO4, filtered, and concentrated in vacuo.
The residue was purified by silica gel column chromatography [Silica Gel 60N: CHCl3–MeOH (30:1)] to
afford (S,S)-8a (24.1 mg, 91% yield). White solid; mp 77–80 °C; [α]D30 −81.4 (c 0.24, CHCl3); 1H NMR
(500 MHz, CDCl3) δ 0.95 (d, J = 6.8 Hz, 3H), 1.02 (d, J = 6.9 Hz, 3H), 1.61–1.73 (m, 1H), 2.67 (s, 3H),
3.01 (s, 3H), 3.09 (dd, J = 7.7, 14.1 Hz, 1H), 3.38 (dd, J = 4.4, 14.1 Hz, 1H), 3.55 (d, J = 7.1 Hz, 1H),
4.12 (dd, J = 4.4, 7.7 Hz, 1H), 7.20–7.35 (m, 5H); 13C NMR (125 MHz, CDCl3) δ 19.6, 20.6, 33.5, 33.9,
35.8, 40.3, 65.1, 68.9, 127.3, 128.9, 129.5, 137.1, 165.8, 166.4; IR (KBr) 2934, 2874, 1662, 1477, 1456,
1402 cm−1; ESIMS m/z: calcd for C16H22N2NaO2 [M+Na]+, 297.1579; found, 297.1578.
2.3.2. (3S,6S)-3-Benzyl-6-isobutyl-1,4-dimethylpiperazine-2,5-dione [(S,S)-8b]
White solid; mp 144.5–145.5 °C; [α]D30 –5.5 (c 0.38, CHCl3); 1H NMR (500 MHz, CDCl3) δ 0.28 (ddd, J

ACCEPTED MANUSCRIPT
= 5.2, 9.2, 14.1 Hz, 1H), 0.66 (ddd, J = 4.2, 9.3, 14.1 Hz, 1H), 0.72 (d, J = 6.7 Hz, 3H), 0.83 (d, J = 6.5
Hz, 3H), 1.66–1.75 (m, 1H), 2.86 (s, 3H), 2.94 (s, 3H), 3.17 (dd, J = 4.6, 14.0 Hz, 1H), 3.29 (dd, J = 4.9,
13
14.0 Hz, 1H), 3.61 (dd, J = 4.2, 9.2 Hz, 1H), 4.18 (brt, 1H), 7.09–7.14 (m, 2H), 7.22–7.32 (m, 3H); C
NMR (125 MHz, CDCl3) δ 21.5, 22.7, 25.1, 32.5, 32.8, 37.7, 42.4, 60.1, 64.0, 127.6, 128.9, 130.0, 135.7,
165.2, 166.6; IR (KBr) 2953, 2866, 1651, 1491, 1455, 1404 cm−1; ESIMS m/z: calcd for C17H24N2NaO2
[M+Na]+, 311.1735; found, 311.1737.
2.3.3. (3S,6S)-3-Benzyl-1,4,6-trimethylpiperazine-2,5-dione [(S,S)-8c]
30
1
White solid, mp 123.5–127 °C; [α]D −2.0 (c 0.74, CHCl3); H NMR (500 MHz, CDCl3) δ 0.48 (d, J =
7.1 Hz, 3H), 2.82 (s, 3H), 3.03 (s, 3H), 3.15 (dd, J = 4.4, 14.0 Hz, 1H), 3.32 (dd, J = 4.2, 14.0 Hz, 1H),
13
3.69 (q, J = 7.1 Hz, 1H), 4.21 (brt, 1H), 7.07–7.12 (m, 2H), 7.21–7.26 (m, 1H), 7.26–7.31 (m, 2H); C
NMR (125 MHz, CDCl3) δ 17.5, 31.7, 32.7, 37.3, 57.6, 63.5, 127.5, 128.9, 130.1, 135.3, 164.5, 166.8; IR
(KBr) 3476, 3385, 2942, 1651, 1493, 1453, 1405, 1346 cm−1; ESIMS m/z: calcd for C14H18N2NaO2
[M+Na]+, 269.1266; found, 269.1288.
