Practical and Scalable Synthesis of 7-Azetidin-1-yl-4-(hydroxymethyl)coumarin: An Improved Photoremovable Group

Article abstract of DOI:10.1055/s-0036-1591742

7-Substituted 4-methylcoumarin derivatives are widely employed as photoprotecting groups in chemistry and biology. We have recently shown that the 7-azetidinylated version of this photocage releases carboxylic acids in aqueous solution more efficiently than the traditionally used 7-diethylamino variant. Here we present a robust and scalable route to prepare the 7-azetidinylated alcohol, a useful precursor for the photoprotection of a variety of leaving groups, and its use in the preparation of model phosphate, sulfonate, and carbamate derivatives.

Full text of DOI:10.1055/s-0036-1591742

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2017, 49, A–G
paper
A
en
G. Bassolino et al.
Paper
Synthesis
Practical and Scalable Synthesis of 7-Azetidin-1-yl-4-(hydroxy-
methyl)coumarin: An Improved Photoremovable Group
Giovanni Bassolino
Elias A. Halabi
Pablo Rivera-Fuentes*
0
0
0
0
0-
0
0
1
8-
5
5
8
2-
8
2
8
Laboratorium für Organische Chemie, ETH Zürich, HCI G329,
Vladimir-Prelog-Weg 3, 8093 Zürich, Switzerland
pablo.rivera-fuentes@org.chem.ethz.ch
Published as part of the Bürgenstock Special Section 2017
Future Stars in Organic Chemistry
Received: 25.10.2017
Accepted after revision: 27.11.2017
Published online: 21.12.2017
stitution approach can be used to improve the efficiency of
other photochemical processes, showing that the φPA of a
series of 7-substituted 4-methylcoumarin esters increases
when an azetidin-1-yl ring (1a–d) is used in place of a
diethylamino group (2a–d, Scheme 1).
DOI: 10.1055/s-0036-1591742; Art ID: ss-2017-z0688-op
Abstract 7-Substituted 4-methylcoumarin derivatives are widely em-
ployed as photoprotecting groups in chemistry and biology. We have
recently shown that the 7-azetidinylated version of this photocage re-
leases carboxylic acids in aqueous solution more efficiently than the tra-
ditionally used 7-diethylamino variant. Here we present a robust and
scalable route to prepare the 7-azetidinylated alcohol, a useful precur-
sor for the photoprotection of a variety of leaving groups, and its use in
the preparation of model phosphate, sulfonate, and carbamate deriva-
tives.
R1
O
O
+
R1
O
O
O
7
MeCN-PBS, 7:3
hν (405 nm)
R2COOH
4
1a–d
2a–d
3
OH
O
R2
R2
R1
=
=
NHBoc
OMe
CF3
Keywords azetidine, bioorganic chemistry, chromophores, photo-
chemistry, protecting groups, uncaging
a
b
c
d
φ
φ
PA = 0.014(5)
φ
φ
PA = 0.016(5)
φ
φ
PA = 0.006(2)
φ
φ
PA = 0.014(2)
1
2
N
Coumarin derivatives are widely used as fluorophores1,2
and photocleavable (also known as ‘caging’) groups.3,4
4-Methylcoumarin derivatives substituted at the 7-position
(for the numbering, see Scheme 1) with an electron-rich
group have been used extensively as photoremovable pro-
tecting groups in biology,5,6 synthetic chemistry,7 and mate-
rials science,8,9 even though their quantum yields of photo-
activation (φPA) are generally low.4,10–15 Photoactivation of
poor leaving groups, such as alcohols and amines, usually
relies on their conversion into carbonates or carbamates to
drive their release through the irreversible formation of
carbon dioxide.3,11,16
PA = 0.003(1)
PA = 0.0035(8)
PA = 0.0039(8)
PA = 0.0052(2)
N
Scheme 1 Quantum yield of photoactivation (φPA) of 7-azetidin-1-yl
(1a–d) and 7-diethylamino (2a–d) coumarin esters19 (PBS = phosphate-
buffered saline, pH 7.4)
Although these esters could be prepared from couma-
rinyl alcohol 3, in our original report they were prepared by
direct nucleophilic substitution, employing carboxylates
and coumarinyl bromide 4 (Scheme 2).19 Compound 3,
however, would be a versatile intermediate for the prepara-
tion of other biologically relevant compounds, including
sulfonates, phosphates, and carbamates. Such compounds
have been used to produce photoinduced pH jumps,20 to
achieve control of biologically relevant processes such as
DNA transcription,13 or to release bioactive molecules.16,21
Lavis and co-workers17 and Xu and co-workers18 report-
ed that azetidin-1-yl and aziridin-1-yl rings can improve
the fluorescence quantum yield of a wide variety of fluoro-
phores when used in place of dialkylamino or larger ring
substituents. We have recently proved19 that the same sub-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

