Photochemical synthesis of cyclic peptide models from phthalimido acetamides and phthaloyl dipeptide esters

Article abstract of DOI:10.1016/j.tetlet.2018.02.074

The photochemistry of several phthalimido acetamides and phthaloyl dipeptide esters has been investigated. Their photocyclization ability strongly depended on the substitution pattern of the amide linker group. While secondary amide-derived starting materials were largely unreactive, the corresponding tertiary amide-linked derivatives furnished the desired cyclic peptide model compounds in acceptable to good yields (41–80%). The structurally related ester-linked model derivatives also remained unreactive upon irradiation. Preferential hydrogen-abstraction from the E-cis-substituent is suggested to explain the observed differences in cyclization ability.

Full text of DOI:10.1016/j.tetlet.2018.02.074

Tetrahedron Letters xxx (2018) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Photochemical synthesis of cyclic peptide models from phthalimido
acetamides and phthaloyl dipeptide esters
a
a
a
a
a
b
Katrin Krüger , Virginia Lübke , Jonathan Pettinger , Luke Ashton , Laetitia Bonnet , Cherie A. Motti ,
c,1
a,
⇑
Johann Lex , Michael Oelgemöller
a
James Cook University, College of Science and Engineering, Townsville, Queensland 4811, Australia
Australian Institute of Marine Science (AIMS), Biomolecular Analysis Facility, Townsville, Queensland, Australia
University of Cologne, Institute of Organic Chemistry, Greinstrasse 4, D-50939 Cologne, Germany
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The photochemistry of several phthalimido acetamides and phthaloyl dipeptide esters has been investi-
gated. Their photocyclization ability strongly depended on the substitution pattern of the amide linker
group. While secondary amide-derived starting materials were largely unreactive, the corresponding ter-
tiary amide-linked derivatives furnished the desired cyclic peptide model compounds in acceptable to
good yields (41–80%). The structurally related ester-linked model derivatives also remained unreactive
upon irradiation. Preferential hydrogen-abstraction from the E-cis-substituent is suggested to explain
the observed differences in cyclization ability.
Received 6 January 2018
Revised 21 February 2018
Accepted 26 February 2018
Available online xxxx
Keywords:
Phthalimide
Photochemistry
Photocyclization
Hydrogen-abstraction
Cyclic peptide models
Ó 2018 Elsevier Ltd. All rights reserved.
Due to their biological importance, the photochemistry of
amino acids and peptides has been intensively studied over the
past decades. Photoinduced electron transfer cyclization reactions
increased solubility of the starting material and product in acetone
and the reduced toxicity of this co-solvent, all subsequent irradi-
ations were performed in a 1:1 acetone/water mixture.
7
1
of their related N-protected phthaloyl derivatives have likewise
The secondary amide linked phthalimido acetamides 4a and
4b underwent partial photodecomposition and remaining starting
materials were recovered in 88% and 85% yield, respectively, after
exhaustive irradiation for 48 h (Scheme 2). The rigid Z-trans-con-
formation and the likely formation of a solvent shell around the
secondary amide linker in the starting materials 4a and 4b, in
combination with the required E-cis-conformation in the prospec-
tive products 5a and 5b prevent successful cyclization.8
Compound 5a is also known to be unstable and decomposes by
2
been widely explored. In contrast, the photocyclization of phthal-
3
imido acetamides by hydrogen-abstraction are rather rare. The
photochemistry of a series of phthalimido acetamides and phtha-
loyl dipeptide esters, both readily accessible by standard peptide
4
coupling, was thus explored.
To find the most suitable solvent system, the photocyclization
of phthaloyl dipeptide ester 1a was selected as a model reaction
–10
(Scheme 1; Table 1). Photoirradiation for 3 h with 300 nm light fur-
5
a,11
nished regioisomer 2a as the sole product. The presence of water
had a significant impact on the conversion rates, which increased
with increasing amounts of water. Previous studies have shown
that the presence of water alters the photophysical properties
through hydrogen-bonding and subsequently enhances photo-
polymerization.
In contrast, tertiary phthalimido acetamides underwent suc-
cessful cyclization reactions. A similar conformationally controlled
photocyclization of secondary vs. tertiary aryl enamides has been
1
2
reported by Rigby and co-workers. Irradiation times varied sig-
nificantly depending on the N, N-disubstitution pattern and amino
acid linker (Scheme 3; Table 2).
