Alicyclic β- And γ-amino acids: Useful scaffolds for the stereocontrolled access to amino acid-based carbocyclic nucleoside analogs

Article abstract of DOI:10.3390/molecules24010161

Stereocontrolled synthesis of some amino acid-based carbocyclic nucleoside analogs containing ring C=C bond has been performed on β- and γ-lactam basis. Key steps were N-arylation of readily available β- or γ-lactam-derived amino ester isomers and amino a

Full text of DOI:10.3390/molecules24010161

molecules
Article
Alicyclic β- and γ-Amino Acids: Useful Scaffolds for
the Stereocontrolled Access to Amino Acid-Based
Carbocyclic Nucleoside Analogs
Attila Márió Remete 1,2 and Loránd Kiss 1,2,
*
1
Institute of Pharmaceutical Chemistry, University of Szeged, Eötvös u. 6, H-6720 Szeged, Hungary;
remete.attila@pharm.u-szeged.hu
Interdisciplinary Excellence Centre, Institute of Pharmaceutical Chemistry, University of Szeged, Eötvös u. 6,
H-6720 Szeged, Hungary
2
*
Correspondence: kiss.lorand@pharm.u-szeged.hu; Tel.: +36-308904092
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 10 December 2018; Accepted: 23 December 2018; Published: 3 January 2019
Abstract: Stereocontrolled synthesis of some amino acid-based carbocyclic nucleoside analogs
containing ring C=C bond has been performed on
β- and γ-lactam basis. Key steps were
N-arylation of readily available - or -lactam-derived amino ester isomers and amino alcohols with
β
γ
5-amino-4,6-dichloropyrimidine; ring closure of the formed adduct with HC(OMe)3 and nucleophilic
displacement of chlorine with various N-nucleophiles in the resulting 6-chloropurine moiety.
Keywords: amino acids; nucleoside analogs; carbocycles; lactams
1. Introduction and Aims
In carbocyclic nucleoside analogs, a methylene group replaces the oxygen atom in the
carbohydrate ring, thereby increasing stability towards hydrolases and phosphorylases. The synthesis
of these molecules is an area of considerable interest to medicinal chemistry, thanks to their bioactivity.
Within natural products, neplanocin A is an antitumor antibiotic, while aristeromycin has antibacterial
and antiviral activities. With respect to synthetic compounds, (-)-carbovir (
anti-HIV activity (Scheme 1) while entecavir inhibits the hepatitis B virus [
analogs with a 6-membered ring received less attention. In their case, antiviral activity usually
requires the presence of a C=C bond in the ring [ ] (see and ), enabling the base to occupy a
pseudoaxial position [ ], but some (2-aminocyclohexyl)methanol derivatives (for example, ]) also
exhibit bioactivity (Scheme 1).
1) and abacavir (
2) show
1–6]. Carbocyclic nucleoside
1–
3,5
,7,8
3
4
1,5
5 [9
Scheme 1. Some bioactive nucleoside analogs.
Molecules 2019, 24, 161; doi:10.3390/molecules24010161
www.mdpi.com/journal/molecules

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Cyclic
be found in natural products, such as peptidyl nucleoside antibiotics amipurimycin (
), blasticidin S ( ) or gougerotin ( ), and related analogous derivatives (Scheme 2) [15
latter three nucleoside analogs, the sugar ring was replaced with a cyclic -amino acid unit. Cyclic
-amino acids are also promising building blocks of new bioactive peptides [14 21] and many simple
β
-amino acids have gained significant attention in the last few decades [10
–
14]. They can
6
), chryscandin
17]. In the
(7
8
9
–
β
β
–
representatives show relevant biological activity (Scheme 3), such as the analgesic drug tilidine (10) or
antifungal antibiotics cispentacin and icofungipen (11) [10–13].
Highly-functionalized cyclic γ-amino acid derivatives possessing multiple stereogenic centers are
also of considerable importance in drug research. Neuraminidase inhibitors Peramivir (12, Scheme 3),
Zanamivir and Oseltamivir and their modified analogs are used in the treatment of influenza [22],
while Gabapentin [23] and CPP-115 [24] (13, Scheme 3) are anticonvulsant drugs.
Scheme 2. Some bioactive β-amino acid-based nucleoside analogs.
Scheme 3. Examples of bioactive cyclic amino acid derivatives.
2. Results and Discussion
Taking into account the importance of carbocyclic nucleoside analogs and the bioactivity of
peptidyl nucleoside antibiotics containing
nucleoside analogs with an amino acid moiety on a
the first synthesis of carbovir from unsaturated
The synthesis of some 6-membered carbocyclic nucleoside analogs containing
also planned.
β
-amino acids, our aim was the synthesis of new carbocyclic
- and -lactam basis. This pathway is similar to
27].
-amino alcohol was
β
γ
γ
-lactam (±)-14 (also known as Vince lactam) [25
–
γ