B
G. Bassolino et al.
Paper
Synthesis
RuPhos/RuPhos Pd G3, with Cs2CO3 as a base, in 1,4-dioxane
or toluene.17 In either solvent, the major product 8 was
formed, presumably by a Tsuji–Trost-type pathway, with
the azetidinyl ring substituting the allylic hydroxy group of
the 4-methyl, instead of the aromatic triflate. This product
was identified by LC/MS analysis of the reaction mixture
(m/z = 364; see Figure S1, Supporting Information). The de-
sired alcohol 3 was formed only in trace amounts.
KF, RCOOH
DMF, 50 °C
N
O
O
N
O
O
O
4
1a–d
Br
O
R
R =
OMe
a 56%
b 40%
To disfavor coordination of palladium at the 4-hydroxy-
methyl group, a triisopropylsilyl group was installed before
the Buchwald–Hartwig cross-coupling (Scheme 4). With
this new substrate 9, no Tsuji–Trost product 8 was observed
during the cross-coupling reaction, but compound 10 was
obtained in low yields (30%) when the reaction was per-
formed on a small scale (15 mg). Upon scaling up to 220 mg,
compound 10 was obtained only as a minor product.
1H NMR analysis revealed that the small amounts of prod-
uct 10 that formed under these conditions underwent fur-
ther nucleophilic ring opening of the azetidine ring (see
Figure S2, Supporting Information).
NHBoc
d 66%
CF3
c 30%
Scheme 2 Previous preparation of 7-azetidin-1-yl-4-methylcoumarin
esters 1a–d19
Oftentimes, new tools in chemical biology are not wide-
ly adopted because their syntheses represent a consider-
able effort. We considered that a simple and reliable syn-
thesis of building block 3 would facilitate the application of
azetidinylcoumarins as protecting groups, even in laborato-
ries in which synthetic chemistry is not the main area of
expertise. Here we report the results of exploring several
potential synthetic routes and identify a reproducible and
scalable protocol for the production of intermediate 3. Fur-
thermore, compound 3 was used to prepare model sulfon-
ate, phosphate, and carbamate derivatives, highlighting the
versatility of this intermediate. Finally, we demonstrate
that photolysis of these compounds proceeds smoothly, re-
leasing only the photoprotected leaving group and alcohol
3.
At the outset, we tried to install the azetidine substitu-
ent after introducing the 4-hydroxymethyl group on the
coumarin skeleton (Scheme 3). After optimization of re-
ported protocols, compound 6 was obtained from resorci-
nol upon hydrolysis of the intermediate chloride 5.8,22,23
Selective triflation of the 7-hydroxy substituent to yield in-
termediate 7 was also carried out according to a reported
procedure.24 To install the azetidine ring, we aimed to opti-
mize the conditions reported by Lavis and co-workers:
NH
RuPhos
RuPhos Pd G3
1,4-dioxane
Cs2CO3
N
O
O
TfO
O
O
TfO
O
O
TIPS-OTf
imidazole
CH2Cl2
7
75%
9
10
OTIPS
OH
OTIPS
Scheme 4 Buchwald–Hartwig coupling of 9
Because installing the azetidine at this late stage was
not practical, we decided to explore alternative routes em-
ploying the azetidinylcoumarin 11 reported by Lavis and
co-workers.17
The first routes tested were the thermal hydrolysis and
photolysis of bromide 4 (Scheme 5), an intermediate which
had already been prepared in the synthesis of esters 1a–d.