5
chemical reactivity. No significant differences between acetone
and acetonitrile mixtures were found in terms of conversion rates,
although acetone is known to function as a triplet-sensitizer, while
The symmetrical glycine-derived tertiary phthalimido aceta-
mides 6a-e furnished the desired cyclic peptide models 7a-e in
6
irradiation in acetonitrile operates via direct excitation. Due to the
1
3
moderate to good yields (51–75%). The phthalimides 6b, 6d, 6e
and 6i furthermore resulted in the formation of diastereoisomeric
⇑
Corresponding author.
E-mail address: michael.oelgemoeller@jcu.edu.au (M. Oelgemöller).
Deceased.
mixtures. Likewise, the branched
a-amino acid containing starting
materials 6f and 6g yielded the corresponding cyclization products
1
https://doi.org/10.1016/j.tetlet.2018.02.074
040-4039/Ó 2018 Elsevier Ltd. All rights reserved.
0
Please cite this article in press as: Krüger K., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.02.074

2
K. Krüger et al. / Tetrahedron Letters xxx (2018) xxx–xxx
7
f and 7g as diastereoisomeric mixtures in 45% and 41% yield,
respectively. No competing hydrogen-abstraction from the amino
acid side-chain was observed for the alanine-derived 6f (within
1
4
the detection limit of NMR spectroscopy). The b-alanine-based
phthalimido acetamides 6h and 6i likewise cyclized successfully
and gave the tricyclic compounds 7h and 7i in acceptable yields
of 53% and 49%, respectively. In contrast, longer phthalimido acet-
amides (n>2) are known to undergo hydrogen-abstraction from the
Scheme 1. Solvent study using phthaloyl dipeptide ester 1a.
3
a
Table 1
internal
c-position.
For all cyclization products, the newly
Crude product compositions for the photocyclization of 1a in different solvent
systems ([1a] = 12.5 mM).
formed CAOH group was observed as a characteristic singlet
between 80 and 90 ppm in the C NMR spectra. Due to the
13
Entry
Solvent system
vol%)
Composition (%)a
absence of a suitable abstractable hydrogen atom, the N, N-
diphenyl derivative 7j remained largely photostable and was
recovered in 81% yield.
Three asymmetrically substituted tertiary phthalimido aceta-
mides 8a–c were furthermore investigated (Scheme 4) and a
(
1a
2a
3a
a
b
c
d
e
f
acetone (100)
95
69
20
94
74
17
5
n.d.b
n.d.b
acetone/water (83:17)
acetone/water (50:50)
acetonitrile (1 0 0)
31
80
6
26
83
b
n.d.
n.d.b
correlation between photocyclization efficiency and amide
b
8,15
acetonitrile/water (83:17)
n.d.
conformations was again noted.
Amide 8a presumably exists
acetonitrile/water (50:50)c
n.d.b
exclusively in the E-cis-conformation due to favourable CH-p-
a
After 3 h irradiation in a test tube (18 mL) as determined by 1H NMR spectro-
16
interactions. As a result, compound 8a remained largely unreac-
scopic analysis of the crude mixture (±3%).
tive even after prolonged irradiation for 24 h and was recovered in
b
n.d. = not detected (<3%).
Solution turbid during irradiation.
8
3% yield. In contrast, the tertiary amides 8b and 8c populate both
c
amide conformers and irradiation gave the regioisomeric cycliza-
tion products 9/10b and 9/10c, respectively.
Scheme 2. Attempted photocyclization of secondary phthalimido acetamides 4a
and 4b.
Scheme 3. Photocyclization of symmetrical tertiary phthalimido acetamides 6a–j.
Scheme 4. Photocyclization of asymmetrical tertiary phthalimido acetamides 8a–c.
Table 2
Experimental details for the photocyclization of symmetrically substituted tertiary phthalimido acetamides 6a–j (c = 10 mM).