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Our synthetic work started with the opening of the heteroring of racemic Vince lactam (±)-14
[28].
Construction of the nucleobase part on the resulting amino ester (±)-15 was accomplished in three
steps. First, compound (±)-15 was subjected to N-arylation with 5-amino-4,6-dichloropyrimidine
to furnish (±)-16. This process was accompanied by C=C bond migration thanks to the basic
conditions, enabling the formation of a more stable conjugated π-system. Then, reaction with trimethyl
orthoformate generated the second heteroring. The remaining chlorine atom of the obtained nucleoside
analog (±)-17 was then replaced with N-nucleophiles to obtain adenosine analogs (±)-18 (±)-19
,
and (±)-20 (Scheme 4). It is worth to note that compound (±)-19 contains a cyclopropylamino group
similar to abacavir, while the azido group of compound (±)-20 enables many further transformations
(e.g., triazole formation).
Scheme 4. Synthesis of cyclic γ-amino acid-based nucleoside analogs.
We continued our synthetic work with ethyl cis
-lactam (±)-21 29 30]. Lactam ring opening, construction of the nucleobase moiety, and aromatic
-amino
32], azidonucleoside (±)-31 was prepared in a similar way (Schemes 5
β
-amino ester hydrochloride (±)-22 obtained from
β
[ ,
nucleophilic substitution resulted in nucleoside analogs (±)-25 and (±)-26. From ethyl trans
β
ester hydrochloride (±)-28
[31,
and 6).

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Scheme 5. Synthesis of
β-amino acid-based nucleoside analogs from ethyl cis-2-aminocyclohex-
4-enecarboxylate hydrochloride (±)-22.
Scheme 6. Synthesis of
β-amino acid-based nucleoside analogs from ethyl trans-2-aminocyclohex-
4-enecarboxylate hydrochloride (±)-28.
Note that the synthetic protocol took place with stereocontrol in both cases. Since the configuration
of the chiral centers are not affected during the syntheses, their integrity is conserved and therefore,
the cis-amino acid starting material led to the corresponding carbanucleoside analog in which the
relative configuration of the groups is cis, while the trans-amino acid provided the carbocyclic
nucleobase analog with trans relative steric arrangement of the ester and the heterocycle.
Analogous treatment of ethyl cis-2-aminocyclohex-3-enecarboxylate hydrochloride (±)-33
(a regioisomer of (±)-22), obtained from
β
-lactam (±)-32
[
33,
34], resulted in nucleoside analog (±)-35
,
the C=C regioisomer of compound (±)-24 (Scheme 7).

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Scheme 7. Synthesis of
3-enecarboxylate hydrochloride (±)-33.
β
-amino acid-based nucleoside analog (±)-35 from ethyl cis-2-aminocyclohex-
In order to synthesize compounds with a five-membered carbocycle, the strategy was also
extended to
β
-lactam (±)-36. Nucleoside analog (±)-39 was obtained successfully using the protocol
described above for the six-membered analogs, although both nucleobase construction steps had lower
yields (Scheme 8).
Scheme 8. Synthesis of
β-amino acid-based nucleoside analog (±)-39 with a 5-membered carbocycle.
Taking into account the bioactivity of compounds
3
,
(±)-4 and (±)-5, the synthesis of similar
molecules was attempted. Reduction of
-amino alcohols (±)-41 and (±)-45, which were further reacted with 5-amino-4,6-dichloropyrimidine.
β
-amino acids (±)-40 and (±)-44 with LiAlH4 [32] afforded
γ
Ring closing with trimethyl orthoformate in the last step yielded, through stereocontrol, nucleoside
analogs (±)-43 and (±)-47 (Scheme 9). These compounds show high structural similarity to bioactive
compound (±)-5.