Briefly, compound 4 was obtained from the corresponding
triflate by Buchwald–Hartwig cross-coupling to provide
11,17 followed by bromination at the 4-methyl substituent
by sequential deprotonation with lithium hexamethyldisi-
lazide and treatment of the anion with N-bromosuccinim-
ide.25–30 Neither the thermal nor the photochemical method
yielded the alcohol 3 in satisfactory yields and purities. In
Cl
O
O
H2O
MsOH
toluene
HO
O
O
HO
O
O
HO
OH
MW, 160 °C
1
60%
86%
all cases, LC/MS and H NMR analyses (see Figure S3, Sup-
O
5
6
OH
porting Information) of the reaction mixtures or the isolat-
ed products revealed the attack of a bromide ion at the
azetidine ring, yielding 7-(3-bromopropylamino)coumarin
12. Attempts to intercept the formed bromide anion with
silver salts did not change the outcome of the reaction.
For an alternative halogenated intermediate, m-azetidin-
1-ylphenol (13) was used in an attempt to prepare 7-azetidin-
1-yl-4-(chloromethyl)coumarin (14) by Pechmann conden-
sation with ethyl 4-chloroacetoacetate promoted by
Ti(OiPr)3Cl at 95 °C (Scheme 6). This reaction has been suc-
cessfully applied to the synthesis of julolidine-fused ana-
Cl
HO
O
O
Tf
Tf
TfO
O
O
DIPEA
MeCN
N
60 %
6
7
OH
NH
OH
RuPhos
RuPhos Pd G3
Cs2CO3
1,4-dioxane or toluene
80 °C
N
O
O
TfO
O
O
TfO
O
O
7
3
8
OH
N
OH
major product
traces
Scheme 3 Route to compound 3 installing the azetidine in the last
step
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

C
G. Bassolino et al.
Paper
Synthesis
azetidine
RuPhos
RuPhos Pd G3
toluene
O
Cs2CO3, 80 °C
The involvement of the trifluoroacetate 15 as an inter-
mediate is supported by a comparison of LC/MS analyses of
the reaction of 4 in the presence of KF and the addition of
both TFA and H2O (Figure 1, top), only H2O (Figure 1, mid-
dle), and only TFA from a newly opened bottle (Figure 1,
bottom). A significant amount of alcohol 3 was detected
only in the first case. In the absence of TFA, only unreacted
starting material was observed, whereas without H2O only
traces of alcohol were formed. These experiments suggest
that perhaps bromide 4 and trifluoroacetate 15 exist in an
equilibrium that is strongly shifted towards the bromide,
and water-driven, irreversible hydrolysis of 15 opens a
pathway to the alcohol 3.
i. LiHMDS
THF
–40 °C
ii. NBS
N
O
O
N
O
O
TfO
O
THF, –78 °C
65 %
70 %
11
4
Br
1,4-dioxane
(or MeCN)/H2O
100 °C or
H
N
O
O
N
O
O
+
N
O
O
hν (350 nm)
Br
4
3
12
Br
OH
OH
Scheme 5 Thermal hydrolysis and photolysis of bromide 4
logues.31–33 Unfortunately, this route was unsuccessful be-
cause at these high temperatures even the chloride ion, a
poor nucleophile, attacked the azetidine to open the ring, as
determined by 1H NMR and LC/MS analyses (see Supporting
Information).
Verkade’s base
Pd(OAc)2
toluene
LiHMDS
80 °C
Ti(OiPr)3Cl
toluene
O
N
OH
N
O
O
Br
OH
O
O
NH
75%
13
14
Cl
Cl
Scheme 6 Tested route to compound 14
Finally, as we were able to form esters by nucleophilic
displacement of the bromide in compound 4 at lower tem-
peratures (Scheme 2), we aimed to obtain alcohol 3 by
hydrolysis of an ester intermediate (Scheme 7). Anticipating
the need to use mild hydrolysis conditions to avoid nucleo-
philic opening of the azetidine ring, we decided to use tri-
fluoroacetic acid (TFA) to form the intermediate ester. De-
rivative 15 was so prone to hydrolysis that even adventi-
tious H2O present in an open TFA bottle was enough to
hydrolyze the intermediate ester. In fact, we did not isolate
or observe trifluoroacetate 15; only alcohol 3 and bromide
4 were detected by LC/MS analysis of the reaction mixture.
Under optimized reaction conditions, no significant attack
at the azetidine ring by any of the nucleophiles present in
the reaction mixture was observed. However, when the re-
action was carried out with more equivalents of TFA–H2O
than KF, LC/MS analysis of the reaction mixture revealed
the presence of a product arising from nucleophilic attack
of a bromide ion at the azetidinyl ring.