2.4. X-ray diffraction studies of (S,S)-5a and (S,S)-7a
2.4.1. (3S,6S)-6-Benzyl-3-isopropyl-1-methylpiperazine-2,5-dione [(S,S)-5a]
A colorless prism crystal of (S,S)-5a having approximate dimensions of 0.400 × 0.300 × 0.300 mm was
°
mounted on a glass fiber. The data were collected at a temperature of −180 ± 1 C to a maximum 2θ value
°
of 54.8 . A total of 44 oscillation images were collected. A sweep of data was done using ω scans from
°
°
°
°
°
130.0 to 190.0 in 5.0 step, at χ = 45.0 and ϕ = 80.0 . The exposure rate was 130.0 [sec./ ]. A second
°
°
°
°
sweep was performed using ω scans from 0.0 to 160.0 in 5.0 step, at χ = 45.0 and ϕ = 260.0 . The
°
exposure rate was 130.0 [sec./ ]. The crystal-to-detector distance was 127.40 mm. Readout was
performed in the 0.100 mm pixel mode. The absolute configuration was deduced from the synthetic
pathway.
Deposition number: CCDC-1429971
Empirical formula: C15H20N2O2
Formula weight: 260.34
Crystal system, Space group: orthorhombic, P 212121 (No. 19)
Unit cell dimensions: a = 8.1071(3) Å, b = 12.3418(4) Å, c = 13.9765(5) Å
Volume: 1398.44(8) Å3
Z, Calculated density: 4, 1.236 g/cm3
Absorption coefficient ꢀ(Mo-Kα): 0.826 cm−1

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Final R indices [I > 2.00σ(I)]: R1 = 0.0340, wR2 = 0.1042.
2.4.2. (3S,6S)-3-Benzyl-6-isopropyl-1-methylpiperazine-2,5-dione [(S,S)-7a]
A colorless block crystal of (S,S)-7a having approximate dimensions of 0.300 × 0.250 × 0.200 mm was
°
mounted on a glass fiber. The data were collected at a temperature of −150 ± 1 C to a maximum 2θ value
°
of 54.8 . A total of 44 oscillation images were collected. A sweep of data was done using ω scans from
°
°
°
°
°
130.0 to 190.0 in 5.0 step, at χ = 45.0 and ϕ = 0.0 . The exposure rate was 130.0 [sec./ ]. A second
°
°
°
°
sweep was performed using ω scans from 0.0 to 160.0 in 5.0 step, at χ = 45.0 and ϕ = 180.0 . The
°
exposure rate was 130.0 [sec./ ]. The crystal-to-detector distance was 127.40 mm. Readout was
performed in the 0.100 mm pixel mode. The absolute configuration was deduced from the synthetic
pathway.
Deposition number: CCDC-1429972
Empirical formula: C15H20N2O2
Formula weight: 260.34
Crystal system, Space group: orthorhombic, P 212121 (No. 19)
Unit cell dimensions: a = 8.4741(5) Å, b = 12.8041(7) Å, c = 12.7997(9) Å
Volume: 1388.8(2) Å3
Z, Calculated density: 4, 1.245 g/cm3
Absorption coefficient ꢀ(Mo-Kα): 0.832 cm-1
Final R indices [I > 2.00σ(I)]: R1 = 0.0326, wR2 = 0.0996.
3. Results and discussion
3.1. Synthesis of N-methylated and N,N’-dimethylated 2,5-DKPs
N1-methylated 2,5-DKPs (S,S)-5a–c, N4-methylated 2,5-DKPs (S,S)-7a–c, and N,N’-dimethylated 2,5-
DKPs (S,S)-8a–c were synthesized as shown in Scheme 1. Methyl, isopropyl, and isobutyl groups, which
came from
L
-alanine,
L
-valine, and
L
-leucine, were chosen as 3-alkyl substituents of (S,S)-1a–c.