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Scheme 9. Synthesis of unsaturated carbanucleoside isomers (±)-43 and (±)-47 analogs of carbocyclic
nucleoside (±)-5.
3. Conclusions and Outlook
A stereocontrolled synthetic pathway was developed to prepare new carbocyclic nucleoside
analogs containing a ring olefin bond with a -amino acid, -amino acid or -amino alcohol
moiety from readily available - and -lactams (across the amino acid isomers). The structure
β
γ
γ
β
γ
of the starting cycloalkene amino acids determined the configuration of the stereogenic centers
of the products. 6- Nucleoside analogs containing the chloropurine moiety proved to be useful
intermediates in various reactions with nucleophiles to access substituted nucleobases. Taking into
consideration our widespread experiences in selective and controlled functionalization of versatile
unsaturated cyclic amino acid derivatives [35–38], further studies in order to investigate the possible
functionalization of the ring olefin bond of product nucleoside analogs are currently being investigated
in our laboratory. Furthermore, based on our experiences in enzymatic resolution of various bicyclic
β- and γ-lactams [39,40], as well as on enzymatic ester hydrolysis methodologies [41], synthesis of
enantiomerically pure substances will be performed.
4. Materials and Methods
4.1. General Information
Chemicals were purchased from Sigma–Aldrich (Budapest, Hungary). Solvents were used as
received from the suppliers. Amino ester hydrochlorides (±)-15
[
28], (±)-22
[
29
,
30], (±)-28
[31,32],
(±)-33
[
33
,
34], (±)-37
[
42] and
γ
-amino alcohols (±)-41
,
(±)-45
[
32] were synthesized according
to literature. The 1H-NMR and 13C-NMR spectra of all new compounds are available in
Supplementary Materials.
4.1.1. General Procedure for N-Arylation of Amino Ester Hydrochlorides with
5-Amino-2,6-Dichloropyrimidine
To a solution of the amino ester hydrochloride (10 mmoles) in EtOH (30 mL), 5-amino-2,
6-dichloropyrimidine (10 mmoles) and Et3N (30 mmoles) were added, then the mixture was treated at
reflux temperature for 20 h. After cooling to room temperature, the reaction mixture was concentrated

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under reduced pressure and the residue was taken up in EtOAc (100 mL). The organic layer was
washed with water (3 50 mL), dried with Na2SO4, and concentrated under reduced pressure.
The crude product was purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 2:1).
×
4.1.2. General Procedure for N-Arylation of
γ-Amino Alcohols with 5-Amino-2,6-Dichloropyrimidine
To a solution of the -amino alcohol (8 mmoles) in EtOH (25 mL), 5-amino-2,6-dichloropyrimidine
γ
(8 mmoles) and Et3N (24 mmoles) were added, then the mixture was kept at boiling temperature
for 20 h. After cooling to room temperature, the reaction mixture was concentrated under reduced
pressure and the residue was taken up in EtOAc (100 mL). The organic layer was washed with water
(3
×
40 mL), dried with Na2SO4 and concentrated under reduced pressure. The crude product was
purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 1:2).
4.1.3. General Procedure for the Formation of the Purine Skeleton of Amino Ester Nucleoside Analogs
To a solution of amino ester (2 mmoles) in trimethyl orthoformate (5 mL), a catalytic amount of
◦
methanesulfonic acid or p-TsOH (30 mg) was added. After stirring at 20 C for 6 h, the reaction mixture
was diluted with EtOAc (25 mL) and washed with saturated aqueous NaCl solution (3
The organic phase was dried with Na2SO4 and concentrated under reduced pressure. The crude
×
15 mL).
product was purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 1:1).
4.1.4. General Procedure for the Formation of the Purine Skeleton of Amino Alcohol
Nucleoside Analogs
To a solution of amino alcohol nucleoside analog (1 mmol) in trimethyl orthoformate (4 mL),
a catalytic amount of p-TsOH (20 mg) was added. After stirring at 20 ◦C for 6 h, the reaction mixture
was diluted with EtOAc (20 mL) and washed with saturated aqueous NaCl solution (3
organic phase was dried with Na2SO4 and concentrated under reduced pressure. The crude product
×
15 mL). The
was purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 1:2).
4.1.5. General Procedure for the Introduction of the Azido Group
To a solution of 6-chloropurinyl nucleoside analog (150 mg) in THF/H2O (10 mL, 4:1), sodium
azide (4 eq.), acetic acid (3 drops), and Et3N (4 drops) were added. After heating at reflux temperature
for 20 h, the reaction mixture was diluted with EtOAc (20 mL) and washed with water (2
The organic phase was dried with Na2SO4 and concentrated under reduced pressure. The crude
×
15 mL).
product was purified by column chromatography on silica gel (eluent: n-hexane-EtOAc 1:2).
4.1.6. General Procedure for the Introduction of the Cyclopropylamino Group
To a solution of 6-chloropurinyl nucleoside analog (150 mg) in EtOH (10 mL), cyclopropylamine
(4 eq.) was added. After the mixture was kept at boiling temperature for 12 h, the reaction mixture was
concentrated under reduced pressure. The crude product was purified by column chromatography on
silica gel (eluent: n-hexane-EtOAc 1:1).
4.2. Synthesis of Methylamino Compound (±)-18
To a solution of 6-chloropurinyl nucleoside analogue (±)-17 (150 mg) in EtOH (10 mL), MeNH2
(4 eq.) was added. After heating under reflux for 20 h, the reaction mixture was concentrated under
reduced pressure. The crude product was purified by column chromatography on silica gel (eluent:
n-hexane-EtOAc 1:1).