Entry
n
R1
R2
R3
R4
Time (h)
Diastereoisomeric ratio (%)a
Yield 7 (%)
a
b
c
d
e
f
g
h
i
1
1
1
1
1
1
1
2
2
1
H
H
H
H
H
H
H
CH
H
(CH
H
H
H
H
CH
3
15
24
36
15
7
18
18
7
—
84:16
—
65:35
50:50
54:46
57:43
—
51
58
57
66
75
45
41
53
CH
CH
Ph
H
H
H
H
CH
Ph
3
C
2
H
5
3
3
i-Pr
CH
2
Ph
H
2
)
3
(S)–CH
(rac)-Ph
H
H
H
3
CH
3
CH
3
CH
3
3
C
2
H
5
7
24
80:20
—
49
n.r. (81%c)
b
j
Ph
a
b
c
Diastereoisomeric ratio determined by 1H NMR spectroscopic analysis of the crude mixture (±3%).
n.r. = no reaction.
Recovered starting material.
Please cite this article in press as: Krüger K., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.02.074

K. Krüger et al. / Tetrahedron Letters xxx (2018) xxx–xxx
3
The N-methyl, N-benzylated derivative 8b predominantly
cyclized at the N-methyl position to 9b, whereas the isoquino-
line-derived starting material 8c preferentially reacted at its ben-
zylic methylene position to 9c. The regioselectivity thus likely
depends on rotational restrictions, conformational and steric fac-
tors as well as radical stability.1
5,17
Similar regioselectivities have
1
8
been described for other photocyclizations to d-lactams.
The photocyclization protocol was subsequently applied to the
related phthaloyl dipeptide esters. In line with the corresponding
phthalimido acetamides, the secondary phthaloyl dipeptide esters
Fig. 1. Single crystal structure of 2a.
1
1a and 11b did not cyclize, even upon exhaustive irradiation for
2
4 h, and were recovered in 88% and 90% yield, respectively
(Scheme 5).
Photocyclization was again restored for the tertiary phthaloyl
dipeptide esters 1a–d (Scheme 6).19 The sarcosine-based starting
materials 1a and 1b regioselectively cyclized at the N-methyl posi-
tion and only gave the cyclic peptide model compounds 2a and 2b
in isolated yields of 53% and 65%, respectively. The symmetrical
diester compound 1c produced a diastereoisomeric mixture of
cyclization product 2c. The N-benzylated substrate 1d furnished
a pair of regioisomers 2d and 3d, although cyclization preferen-
tially occurred at the N-benzyl position. Within the detection limit
of NMR spectroscopy, both regioisomers 2d and 3d were formed as
single diastereoisomers.
Scheme 7. Simplified mechanistic scenario.
The structure of 2a was furthermore determined by single crys-
tal X-ray structure analysis (Fig. 1).20 The compound forms a net-
work via hydrogen bonding between the CAOH and the amide
C@O group of independent molecules (see Supplementary data).
In line with previous mechanistic studies,3 the photocycliza-
tion is proposed to operate via triplet sensitization by acetone
,6
Scheme 8. Attempted photocyclization of phthalimido esters 13a and 13b.
and subsequent hydrogen transfer from the remote N-sub-
2
1
stituent.
An alternative photoinduced electron-transfer from
2
2
the amide linker is energetically not feasible. The strong depen-
dence on N, N-disubstitution suggests that successful photocycliza-
tion occurs predominantly from the E-cis-substituent due to its
closer proximity to the phthalimide group (Scheme 7), as found
in the X-ray structures of several related derivatives. The impor-
tance of conformational flexibility and preorientation that generate
favorable geometries for successful photocyclizations have been
2
3
Scheme 5. Attempted photocyclization of secondary phthaloyl dipeptide esters 11a
and 11b.
2
e,17b
recognized earlier.
This assumption was furthermore confirmed by the lack of pho-
tocyclization efficiency for the corresponding ester-linked ana-
logues 13a and 13b (Scheme 8), in which the ester group
exclusively adopts the trans-conformation as confirmed by X-ray
2
4
structure analysis of related esters. Both compounds were recov-
ered in 92% and 80% yield, respectively.
In conclusion, tertiary phthalimido acetamides and phthaloyl
dipeptide esters undergo photocyclization via hydrogen-abstrac-
tion. The cyclization efficiency depends on the N, N-disubstitution
pattern and amino acid linker. In contrast, the corresponding sec-
ondary analogues as well as ester linked derivatives remained lar-
gely photostable.