-phenylalanine methyl ester
Condensation of Boc-
L
-α-amino acids (S)-2a–c with N-methyl-
L
hydrochloride [(S)-3d] using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC•HCl)
as a coupling reagent in the presence of 1-hydroxybenzotriazole (HOBt) and triethylamine afforded the
dipeptides (S,S)-4a–c. Subsequently, microwave-assisted removal of the Boc group followed by
intramolecular cyclization furnished the N1-methylated 2,5-DKPs (S,S)-5a–c by a one-pot reaction [34].

ACCEPTED MANUSCRIPT
N4-methylated 2,5-DKPs (S,S)-7a–c were also synthesized in a similar way. Furthermore, N-methylation
of (S,S)-7a–c afforded N,N’-dimethylated 2,5-DKPs (S,S)-8a–c.
Me
(S)
HCl • HN
CO2Me
EDC • HCl (1.5 mol eq)
HOBt (1.1 mol eq)
Et3N (1.1 mol eq)
H
Bn
(S)-3d (1.1 mol eq)
H
(S)
Boc
N
CO2H
H
R
CH2Cl2
(S)-2a
-
c
rt, 24 h
O
H
O
H
Me
H
Bn
H
(S)
N
R
(S)
Boc
N
N
CO2Me
HN
(S)
Me
H2O–MeOH (3 : 1) a
(S)
H
R
Bn
b
MW (170 °C)
O
(S,S)-4a-c: 62-93%
10 min
(S,S)-5a-c: 87-89%
Me
(S)
HCl • HN
CO2Me
EDC • HCl (1.5 mol eq)
HOBt (1.1 mol eq)
Et3N (1.1 mol eq)
H
R
c
H
(S)
Boc
Boc
N
CO2H
(S)-3a
-
(1.1 mol eq)
H
Bn
(S)-2d
CH2Cl2
rt, 24 h
O
H
O
H
R
H
(S)
NH
R
(S)
N
H
(S)
N
CO2Me
N
(S)
Me
H2O–MeOH (3 : 1)
Me
H
Bn
Bn
b
MW (170 °C)
O
(S,S)-6a : 77-90%
-c
10 min
(S,S)-7a-c: 87-92%
O
H
MeI (1.5 mol eq)
NaH (1.5 mol eq)
Me
N
R
(S)
a
: R = i-Pr
: R = i-Bu
: R = Me
H
(S)
N
DMF
Me
b
Bn
O
0 °C, 30-40 min
c
(S,S)-8a-c: 52-91%
Scheme 1. Synthesis of N1-methylated 2,5-DKPs (S,S)-5a–c, N4-methylated 2,5-DKPs (S,S)-7a–c, and
N,N’-dimethylated 2,5-DKPs (S,S)-8a–c.
a H2O was used alone as the solvent in the reaction of (S,S)-4a.
b InitiatorTM 60 (Biotage AB).
3.2. Crystallographic studies of N1- and N4-methylated 2,5-DKPs
Conformations of the 2,5-DKPs (S,S)-5a and (S,S)-7a in the solid state were examined by single crystal
X-ray crystallography as shown in Figures 2 and 3. Unfortunately, the results showed that N1-methylated
2,5-DKP (S,S)-5a adopted a folded conformation, and the perpendicular distance (2.3778 Å) of the
methine hydrogen atom [H(10)] of the isopropyl group from the plane of the benzene ring (Pbenzene) was
clearly shorter than the conventional van der Waals limit (2.9 Å: 1.2 Å for C-H plus 1.7 Å for a half

ACCEPTED MANUSCRIPT
thickness of the aromatic molecule) (Figure 2) [35–38]. The C(12)–H(10)•••Cbenzene (the centroid of the
benzene ring) angle of 173.74° was also observed. Therefore, the crystal structure of N1-methylated 2,5-
DKP (S,S)-5a strongly suggested the existence of an intramolecular CH/π interaction [35–43] as a
stabilizing factor for the folded conformation of (S,S)-5a. Intermolecular CH/π interactions of (S,S)-5a
leading to a molecular packing were not observed. However, N4-methylation of (S,S)-1a successfully
changed the folded conformation to the extended one. As shown in Figure 3, the crystal structure of N4-
methylated 2,5-DKP (S,S)-7a was found to exist in an extended conformation in which the benzyl moiety
was apart from the 2,5-DKP ring [44].