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Ethyl (S*)-4-((5-amino-6-chloropyrimidin-4-yl)amino)cyclopent-1-ene-1-carboxylate, (±)-16.
Brownish white solid, m.p. 121–123 ◦C, 40%; 1H-NMR (CDCl3, 400 MHz):
δ
(ppm) = 1.30 (t, 3H, CH3,
J = 7.14 Hz), 2.43–2.59 (m, 2H, CH2), 3.00–3.16 (m, 2H, CH2), 3.44 (brs, 2H, NH2), 4.17–4.25 (m, 2H,
OCH2), 4.79–4.86 (m, 1H, H-4), 5.08 (d, 1H, N-H, J = 5.76 Hz), 6.76-6.78 (m, 1H, H-2), 8.08 (s, 1H, Ar-H);
13C-NMR (DMSO, 100 MHz):
164.9; MS (ES, pos) m/z = 283 (M + 1).
δ (ppm) = 15.0, 39.3, 41.2, 51.3, 60.7, 124.6, 135.1, 137.7, 142.6, 146.4, 152.0,
Ethyl (S*)-4-(6-chloro-9H-purin-9-yl)cyclopent-1-ene-1-carboxylate, (±)-17.
◦
1
Yellowish white solid, m.p. 83–85 C, 81%; H-NMR (DMSO, 400 MHz):
δ = 1.20 (t, 3H, CH3,
J = 7.08 Hz), 2.93–3.05 (m, 2H, CH2), 3.06–3.20 (m, 2H, CH2), 4.09–4.17 (m, 2H, OCH2), 5.38–5.47
(m, 1H, H-4), 6.70–6.81 (m, 1H, H-2), 8.69 (s, 1H, Ar-H), 8.74 (s, 1H, Ar-H); 13C-NMR (CDCl3, 100 MHz):
δ
(ppm) = 14.6, 39.2, 40.9, 54.2, 61.2, 132.2, 135.5, 140.1, 143.5, 151.6, 151.8, 152.3, 164.2; MS (ES, pos)
m/z = 293 (M + 1).
Ethyl (S*)-4-(6-(methylamino)-9H-purin-9-yl)cyclopent-1-ene-1-carboxylate, (±)-18.
Yellow oil, 68%; 1H-NMR (CDCl3, 400 MHz):
(ppm) = 1.32 (t, 3H, CH3, J = 7.12 Hz), 2.86–3.03 (m, 2H,
δ
CH2), 3.10–3.29 (m, 5H, NCH3 and CH2), 4.20–4.31 (m, 2H, OCH2), 5.41–5.50 (m, 1H, H-4), 6.16 (brs,
1H, N-H), 6.86–6.89 (m, 1H, H-2), 7.75 (s, 1H, Ar-H), 8.41 (s, 1H, Ar-H); 13C-NMR (CDCl3, 100 MHz):
δ
(ppm) = 14.6, 39.3, 41.1, 50.6, 58.9, 61.1, 120.4, 135.5, 136.1, 137.7, 140.5, 153.4, 155.9, 164.5; MS (ES,
pos) m/z = 288 (M + 1).
Ethyl (S*)-4-(6-(cyclopropylamino)-9H-purin-9-yl)cyclopent-1-ene-1-carboxylate, (±)-19.
1
Yellow oil, 53%; H-NMR (CDCl3, 500 MHz):
δ (ppm) = 0.64–0.69 (m, 2H, CH2), 0.90–0.97 (m, 2H,
CH2), 1.32 (t, 3H, CH3, J = 7.13 Hz), 2.86–3.08 (m, 3H, CH2), 3.18–3.32 (m, 2H, CH2, CH), 4.21–4.28
(m, 2H, OCH2), 5.35–5.43 (m, 1H, H-4), 6.03 (brs, 1H, N-H), 6.86-6.91 (m, 1H, H-2), 7.75 (s, 1H, Ar-H),
8.48 (s, 1H, Ar-H); 13C-NMR (CDCl3, 126 MHz):
δ
(ppm) = 7.4, 14.3, 23.7, 38.9, 40.8, 52.9, 60.7, 119.9,
135.1, 137.6, 140.2, 153.2, 155.8, 164.1; MS (ES, pos) m/z = 314 (M + 1).