Acknowledgements
This research was financially supported by James Cook Univer-
sity (JCU-CRIG 2011) and the Australian Research Council (ARC,
Discovery Project, DP130100794). The authors thank Assoc.-Prof.
Bruce Bowden and Dr. Murray Davies (JCU) for technical assistance.
A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.tetlet.2018.02.074.
Scheme 6. Photocyclization of tertiary phthaloyl dipeptide esters 1a–d.
Please cite this article in press as: Krüger K., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.02.074

4
K. Krüger et al. / Tetrahedron Letters xxx (2018) xxx–xxx
7
1
.73-7.78 (br m, 4 H, Harom). 13C NMR (75 MHz, acetone-d
C, NCH ), 41.7 (t, 1C, CH ), 57.5 (t, 1C, CH ), 83.9 (s, 1C, COH), 123.1 (d, 1C,
6
): d (ppm) = 34.9 (q,
References
3
2
2
CHarom), 123.9 (d, 1C, CHarom), 130.6 (d, 1C, CHarom), 132.4 (s, 1C, Cqarom), 133.1
1
. (a) Lundeen RA, Janssen EM-L, Chu C, McNeill K. Chimia. 2014;68:812–817;
(
(
(
d, 1C, CHarom), 147.1 (s, 1C, Cqarom), 164.3 (s, 1C, C@O), 165.8 (s, 1C, C@O). IR
(
(
(
b) Hansen SG, Skrydstrup T. Top Curr Chem. 2005;263:135–162;
c) Schaich KM, Pryor WA. CRC Crit Rev Food Sci Nutr. 1980;13:131–159;
d) Vladimirov YA, Roshchupkin DI, Fesenko EEJ. Photochem Photobiol.
À1
ATR):
m
(cm ) = 3201, 2923, 1698, 1624, 1409, 1136, 946, 873, 685. HR-MS
+
+
12 2 3
ESI, positive ions): Calcd. [M + Na] : 255.0740 for C12H N O + Na . Found [M
+
+
Na] : 255.0721.
4. (a) Griesbeck AG. Chimia. 1998;52:272–283;
b) Griesbeck AG. Liebigs Ann Chem. 1996;1951–1955.
5. (a) Laursen JS, Engel-Andreasen J, Fristrup P, Harris P, Olsen CA. J Am Chem Soc.
013;135:2835–2844;
1
970;11:227–246.
. (a) Liu Y, Tan G, Wei S, et al. Chin J Org Chem. 2011;31:480–485;
b) Jin Y, Tan G, Zhao J, Zhang P, Wang B, Zheng P. Chin
010;30:860–864;
1
1
2
(
(
2
J Org Chem.
2
(
(
c) Yoon UC, Jin YX, Oh SW, et al. J Am Chem Soc. 2003;125:10664–10671;
d) Griesbeck AG, Heinrich T, Oelgemöller M, Molis A, Lex J. J Am Chem Soc.
(
(
b) Sui Q, Borchardt D, Rabenstein DL. J Am Chem Soc. 2007;129:12042–12048;
c) Taylor CM, Hardré R, Edwards PJB, Park JH. Org. Lett.. 2003;5:4413–4416.
2
(
002;124:10972–10973;
e) Griesbeck AG, Heinrich T, Oelgemöller M, Molis A, Heidtmann A. Helv Chim
Acta. 2002;85:4561–4578;
f) Griesbeck AG, Henz A, Kramer W, et al. Helv Chim Acta. 1997;80:912–933.
. (a) Takechi H, Machida M, Kanaoka Y. Liebigs Ann Chem. 1986;859–868;
b) Machida M, Takechi H, Shishido Y, Kanaoka Y. Synthesis. 1982;1078–1080.
1
1
6. Ozawa T, Isoda Y, Watanabe H, et al. Magn Reson Chem. 1997;35:323–332.
7. (a) Menon AS, Henry DJ, Bally T, Radom L. Org Biomol Chem.
2
011;9:3636–3657;
b) Curran DP, Porter NA, Giese B. Stereochemistry of Radical
Reactions. Weinheim: Wiley-VCH; 1996;
c) Wagner PJ. Acc Chem Res. 1983;16:461–467.
8. (a) Griesbeck AG, Heckroth H, Schmickler H. Tetraedron Lett.