C(9)
C(8)
C(10)
C(7)
2.3778 Å
H(10)
C(14)
C(6)
C(5)
C(11)
C(13)
N1-methylated
C(12)
O(2)
C(2)
N(2)
C(3)
C(4)
C(15)
C(1)
N(1)
O(1)
Fig. 2. ORTEP drawing of N1-methylated 2,5-DKP (S,S)-5a with 50% probability ellipsoids. Selected
distances (Å) and angles (°) are: H(10)•••Pbenzene = 2.3778, C(12)•••Pbenzene = 3.3745(12), C(5)•••C(12) =
4.2910(15), C(12)–H(10)••• Cbenzene = 173.74, C(2)–C(5)–C(6) = 113.63(8), C(4)–C(12)–H(10) = 106.0.
C(10)
C(11)
C(9)
C(8)
C(14)
H(10)
C(13)
C(5)
C(7)
C(6)
C(12)
O(2)
N(2)
C(4)
C(3)
C(2)
N(1)
O(1)
C(1)
C(15)
N4-methylated

ACCEPTED MANUSCRIPT
Fig. 3. ORTEP drawing of N4-methylated 2,5-DKP (S,S)-7a with 50% probability ellipsoids. Selected
distances (Å) and angles (°) are: C(5)•••C(12) = 3.7541(16) , C(2)–C(5)–C(6) = 112.48(9), C(4)–C(12)–
H(10) = 107.2.
3.3. 1H NMR analysis of N-methylated and N,N’-dimethylated 2,5-DKPs
Next, we investigated the conformational aspects of N1-methylated 2,5-DKPs (S,S)-5a–c, N4-methylated
2,5-DKPs (S,S)-7a–c, and N,N’-dimethylated 2,5-DKPs (S,S)-8a–c in solution by 1H NMR spectroscopy.
Selected chemical shifts of (S,S)-5a–c, (S,S)-7a–c, and (S,S)-8a–c in 1H NMR spectra (500 MHz, CDCl3)
are given in Figure 4. Protons of the cis-substituent (on the same side of the 2,5-DKP ring as the benzyl
moiety) of (S,S)-5a–c were found to appear at higher magnetic fields than those of (S,S)-7a–c. In
particular, the isopropyl protons (HA and MeA) of (S,S)-5a, isobutyl protons (HC and HD) of (S,S)-5b, and
methyl protons (MeE) of (S,S)-5c exhibited remarkable upfield chemical shifts. The trans-protons HB (on
the opposite side of the 2,5-DKP ring with the benzyl moiety) of (S,S)-5a–c and (S,S)-7a–c did not show
large differences in chemical shifts. The upfield chemical shifts of protons of the cis-substituent of (S,S)-
5a–c could be attributed to the strong shielding effect of the benzene ring of the benzyl moiety. Therefore,
it was suggested that N1-methylated 2,5-DKPs (S,S)-5a–c adopted a folded conformation and the protons
of the cis-substituent were close to the benzene ring in solution. On the other hand, N4-methylated 2,5-
DKPs (S,S)-7a–c seemed to prefer an extended conformation because the signals of protons of the cis-
substituent were not observed in higher magnetic fields such as that of (S,S)-5a–c. However, N,N’-
dimethylated 2,5-DKPs (S,S)-8a–c showed upfield chemical shifts of protons of the cis-substituent
compared to those of (S,S)-7a–c, which suggests the predominance of the folded conformation in (S,S)-
8a–c relative to (S,S)-7a–c.