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Ethyl (S*)-4-(6-azido-9H-purin-9-yl)cyclopent-1-ene-1-carboxylate, (±)-20.
◦
White solid, m.p. 145–147 C, 68%; 1H-NMR (DMSO, 400 MHz):
δ
(ppm) = 1.22 (t, 3H, CH3,
J = 7.08 Hz), 2.97–3.09 (m, 2H, CH2), 3.22–3.28 (m, 2H, CH2), 4.12–4.23 (m, 2H, OCH2), 5.53–5.64
(m, 1H, H-4), 6.86–6.89 (m, 1H, H-2), 8.66 (s, 1H, Ar-H), 10.07 (s, 1H, Ar-H); 13C-NMR (DMSO,
100 MHz):
δ (ppm) = 15.0, 34.9, 39.2, 55.1, 61.0, 121.2, 134.5, 136.3, 141.8, 142.8, 143.8, 146.3, 164.5; MS
(ES, pos) m/z = 300 (M + 1).
Ethyl (1R*,6S*)-6-((5-amino-6-chloropyrimidin-4-yl)amino)cyclohex-3-ene-1-carboxylate, (±)-23.
◦
White solid, m.p. 120–121 C, 42%; 1H-NMR (CDCl3, 400 MHz):
δ (ppm) = 1.28 (t, 3H, CH3, J = 7.12 Hz),
2.26–2.38 (m, 1H, CH2), 2.42–2.56 (m, 2H, CH2), 2.62–2.73 (m, 1H, CH2), 2.95–3.03 (m, 1H, H-1), 3.39
(brs, 2H, NH2), 4.11–4.24 (m, 2H, OCH2), 4.74–4.85 (m, 1H, H-6), 5.66–5.87 (m, 3H, H-3, H-4, N-H),
8.08 (s, 1H, Ar-H); 13C-NMR (DMSO, 100 MHz):
δ (ppm) = 14.8, 25.5, 30.4, 41.4, 47.1, 60.6, 124.6, 125.4,
125.9, 138.0, 146.1, 152.2, 173.4; MS (ES, pos) m/z = 297 (M + 1), 299 (M + 3).
Ethyl (1R*,6S*)-6-(6-chloro-9H-purin-9-yl)cyclohex-3-ene-1-carboxylate, (±)-24.
1
Yellow oil, 76%. H-NMR (DMSO, 400 MHz):
δ (ppm) = 0.98 (t, 3H, CH3, J = 7.12 Hz), 2.31–2.52 (m,
2H, CH2), 2.74–2.84 (m, 2H, CH2), 3.28–3.33 (m, 1H, H-1), 3.83–3.90 (m, 2H, OCH2), 5.25–5.32 (m, 1H,
H-6), 5.85–5.89 (m, 2H, H-3, H-4), 8.56 (s, 1H, Ar-H), 8.78 (s, 1H, Ar-H). 13C-NMR (DMSO, 100 MHz):
δ
(ppm) = 14.5, 25.4, 29.3, 42.0, 51.2, 61.2, 125.3, 126.4, 131.2, 146.6, 149.9, 152.2, 153.0, 172.4; MS (ES,
pos) m/z = 307 (M + 1).
Ethyl (1R*,6S*)-6-(6-azido-9H-purin-9-yl)cyclohex-3-ene-1-carboxylate, (±)-25.
◦
White solid, m.p. 130–132 C, 51%; 1H-NMR (DMSO, 400 MHz):
δ (ppm) = 1.03 (t, 3H, CH3,
J = 7.10 Hz), 2.34–2.65 (m, 2H, CH2), 2.78–3.00 (m, 2H, CH2), 3.31–3.43 (m, 1H, H-1), 3.82–4.00
(m, 2H, OCH2), 5.40-5.48 (m, 1H, H-6), 8.54 (s, 1H, Ar-H), 10.13 (s, 1H, Ar-H); 13C-NMR (DMSO,
100 MHz):
δ (ppm) = 14.6, 25.4, 29.7, 42.3, 51.5, 61.3, 125.3, 126.4, 129.4, 130.4, 135.3, 142.8, 147.9, 170.5;
MS (ES, pos) m/z = 314 (M + 1).