Figure 1 LC/MS analyses of the reaction of bromide 4 with both TFA
and H2O (top), only H2O (middle), or only TFA (bottom). Analyses were
undertaken between 2.3 and 3 hours after addition of the reactants.
Taking these results into account, the route depicted in
Scheme 8 is recommended for the preparation of alcohol 3.
Whereas it is possible to purify bromide 4 by column chro-
matography, it is not necessary for the subsequent step to
proceed with good yields. Indeed, we found it more practi-
cal to use the crude of the bromination reaction for the
hydrolysis step because the difference in retention factors
between alcohol 3 (Rf = 0.25, silica gel, CH2Cl2–Et2O, 7:3)
and bromide 4 (Rf = 0.55, silica gel, CH2Cl2–EtOAc, 95:5) is
much larger than the difference between the latter and the
parent compound 11 (Rf = 0.42, silica gel, CH2Cl2–EtOAc,
95:5). This route has been scaled up to 0.4 g of starting ma-
terial 11 without loss in isolated yield or purity of alcohol 3.
With multi-milligram quantities of alcohol 3 in hand, it
was used to prepare the model photoprotected sulfonate,
phosphate, and carbamate derivatives depicted in Scheme
9. These compounds were synthesized to demonstrate the
N
O
O
N
O
O
N
O
O
O
TFA, KF
DMF, 40 °C
H2O
80%
4
15
3
Br
OH
O
CF3
Scheme 7 One-pot esterification and hydrolysis of bromide 4
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

D
G. Bassolino et al.
Paper
Synthesis
i. LiHMDS
THF
–40 °C
ii. NBS
THF
TFA, KF
H2O
DMF
N
O
O
N
O
O
N
O
O
–78 °C
40 °C
67% (2 steps)
3
11
4
OH
Br
Scheme 8 Optimized synthesis of alcohol 3
versatility of building block 3 and to test the stability of the
azetidine ring under the reaction conditions of these trans-
formations. In all cases, the yields of these reactions were
comparable to those reported when the same procedures
were performed employing 7-(diethylamino)-4-(hy-
droxymethyl)coumarin as starting material.16,20 These re-
sults confirm that the azetidinylated coumarin can be used
to prepare photoprotected versions of a variety of biologi-
cally relevant compounds.
THF
DMAP
OP(OEt)2Cl
Figure 2 LC/MS analyses of the mixtures of photolyzed compounds
N
O
O
N
O
O
CH3SO2
Et3N, CH2Cl2
N
O
O
16–18. All indicated m/z values corresponds to the [M + H]+ ions.
38%
77%
16
3
17
O
P
OH
O
S
reliable synthesis will encourage other scientists to adopt
this photoremovable group, which is much more efficient
than its diethylamino-substituted counterpart.
EtO
O
O
O
i. DSC
MeCN
ii. benzylamine
EtO
39%
N
O
O
O
18
All reagents were purchased from commercial sources and used as re-
ceived. Anhydrous solvents were procured from Acros Organics and
used as received. Preparative HPLC was carried out using an Hitachi
instrument equipped with an autosampler and a diode array detector
and employing an ACE Excel 5 C18-Amide column (25 cm length, 10
mm internal diameter). Automated flash chromatography was per-
formed using a Büchi Reveleris PREP system. LC/MS chromatograms
were recorded on a Waters Acquity UPLC system equipped with a Wa-
ters Acquity BEH C18 column (5 cm length, 2.1 mm internal diame-
ter), a photodiode array detector, and an SQ Detector 2 ZSpray (ESI).
The samples were eluted with a MeCN–H2O gradient running from 2%
to 98% MeCN, with a constant 0.1% HCOOH. NMR spectra were ac-
quired on Bruker AV300 or Bruker 400 instruments. 1H NMR chemical
shifts are reported in ppm relative to SiMe4 (δ = 0) and were refer-
enced internally with respect to residual protons in the solvent
(δ = 2.50 for DMSO-d6, δ = 7.26 for CDCl3). Coupling constants are re-
ported in Hz and multiplicities of signals are described as singlet (s),
doublet (d), triplet (t), quartet (q), quintuplet (quint), or multiplet (m).
13C NMR chemical shifts are reported in ppm relative to SiMe4 (δ = 0)
and were referenced internally with respect to the solvent signal
(δ = 39.5 for DMSO-d6, δ = 77.2 for CDCl3). Peak assignments are
based on calculated chemical shift, multiplicity, and 2D experiments.