ACCEPTED MANUSCRIPT
(a)
δ
3.60
HB
δ
3.87
HB
δ
0.49
O
O
δ
0.50
MeE
MeA
HA
MeB
Me
Me
N
N
δ
1.35–1.46
(S)
(S)
H
(S)
H
(S)
HN
HN
δ
0.85
O
O
H
H
H
H
(S,S)-5a
(S,S)-5c
δ
0.70
δ
3.74
δ
1.28–1.40
MeC
HE
D
O
HB
Me
0.75
δ
Me
N
HC
HD
1.03
(S)
H
(S)
δ
–0.22
HN
δ
O
H
H
(S,S)-5b
(b)
δ
3.75
HB
δ
3.78
HB
δ
1.06
O
O
MeA
HA
MeB
δ
1.01
MeE
NH
NH
H
(S)
δ
2.25
(S)
(S)
H
N
N
(S)
Me
Me
δ
1.19
O
O
H
H
H
H
(S,S)-7a
(S,S)-7c
δ
0.88
δ
3.73
δ
1.77–1.88
MeC
HE
D
O
HB
Me
δ
0.94
NH
HC
HD
1.33
(S)
δ
1.15
H
N
(S)
Me
δ
O
H
H
(S,S)-7b
(c)
δ
3.55
HB
δ
3.69
HB
δ
0.95
O
O
MeA
HA
MeB
δ
0.48
MeE
Me
Me
N
N
δ
1.61–1.73
(S)
(S)
H
(S)
H
(S)
N
N
Me
Me
δ
1.02
O
O
H
H
H
H
(S,S)-8a
(S,S)-8c
δ
0.72
δ
3.61
δ
1.66–1.75
MeC
HE
D
O
HB
Me
δ
0.83
Me
N
HC
HD
0.66
(S)
H
(S)
δ
0.28
N
Me
δ
O
H
H
(S,S)-8b
Fig. 4. Selected chemical shifts of (a) N1-methylated 2,5-DKPs (S,S)-5a–c, (b) N4-methylated 2,5-DKPs
(S,S)-7a–c, and (c) N,N’-dimethylated 2,5-DKPs (S,S)-8a–c in 1H NMR (500 MHz, CDCl3) analysis.
The vicinal coupling constants (J1 and J2) between the benzylic protons and the adjacent methine proton
of N1-methylated 2,5-DKPs (S,S)-5a–c, N4-methylated 2,5-DKPs (S,S)-7a–c, and N,N’-dimethylated 2,5-
DKPs (S,S)-8a–c are listed in Table 1. The results showed that similar values of the vicinal coupling

ACCEPTED MANUSCRIPT
constants (J1 = 3.7–4.7 Hz, J2 = 4.6–5.0 Hz) were observed for (S,S)-5a–c. Therefore, a gauche
relationship was presumed to exist in the CDCl3 solution between the two benzylic hydrogens and the
adjacent methine hydrogen, indicating that the N1-methylated 2,5-DKPs (S,S)-5a–c adopted the folded
conformation [14]. On the other hand, it was found that J1 and J2 were quite different (J1 = 3.2–3.9 Hz, J2
= 7.5–11.4 Hz) in N4-methylated 2,5-DKPs (S,S)-7a–c. The large difference in the vicinal coupling
constants suggested that one of the two benzylic hydrogens and the adjacent methine hydrogen could be
in a gauche relationship, while the other benzylic hydrogen and adjacent methine hydrogen were in an
anti-relationship [14]. Therefore, it was strongly suggested that the extended conformation of (S,S)-7a–c
was predominant in CDCl3 solution. The values of the vicinal coupling constants for N,N’-dimethylated
2,5-DKPs (S,S)-8a–c were similar to those for (S,S)-5a–c, and the preference for the folded conformation
of (S,S)-8a–c was estimated. It is remarkable that differences of the vicinal coupling constants between
(S,S)-5a–c and (S,S)-7a–c were correlated to the bulkiness of 3-alkyl substituents (methyl, isopropyl, and
isobutyl groups) of the corresponding 2,5-DKPs. In the case of (S,S)-5a and (S,S)-7a, a methine carbon is
directly connected to the 3-position of the 2,5-DKP ring and a larger difference of vicinal coupling
1
constants (J1 = 3.2 Hz, J2 = 11.4 Hz) is observed. Thus, the H NMR spectra of (S,S)-5a and (S,S)-7a
showed a good agreement with the conformations in the solid state established by X-ray crystallography,
as depicted in Figures 2 and 3.