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Ethyl (1R*,6S*)-6-(6-(cyclopropylamino)-9H-purin-9-yl)cyclohex-3-ene-1-carboxylate, (±)-26.
White solid, m.p. 128–129 ◦C, 62%; 1H-NMR (CDCl3, 400 MHz):
δ
(ppm) = 0.61–0.65 (m, 2H, CH2),
0.88–0.93 (m, 2H, CH2), 1.20 (t, 3H, CH3), 2.48–2.55 (m, 2H, CH2), 2.70–2.74 (m, 2H, CH2), 2.99–3.04
(m, 1H, H-1), 3.14–3.20 (m, 1H, CH), 3.97–4.08 (m, 2H, OCH2), 5.30–5.38 (m, 1H, H-6), 5.91–5.98 (m,
2H, H-3, H-4), 6.04 (brs, 1H, N-H), 7.98 (s, 1H, Ar-H), 8.44 (s, 1H, Ar-H); 13C-NMR (CDCl3, 126 MHz):
δ
(ppm) = 7.4, 13.9, 23.7, 25.2, 29.8, 42.0, 49.2, 61.0, 119.3, 124.8, 125.9, 139.0, 148.6, 153.0, 155.7, 172.1;
MS (ES, pos) m/z = 328 (M + 1).
Ethyl (1S*,6S*)-6-((5-amino-6-chloropyrimidin-4-yl)amino)cyclohex-3-ene-1-carboxylate, (±)-29.
◦
White solid, m.p. 120–121 C, 42%; 1H-NMR (CDCl3, 400 MHz):
δ (ppm) = 1.19 (t, 3H, CH3, J = 7.12 Hz),
2.03–2.12 (m, 1H, CH2), 2.34–2.43 (m, 1H, CH2), 2.56–2.75 (m, 2H, CH2), 2.83–2.92 (m, 1H, H-1), 3.47
(brs, 2H, NH2), 4.06–4.16 (m, 2H, OCH2), 4.62–4.69 (m, 1H, H-6), 5.13 (d, 1H, N-H, J = 8.12 Hz),
5.66–5.78 (m, 2H, H-3, H-4), 8.09 (s, 1H, Ar-H); 13C-NMR (CDCl3, 100 MHz):
δ
(ppm) = 14.5, 27.5, 31.9,
45.2, 48.5, 61.3, 122.1, 124.2, 125.1, 143.7, 150.1, 154.8, 174.1; MS (ES, pos) m/z = 297 (M + 1).
Ethyl (1S*,6S*)-6-(6-chloro-9H-purin-9-yl)cyclohex-3-ene-1-carboxylate, (±)-30.
Colorless oil, 58%; 1H-NMR (CDCl3, 400 MHz):
(ppm) = 0.96 (t, 3H, CH3, J = 7.12 Hz), 2.53–2.68 (m,
3H, CH2), 3.01–3.07 (m, 1H, CH2), 3.56–3.66 (m, 1H, H-1), 3.86–3.96 (m, 2H, OCH2), 4.94–5.03 (m, 1H,
H-6), 5.78
5.89 (m, 2H, H-3, H-4), 8.17 (s, 1H, Ar-H), 8.76 (s, 1H, Ar-H); 13C-NMR (CDCl3, 100 MHz):
(ppm) = 14.2, 29.0, 30.7, 44.1, 54.5, 61.4, 124.4, 125.6, 132.2, 145.3, 151.5, 151.9, 152.1, 173.0; MS (ES,
pos) m/z = 307 (M + 1).
δ
−
δ
Ethyl (1S*,6S*)-6-(6-azido-9H-purin-9-yl)cyclohex-3-ene-1-carboxylate, (±)-31.
◦
White solid, m.p. 140–141 C, 68%; 1H-NMR (DMSO, 400 MHz):
δ (ppm) = 0.74 (t, 3H, CH3, J = 7.08 Hz),
2.46–2.57 (m, 3H, CH2), 2.86–3.00 (m, 1H, CH2), 3.60–3.66 (m, 1H, H-1), 3.69–3.76 (m, 2H, OCH2),
5.08–5.15 (m, 1H, H-6), 5.80–5.92 (m, 2H, H-3, H-4), 8.79 (s, Ar-H), 10.13 (s, 1H, Ar-H). 13C-NMR
(DMSO, 100 MHz):
δ (ppm) = 14.3, 29.3, 31.9, 44.7, 54.3, 61.1, 125.1, 125.9, 136.5, 139.3, 142.9, 144.6,
146.3, 173.1. MS (ES, pos) m/z = 314 (M + 1).