999;40:3137–3140;
b) Lindemann U, Reck G, Wulff-Molder D, Wessig P. Tetrahedron.
998;54:2529–2544.
9. Selected physical and spectral data for (11b-hydroxy-3,7-dioxo-4,5,7,11b-
tetrahydro-1H,3H-[1,4]diazepino[2,1- ]isoindol-2-yl)acetic acid methyl ester
2b): yellowish solid, R (EtOAc/n-hexane = 10/1) = 0.31, mp 194–196 °C.
NMR (300 MHz, acetone-d ): d (ppm) = 2.66 (m, 1 H, CH ), 2.85 (ddd, J = 14.1,
1.0, 2.7 Hz, 1 H, CH ), 3.24 (ddd, J = 14.1, 11.0, 2.7 Hz, 1 H, CH ), 3.67 (s, 3 H,
OCH ), 3.88 (d, J = 15.4 Hz, 1 H, CH CO ), 4.05 (d, J = 17.6 Hz, 1 H, NCH ), 4.08 (d,
J = 15.4 Hz, 1 H, CH CO ), 4.35 (ddd, J = 14.1, 11.0, 2.7 Hz, 1 H, CH ), 4.48 (d, J =
7.6 Hz, 1 H, NCH ), 6.54 (s, 1 H, OH), 7.48-7.85 (br m, 4 H, Harom). C NMR (75
MHz, acetone-d ): d (ppm) = 33.2 (t, 1 C, CH ), 37.9 (t, 1 C, NCH ), 51.4 (q, 1 C,
OCH ), 52.2 (t, 1 C, CH ), 57.8 (t, 1 C, CH ), 87.6 (s, 1 C, COH), 123.5 (d, 1 C,
CHarom), 123.6 (d, 1 C, CHarom), 130.6 (d, 1 C, CHarom), 132.7 (s, 1 C, Cqarom), 132.9
d, 1 C, CHarom), 146.6 (s, 1 C, Cqarom), 165.3 (s, 1 C, C@O), 171.4 (s, 1 C, C@O),
(
(
3
(
(
4
. Montalbetti CAGN, Falque V. Tetrahedron. 2005;61:10827–10852.
1
1
5
. (a) Yoon UC, Oh SW, Lee SM, Cho SJ, Gamlin J, Mariano PS. J Org Chem.
1
(
1
1
(
999;64:4411–4418;
b) Almeida FCL, Toscano VG, dos Santos O, Politi MJ, Neumann MG, Fo PBJ.
Photochem Photobiol A: Chem. 1991;58:289–294;
c) Folho PB, Toscano VG, Politi MJJ. Photochem Photobiol A: Chem.
988;43:51–58.
. (a) Görner H, Oelgemöller M, Griesbeck AG.
002;106:1458–1464;
b) Görner H, Griesbeck AG, Heinrich T, Kramer W, Oelgemöller M. Chem Eur J.
001;7:1530–1538;
c) Griesbeck AG, Görner HJ. Photochem Photobiol A: Chem. 1999;129:111–119.
. Henderson RK, Jiménez-González C, Constable DJC, et al. Green Chem.
011;13:854–862.
. (a) Joseph AP, Srinivasan N, de Brevern AG. Amino Acids. 2012;43:1369–1381;
(
1
a
1
(
f
H
6
J
Phys Chem A.
6
2
2
(
2
(
1
2
2
3
2
2
2
2
2
2
13
1
2
7
8
6
2
2
2
3
2
2
(
(
b) Avalos M, Babiano R, Barneto JL, et al. J Org Chem. 2001;66:7275–7282;
c) Ramachandra GN, Sasisekharan V. Adv Protein Chem. 1968;23:283–437.
(
–1
1
1
C
74.0 (s, 1C, C@O). IR (ATR):
m (cm ) = 3380, 2922, 1741, 1686, 1646, 1358,
9
. (a) Zielkiewicz J. Phys Chem Chem Phys. 2000;2:2925–2932;
b) Zielkiewicz JJ. J Chem Soc, Faraday Trans. 1998;1713–1719.
0. (a) Endo Y, Ohno M, Hirano M, Itai A, Shudo K.
996;118:1841–1855;
b) Ramakrishnan C, Paul PKC, Ramnarayan K. J Biosci. 1985;8:239–251.