Atom-numbering is indicated in the Supporting Information. 31P NMR
(with 1H decoupling) chemical shifts were referenced upon addition
of triphenylphosphine as internal standard (δ = –4.9 for CDCl3).35 IR
spectra were measured on a Perkin Elmer Spectrum Two FTIR spec-
trophotometer. High-resolution mass spectrometry was conducted
by staff of the Molecular and Biomolecular Analysis Service (MoBiAS,
ETH Zürich) employing a Bruker maXis ESI/NanoSpray-Qq-TOF-MS or
a Bruker solariX ESI/MALDI-FTICR-MS instrument. Elemental analysis
was conducted by staff of MoBiAS (ETH Zürich) employing a LECO
O
N
H
Scheme 9 Synthesis of compounds 16–18 starting from building
block 3 (DSC = N,N′-disuccinimidyl carbonate)
Finally, ~0.3 mM solutions of compounds 16–18 in 7:3
mixtures of MeCN–phosphate-buffered saline (PBS, pH 7.4)
were prepared and then irradiated using 405 nm LEDs to
monitor the photolysis reactions (Figure 2), as described
previously.19 In all cases, the photoreactions proceeded
cleanly to give the photodeprotected products and alcohol
3 or chloride 14. This halogenated coumarin is likely gener-
ated by a chloride ion trapping the intermediate ion pair of
the photolysis12 when the reaction is carried out in PBS,
which has a high concentration (155 mM) of NaCl.34
In summary, we have presented a reproducible and
scalable route to prepare alcohol 3, a fundamental building
block to obtain compounds for the photocontrolled release
of different classes of substances such as phosphates, sul-
fonates, and amines. Whereas we do not exclude that with a
thorough optimization of the reaction parameters other
routes might also successfully lead to alcohol 3, the one we
propose here proved the most robust and scalable in our
experience. We also prepared three photoprotected model
compounds and showed that the photolysis reactions pro-
ceed cleanly. We envision that the availability of a short and
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E
G. Bassolino et al.
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Synthesis
TruSpec Micro instrument. Melting points were measured on a Büchi
Melting Point M-560 instrument and are uncorrected; unless other-
vealed the presence of a dibrominated byproduct (m/z = 374). The
mixture was treated with HCl (0.1 M) and extracted three times with
CH2Cl2. The combined organic phases were dried over MgSO4 and the
solvent was removed under reduced pressure. The product was fixed
on Celite and purified by silica gel column chromatography (CH2Cl2–
EtOAc, 99.5:0.5) to yield an orange solid; yield: 490 mg (70%).
wise given, the temperature gradient used was 20 °C·min–1
.
7-(Azetidin-1-yl)-4-(hydroxymethyl)-2H-chromen-2-one (3)
Compound 11 (400 mg, 1.86 mmol) was added to a flame-dried
multi-neck flask and suspended in anhydrous THF (34 mL). The sus-
pension was stirred for 10 min at r.t., then cooled to –40 °C in an ace-
tone–dry ice bath, and LiHMDS (1 M in THF; 4.7 mL, 4.7 mmol) was
added dropwise to the mixture through a dropping funnel. The solu-
tion was warmed to –30 °C, followed by cooling to –78 °C. A freshly
prepared solution of N-bromosuccinimide (364 mg, 2.05 mmol) in
anhydrous THF (6 mL) was added dropwise through a dropping fun-
nel, ensuring that the temperature did not rise above –70 °C. The
solution was kept stirring at –78 °C for about 1 h, when LC/MS and
TLC analyses revealed the presence of a dibrominated byproduct
(m/z = 374). The reaction was quenched with HCl (0.1 M) and the
mixture repeatedly extracted with CH2Cl2. The combined organic
phases were dried over MgSO4, and the solvent was removed under
reduced pressure until an orange solid was obtained. KF (118 mg, 2.04
mmol) was flame-dried in a separate two-neck flask. The crude of the
bromination reaction was added as a solid, and the system was evacu-
ated and backfilled with nitrogen three times. The solids were sus-
pended in anhydrous DMF (10 mL) and TFA–H2O (4:1, 450 μL) was
added to the flask (2.5 equiv of each). The flask was sealed, and the
system was stirred at 40 °C for 12–15 h, until completion, as indicated
by TLC (silica gel; CH2Cl2–EtOAc, 95:5) by the disappearance of the in-
termediate bromide 4. CH2Cl2 was added and the organic phase was
washed with H2O. The aqueous phase was back-extracted once with
CH2Cl2. The combined organic phases were washed with brine, dried
over MgSO4, and concentrated under reduced pressure until a solid
was obtained. The crude was dissolved in CH2Cl2 and as little MeOH as
possible, and was fixed on Celite. The product was purified by silica
gel column chromatography (CH2Cl2–Et2O, 95:5 → 8:2), and isolated
as a yellow-orange powder; yield: 290 mg (67%).