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Ethyl (1R*,2S*)-2-((5-amino-6-chloropyrimidin-4-yl)amino)cyclohex-3-ene-1-carboxylate, (±)-34.
Brown oil, 55%; 1H-NMR (CDCl3, 400 MHz):
δ
(ppm) = 1.20 (t, 3H, CH3, J = 7.14 Hz), 1.99–2.19 (m,
4H, CH2), 2.99–3.05 (m, 1H, H-1), 4.00–4.17 (m, 2H, OCH2), 5.23–5.26 (m, 1H, H-2), 5.59 (d, 1H, N-H,
J = 9.04 Hz), 5.70–5.78 (m, 1H, H-4), 5.87–5.92 (m, 1H, H-3), 8.06 (s, 1H, Ar-H), 13C-NMR (CDCl3,
100 MHz):
δ (ppm) = 14.5, 22.7, 23.6, 43.4, 46.9, 61.1, 122.4, 127.5, 130.0, 143.4, 149.7, 154.6, 174.2; MS
(ES, pos) m/z = 297 (M + 1).
Ethyl (1R*,2S*)-2-(6-chloro-9H-purin-9-yl)cyclohex-3-ene-1-carboxylate, (±)-35.
◦
Brown solid, m.p. 119–121 C, 68%; 1H-NMR (CDCl3, 400 MHz):
δ (ppm) = 1.03 (t, 3H, CH3, J = 7.14 Hz),
2.00–2.08 (m, 2H, CH2), 2.27–2.33 (m, 1H, CH2), 2.41–2.46 (m, 1H, CH2), 3.12–3.20 (m, 1H, H-1),
3.72–3.87 (m, 2H, OCH2), 5.67–5.70 (m, 1H, H-2), 5.83–5.90 (m, 1H, H-4), 6.28–6.33 (m, 1H, H-3), 8.23
(s, 1H, Ar-H), 8.73 (s, 1H, Ar-H); 13C-NMR (DMSO, 100 MHz):
δ
(ppm) = 14.3, 20.3, 24.4, 44.1, 50.1,
61.0, 123.1, 134.8, 147.5, 147.8, 149.9, 152.3, 152.9, 172.4; MS (ES, pos) m/z = 307 (M + 1), 309 (M + 3).
Ethyl (1R*,2S*)-2-((5-amino-6-chloropyrimidin-4-yl)amino)cyclopent-3-ene-1-carboxylate, (±)-38.
Brownish white solid, m.p. 104–106 ◦C, 34%; 1H-NMR (CDCl3, 400 MHz):
(ppm) = 1.04 (t, 3H, CH3,
δ
J = 7.12 Hz), 2.56–2.67 (m, 1H, CH2), 2.83–2.90 (m, 1H, CH2), 3.44–3.54 (m, 1H, H-1), 3.57 (brs, 2H,
NH2), 3.85–4.02 (m, 2H, OCH2), 5.34 (d, 1H, N-H, J = 8.76 Hz), 5.69–5.76 (m, 2H, H-2, H-4), 5.98–6.01
(m, 1H, H-3), 8.06 (s, 1H, Ar-H); 13C-NMR (CDCl3, 100 MHz):
δ
(ppm) = 14.2, 35.4, 46.4, 58.2, 61.1,
122.6, 130.2, 134.0, 143.0, 149.3, 154.1, 174.0; MS (ES, pos) m/z = 283 (M + 1), 285 (M + 3).
Ethyl (1R*,2S*)-2-(6-chloro-9H-purin-9-yl)cyclopent-3-ene-1-carboxylate, (±)-39.
Yellowish white solid, m.p. 118–119 ◦C, 39%; 1H-NMR (CDCl3, 400 MHz):
(ppm) = 0.72 (t, 3H, CH3,
δ
J = 7.14 Hz), 2.76–2.85 (m, 1H, CH2), 3.14–3.23 (m, 1H, CH2), 3.49–3.56 (m, 1H, H-1), 3.65–3.77 (m, 2H,
OCH2), 5.86–5.89 (m, 1H, H-2), 5.15–5.17 (m, 1H, H-4), 6.44–6.47 (m, 1H, H-3), 7.99–8.01 (m, 1H, Ar-H),
8.78–8.81 (m, 1H, Ar-H); 13C-NMR (CDCl3, 100 MHz):
δ
(ppm) = 13.8, 34.9, 47.2, 60.9, 61.4, 126.8, 134.7,
138.7, 150.0, 152.2, 152.3, 154.2, 170.9; MS (ES, pos) m/z = 293 (M + 1), 295 (M + 3).