+
033, 842, 764, 691. HR-MS (ESI, positive ions): Calcd. [M + Na] : 327.0951
H N O + Na . Found [M + Na] : 327.0948.
16 2 5
(
+
+
15
1
J Am Chem Soc.
2
0. Crystal data for 2a (CCDC-1814682): colourless prisms (from acetone), mp
1
(
1
=
=
2
79–180 °C; C14
H N O , FW = 290.27 g/mol; monoclinic, space group P2 /c; a
1
4
2
5
1
3
10.531(1), b = 12.444(1), c = 11.142(1) Å; b = 109.90(1)°; V = 1372.9(2) Å ; Z
1
1
1. Tan G, Wei S, Zhao J, et al. Imag Sci Photochem. 2010;28:131–136.
2. (a) Rigby JH, Maharoof USM, Mateo ME. J Am Chem Soc. 2000;122:6624–6628;
3
4; dcalc = 1.404 g/cm ; R = 0.0364, R
(F).
w
= 0.0948 for 2604 reflections having F >
r
(
b) Rigby JH, Gupta V. Synlett. 1995;547–548.
2
1. (a) Dala A, Khanna R, Kumar D, Jindal P, Chaudhary A, Kamboj RC. Curr Org
Chem. 2015;19:2156–2195;
1
3. General irradiation procedure: A solution of the phthaloylimido acetamide or
phthaloyl dipeptide ester (1.5 mmol) in acetone/water (150 mL, 1:1 vol%) was
(
b) Hasegawa T, Yamazaki Y, Yoshioka M. Trends Photochem Photobiol.
997;4:27–41.
2. (a) Gallagher S, Hatoum F, Zientek N, Oelgemöller M. Tetrahedron Lett.
010;51:3639–3641;
b) Oelgemöller M, Griesbeck AGJ. Photochem Photobiol C: Rev. 2002;3:109–127.
3. (a) Singh AK, Kishan R, Bahadur V, et al. J Phys Org Chem. 2014;27:490–497;
b) Singh AK, Kishan R, Vijayan N, et al. RSC Adv. 2013;3:14750–14756.
4. (a) Ulbrich D, Daniliuc CG, Haufe G. J Fluorine Chem. 2016;188:65–75;
b) Shendage DM, Fröhlich R, Haufe G. Org Lett. 2004;6:3675–3678;
2
purged with a slow stream of N for 10 min and subsequently irradiated
1
(
Rayonet Photochemical Reactor RPR-200; k = 300 ± 20 nm) at 15–20 °C in a
2
Pyrex Schlenk tube (k ꢀ 300 nm) equipped with a cold finger. The reaction
progress was followed by TLC analysis. Most of the acetone was evaporated and
the reaction mixture was extracted with EtOAc (3 Â 30 mL). The combined
2
(
2
2
organic layers were washed with brine (30 mL), dried over MgSO
4
and
(
2
evaporated. The products were obtained after column chromatography (SiO ;
eluent: EtOAc, mixtures of EtOAc/n-hexane or dichloromethane/methanol).
(
Selected physical and spectral data for 4a-hydroxy-3-methyl-,4,4a-dihydro-
3
a
(c) Xu Q, Wang G, Wang X, Wu T, Pan X, Chan ASC, Yang T. Tetrahedron: Asymm.
2000;11:2309–2314;
3
1
H-3,9a-diaza-fluorene-2,9-dione (7a): colorless solid, R
f
(EtOAc) = 0.52, mp
98-199 °C (ref: 226–228 °C). H NMR (300 MHz, acetone-d ): d (ppm) = 2.98
), 3.52 (d, J = 12.3 Hz, 1 H, NCH ), 3.80 (d, J = 17.9 Hz, 1 H, NCH ),
.89 (d, J = 12.3 Hz, 1 H, NCH ), 4.42 (d, J = 17.9 Hz, 1 H, NCH ), 6.47 (s, 1 H, OH),
3a
1
6
(
d) Peters K, Peters E-M, Griesbeck AG, Oelgemöller M, Bartoschek AZ.
(
s, 3 H, NCH
3
2
2
Kristallogr NCS. 1999;214:107–108.
3
2
2
Please cite this article in press as: Krüger K., et al. Tetrahedron Lett. (2018), https://doi.org/10.1016/j.tetlet.2018.02.074