Table 1
Vicinal coupling constants (J1 and J2) between the benzylic protons and the adjacent methine proton (a)
N1-methylated 2,5-DKPs (S,S)-5a–c, (b) N4-methylated 2,5-DKPs (S,S)-7a–c, and (c) N,N’-dimethylated
2,5-DKPs (S,S)-8a–c in 1H NMR (500 MHz, CDCl3) analysis.
(a), (c)
O
(b)
anti
Me
H
O
gauche
N
N
H
H
Ph
H
R
H
R
Ph
H
H
H
gauche
H
N
N
gauche
O
Me
O
R
a
: R = i-Pr
: R = i-Bu
: R = Me
(S,S)-5a
(S,S)-8a
-
-
c
c
: R = H
: R = Me
(S,S)-7a
-c
b
c

ACCEPTED MANUSCRIPT
Compounds
(S,S)-5a
(S,S)-5b
(S,S)-5c
(S,S)-7a
(S,S)-7b
(S,S)-7c
(S,S)-8a
(S,S)-8b
(S,S)-8c
R
J1
J2
5.0
4.6
4.6
11.4
8.7
7.5
7.7
4.9
4.4
i-Pr
i-Bu
Me
4.7
3.7
3.7
3.2
3.7
3.9
4.4
4.6
4.2
i-Pr
i-Bu
Me
i-Pr
i-Bu
Me
4. Conclusions
A series of novel N1- and/or N4-methylated 2,5-DKPs [(S,S)-5a–c, (S,S)-7a–c, and (S,S)-8a–c] have been
1
prepared and fully characterized. Their conformational aspects have been confirmed by H NMR
spectroscopy in solution. Single crystal X-ray structural analysis of (S,S)-5a and (S,S)-7a has also been
performed. In conclusion, N1-methylation and N1,N4-dimethylation have no influence on the folded
conformation of (S,S)-1a–c. However, we have succeeded in changing the folded conformation of (S,S)-
1a–c to the extended conformation by N4-methylation of the 2,5-DKP ring. This simple chemical
modification could be applied to access novel functional molecules such as organocatalysts and
pharmaceuticals based on the 2,5-DKP ring.
Acknowledgements
This work was supported in part by a Grant for the Regional Innovation Cluster Program (Global Type)
promoted by MEXT.
Appendix A. Supplementary data

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Deposition number CCDC-1429971 for compound (S,S)-5a and CCDC-1429972 for compound (S,S)-7a
contains the supplementary crystallographic data. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Highlights
•
•
A series of novel N-methylated and N,N’-dimethylated 2,5-DKPs were prepared.
1
Conformational analysis of prepared N-methylated and N,N’-dimethylated 2,5-DKPs based on H
NMR spectroscopy were performed.
•
•
Single crystal X-ray structural analysis of prepared N1-methylated and N4-methylated 2,5-DKPs
were performed.
N4-methylation of mono-benzylated 2,5-DKPs, which overcome the intramolecular CH/π
interaction, was found to change their conformation from folded to extended.