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((1R*,6S*)-6-((5-Amino-6-chloropyrimidin-4-yl)amino)cyclohex-3-en-1-yl)methanol, (±)-42.
White solid, m.p. 186–188 ◦C, 60%; 1H-NMR (DMSO, 400 MHz):
δ
(ppm) = 1.96–2.12 (m, 4H, CH2),
2.21–2.30 (m, 1H, H-1), 3.24–3.32 (m, 1H, OCH2), 3.40-3.49 (m, 1H, OCH2), 4.43–4.48 (m, 2H, H-6 and
O-H), 5.16 (brs, 2H, N-H), 5.59–5.70 (m, 2H, H-3, H-4), 6.24 (d, 1H, N-H, J = 7.56 Hz), 7.68 (s, 1H, Ar-H);
13C-NMR (DMSO, 100 MHz):
δ
(ppm) = 25.9, 30.1, 39.8, 47.2, 61.4, 124.5, 125.6, 126.6, 138.0, 146.4, 152.6;
MS (ES, pos) m/z = 255 (M + 1), 257 (M + 3).
((1R*,6S*)-6-(6-Chloro-9H-purin-9-yl)cyclohex-3-en-1-yl)methanol, (±)-43.
White solid, m.p. 109–111 ◦C, 76%, 1H-NMR (CDCl3, 500 MHz):
δ (ppm) = 1.36–1.47 (m, 1H, CH2),
2.05–2.15 (m, 1H, CH2), 2.34–2.44 (m, 1H, H-1), 2.50–2.59 (m, 1H, CH2), 2.73–2.81 (m, 1H, OCH2),
2.93–3.03 (m, 1H, CH2), 3.44–3.53 (m, 1H, OCH2), 4.65 (brs, 1H, OH), 5.31–5.37 (m, 1H, H-6), 5.98–6.06
(m, 2H, H-3, H-4), 8.33 (s, 1H, Ar-H), 8.76 (s, 1H, Ar-H); 13C-NMR (CDCl3, 126 MHz): 22.6, 31.0, 39.7,
48.6, 62.1, 124.7, 127.8, 131.1, 144.6, 151.6, 151.7, 152.5, MS (ES, pos) m/z = 265 (M + 1), 267 (M + 3).
((1S*,6S*)-6-((5-Amino-6-chloropyrimidin-4-yl)amino)cyclohex-3-en-1-yl)methanol, (±)-46.
White solid, m.p. 164–167 ◦C, 64%; 1H-NMR (DMSO, 400 MHz):
δ (ppm) = 1.70–1.84 (m, 1H, CH2),
1.89–2.05 (m, 2H, CH2), 2.14–2.25 (m, 1H, CH2), 2.26–2.36 (m, 1H, H-1), 3.26–3.43 (m, 2H, OCH2),
4.05–4.15 (m, 1H, H-6), 4.36 (t, 1H, O-H, J = 5.32 Hz), 4.99 (brs, 2H, N-H), 5.51-5.67 (m, 2H, H-3, H-4),
6.50 (d, 1H, N-H, J = 7.96 Hz), 7.64 (s, 1H, Ar-H), 13C-NMR (DMSO, 100 MHz):
δ
(ppm) = 28.7, 32.3,
41.3, 48.3, 62.9, 124.2, 125.5, 127.3, 137.7, 146.4, 152.7, MS (ES, pos) m/z = 255 (M + 1), 257 (M + 3).
((1S*,6S*)-6-(6-Chloro-9H-purin-9-yl)cyclohex-3-en-1-yl)methanol, (±)-47.
White solid, m.p. 160–162 ◦C, 70%; 1H-NMR (DMSO, 500 MHz):
δ (ppm) = 2.13–2.29 (m, 2H, CH2),
2.42–2.49 (m, 1H, CH2), 2.53–2.62 (m, 1H, H-1), 2.83–2.95 (m, 1H, CH2), 2.98–3.05 (m, 1H, OCH2),
3.11–3.17 (m, 1H, OCH2), 4.69–4.78 (m, 1H, H-6), 5.68–5.85 (m, 2H, H-3, H-4), 8.73–8.78 (m, 2H, Ar-H),
13C-NMR (DMSO, 126 MHz): 28.5, 31.3, 39.0, 54.1, 61.7, 124.6, 127.2, 131.6, 147.6, 149.4, 151.7, 152.4; MS
(ES, pos) m/z = 265 (M + 1), 267 (M + 3).

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Supplementary Materials: The Supplementary Materials are available online.
Author Contributions: Both authors equally contributed to the manuscript. L.K. designed the experiments. A.R.
performed the analyses and wrote the paper. L.K. reviesed the manuscript.
Funding: We are grateful to the Hungarian Research Foundation (NKFIH No. K 119282) for financial support.
The financial support of the GINOP-2.3.2-15-2016-00034 project is also acknowledged. This research was supported
by the EU-funded Hungarian grant EFOP-3.6.1-16-2016-00008. Ministry of Human Capacities, Hungary grant
20391-3/2018/FEKUSTRAT is also acknowledged.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of all compounds are available in mg amounts from the authors.
2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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(CC BY) license (http://creativecommons.org/licenses/by/4.0/).