Mp 200 °C (when measured with a slower gradient of 5 °C·min–1, a
color change was observed around 185 °C, yielding a solid that does
not melt at 200 °C); Rf = 0.55 (silica gel; CH2Cl2–EtOAc, 95:5).
IR (neat): 1723, 1617, 1529, 1412, 1306, 1230, 1161 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.48 (d, J = 8.7 Hz, 1 H, H-3), 6.33 (dd,
J = 8.7, 2.3 Hz, 1 H, H-2), 6.22 (d, J = 2.3 Hz, 1 H, H-1), 6.17 (s, 1 H, H-
4), 4.40 (s, 2 H, H-5), 4.01 (t, J = 7.3 Hz, 4 H, H-6), 2.45 (quint, J = 7.4
Hz, 2 H, H-7).
13C NMR (100 MHz, CDCl3): δ = 161.6, 156.4, 154.2, 150.6, 125.4,
110.1, 108.0, 107.4, 97.3, 51.8, 27.3, 16.6.
HRMS-ESI: m/z [M + H]+ calcd for C13H13NO2Br: 294.0124; found:
294.0118.
Anal. Calcd for C13H12NO2Br: C, 53.08; H, 4.11; N, 4.76. Found:
C, 53.02; H, 4.18; N, 4.62.
7-(Azetidin-1-yl)-4-methyl-2H-chromen-2-one (11)17
4-Methyl-2-oxo-2H-chromen-7-yl trifluoromethanesulfonate (800
mg, 2.6 mmol), RuPhos (182 mg, 0.4 mmol), RuPhos Pd G3 (334 mg,
0.4 mmol), and Cs2CO3 (1.27 g, 3.9 mmol) were transferred to a flame-
dried Schlenk flask, which was evacuated and backfilled with nitro-
gen three times. Anhydrous toluene (21 mL) was added, followed by
azetidine (0.2 mL, 162 mg, 2.8 mmol). The reaction mixture was
stirred for 15 h at 80 °C, then filtered over Celite, fixed on Celite, puri-
fied by silica gel column chromatography (CH2Cl2–EtOAc, 99:1), and
washed with pentane to yield compound 11 as a bright yellow solid;
yield: 360 mg (65%).
Rf = 0.42 (silica gel; CH2Cl2–EtOAc, 95:5).
1H NMR (400 MHz, CDCl3): δ = 7.38 (d, J = 8.6 Hz, 1 H, H-3), 6.30 (dd,
J = 8.6, 2.3 Hz, 1 H, H-2), 6.21 (d, J = 2.3 Hz, 1 H, H-1), 5.97 (q, J = 1.2
Hz, 1 H, H-4), 3.99 (t, J = 7.3 Hz, 4 H, H-6), 2.43 (quint, J = 7.3 Hz, 2 H,
H-7), 2.34 (d, J = 1.2 Hz, 3 H, H-5).
13C NMR (100 MHz, CDCl3): δ = 162.1, 155.7, 154.1, 153.1, 125.5,
110.4, 109.6, 107.8, 97.2, 51.9, 18.7, 16.6.
Mp 198 °C; Rf = 0.25 (silica gel; CH2Cl2–Et2O, 7:3).
IR (neat): 3386, 1690, 1600, 1530, 1415, 1090 cm–1
1H NMR (400 MHz, DMSO-d6): δ = 7.45 (d, J = 8.7 Hz, 1 H, H-3), 6.36
(dd, J = 8.7, 2.3 Hz, 1 H, H-2), 6.26 (d, J = 2.3 Hz, 1 H, H-1), 6.11 (t,
J = 1.5 Hz, 1 H, H-4), 5.51 (t, J = 5.6 Hz, 1 H, H-8), 4.67 (dd, J = 5.6, 1.5
Hz, 2 H, H-5), 3.93 (t, J = 7.4 Hz, 4 H, H-6), 2.35 (quint, J = 7.4 Hz, 2 H,
H-7).
.
HRMS-ESI: m/z [M + H]+ calcd for C13H14NO2: 216.1019; found:
13C NMR (100 MHz, DMSO-d6): δ = 160.9, 157.0, 155.1, 153.7, 124.9,
216.1019.
107.8, 106.9, 104.6, 96.3, 59.1, 51.5, 15.9.
(7-(Azetidin-1-yl)-2-oxo-2H-chromen-4-yl)methyl Diethyl Phos-
phate (16)
HRMS-ESI: m/z [M + H]+ calcd for C13H14NO3: 232.0968; found:
232.0970.
Compound 3 (50 mg, 0.21 mmol) and DMAP (3 mg, 0.02 mmol) were
added to a flame-dried flask, and the system was evacuated and back-
filled with nitrogen three times. Anhydrous THF (1 mL) and Et3N (46
μL, 0.33 mmol) were added. A solution of diethyl chlorophosphate (35
μL, 0.24 mmol) in THF (1 mL) was added dropwise. The reaction mix-
ture was stirred at r.t. for 12–15 h, until the starting material was con-
sumed, as determined by TLC (silica gel; CH2Cl2–Et2O, 7:3). Upon
completion, CH2Cl2 was added, and the organic phase was washed
with H2O. The aqueous phase was back-extracted once with CH2Cl2.
The combined organic phases were washed with brine, dried over
MgSO4, and concentrated under reduced pressure. The crude was dis-
solved in CH2Cl2 and fixed on Celite. The product was purified by sili-
ca gel column chromatography (CH2Cl2–Et2O, 92:8 → 95:5), followed
by HPLC [solvent A: H2O, solvent B: MeCN; flow: 1 mL·min–1; 0–1
min: 50% A; 1–6.5 min: 50–20% A; 6.5–7.5 min: 20–10% A; 7.5–9 min:
Anal. Calcd for C13H13NO3·0.33 H2O: C, 65.81; H, 5.81; N, 5.90. Found:
C, 65.50; H, 5.68; N, 5.85.
7-(Azetidin-1-yl)-4-(bromomethyl)-2H-chromen-2-one (4)
Compound 11 (500 mg, 2.32 mmol) was added to a flame-dried
multi-neck flask and suspended in anhydrous THF (45 mL). The sus-
pension was cooled to –40 °C in an acetone–dry ice bath, and LiHMDS
(1 M in THF; 5.81 mL, 5.81 mmol) was added dropwise to the mixture
through a dropping funnel. The dropping funnel was rinsed by adding
THF (1 mL). The solution was warmed to –30 °C, followed by cooling
to –78 °C. A solution of N-bromosuccinimide (455 mg, 2.55 mmol) in
anhydrous THF (7 mL) was freshly prepared in a flame-dried flask and
added dropwise, and the mixture was stirred at –78 °C. The reaction
mixture was neutralized after 1 h, when LC/MS and TLC analyses re-
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G

F
G. Bassolino et al.
Paper
Synthesis
10% A; 9–10.5 min: 10–50% A]. Compound 16 was isolated as an oil
that became an off-white powder after cooling overnight at –20 °C;
yield: 30 mg (38%).
nitrogen three times. The flask was covered with aluminum foil. An-
hydrous MeCN was added to the carbonate (1 mL) and to the drop-
ping funnel (2 mL). N,N-Diisopropylethylamine (150 μL, 0.84 mmol)
was added to the suspension of compound 3. The suspension was
added in portions to the carbonate, and anhydrous MeCN (6.5 mL)
was added in portions to the dropping funnel to recover as much of
the solid 3 as possible. The mixture was stirred at r.t. for 12 h, until
TLC analysis (silica gel; CH2Cl2–Et2O, 7:3) revealed the absence of
compound 3. Benzylamine (250 μL, 2.3 mmol) was added to the flask,
and the reaction mixture was stirred for 6 h, until LC/MS analysis re-
vealed that the desired carbamate was the predominant product of
the reaction. CH2Cl2 was added and the organic phase was washed
with H2O. The aqueous phase was back-extracted once with CH2Cl2.
The combined organic phases were washed with brine, dried over
MgSO4, and concentrated under reduced pressure. The crude was dis-
solved in CH2Cl2 and as little MeOH as possible, and was fixed on
Celite. The product was purified by automated silica gel flash column
chromatography (CH2Cl2–EtOAc, 100:0 → 9:1), and isolated as a yel-
low solid; yield: 30 mg (39%).
Mp 100 °C; Rf = 0.1 (silica gel; CH2Cl2–Et2O, 95:5).
IR (neat): 2980, 1726, 1608, 1530, 1419, 1270, 1033 cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.26 (d, J = 8.7 Hz, 1 H, H-3), 6.28 (dd,
J = 8.7, 2.3 Hz, 1 H, H-2), 6.24 (t, J = 1.4 Hz, 1 H, H-4), 6.22 (d, J = 2.3
Hz, 1 H, H-1), 5.16 (dd, J = 6.7, 1.4 Hz, 2 H, H-5), 4.16 (dq, J = 7.7, 7.1,
Hz, 4 H, H-8), 3.99 (t, J = 7.4 Hz, 4 H, H-6), 2.44 (quint, J = 7.4 Hz, 2 H,
H-7), 1.35 (td, J = 7.1, 1.0 Hz, 6 H, H-9).
13C NMR (100 MHz, CDCl3): δ = 161.7, 155.9, 154.0, 149.8, 149.7,
124.2, 108.0, 107.0, 106.8, 97.2, 64.5 (d, J = 6.0 Hz), 64.4 (d, J = 4.4 Hz),
51.8, 16.6, 16.2 (d, J = 6.7 Hz).
31P NMR (162 MHz, CDCl3): δ = –0.5.
HRMS-ESI: m/z [M + H]+ calcd for C17H23NO6P: 368.1258; found:
368.1256.
Anal. Calcd for C17H22NO6P: C, 55.59; H, 6.04; N, 3.81. Found: C, 55.79;
H, 6.26; N, 3.55.
Mp 215 °C; Rf = 0.26 (silica gel; CH2Cl2–EtOAc, 95:5).
IR (neat): 3332, 1718, 1605, 1527, 1419, 1238, 1145 cm–1
.
(7-(Azetidin-1-yl)-2-oxo-2H-chromen-4-yl)methyl Methanesul-
fonate (17)
1H NMR (400 MHz, DMSO-d6): δ = 8.09 (t, J = 6.2 Hz, 1 H, H-8), 7.47 (d,
J = 8.7 Hz, 1 H, H-3), 7.38–7.18 (m, 5 H, H-10, H-11, and H-12), 6.38
(dd, J = 8.7, 2.3 Hz, 1 H, H-2), 6.27 (d, J = 2.2 Hz, 1 H, H-1), 6.03 (t,
J = 1.3 Hz, 1 H, H-4), 5.25 (d, J = 1.4 Hz, 2 H, H-5), 4.24 (d, J = 6.1 Hz, 2
H, H-9), 3.95 (t, J = 7.4 Hz, 4 H, H-6), 2.35 (quint, J = 7.4 Hz, 2 H, H-7).
13C NMR (100 MHz, DMSO-d6): δ = 160.5, 155.7, 155.2, 153.9, 152.1,
139.5, 128.3, 127.0, 126.9, 125.2, 108.0, 106.4, 105.0, 96.3, 61.1, 51.5,
43.9, 15.9.
Compound 3 (50 mg, 0.21 mmol) was flame-dried in a two-neck flask
equipped with a dropping funnel, and the system was evacuated and
backfilled with nitrogen three times. Anhydrous CH2Cl2 (8 mL) and
Et3N (60 μL) were added, and the system was cooled in an ice bath. A
solution of methanesulfonyl chloride (25 μL, 0.32 mmol) in CH2Cl2 (2
mL) was added dropwise. The reaction mixture was stirred at 0 °C for
45 min, until completion, as determined by TLC (silica gel; CH2Cl2–
Et2O, 7:3) by the disappearance of the starting material. CH2Cl2 was
added, and the organic phase was washed with H2O. The aqueous
phase was back-extracted once with CH2Cl2. The combined organic
phases were washed with brine, dried over MgSO4, and concentrated
under reduced pressure until a solid was obtained. The crude was dis-
solved in CH2Cl2 and as little MeOH as possible, and was fixed on
Celite. The product was purified by silica gel column chromatography
(CH2Cl2–Et2O, 95:5), and isolated as an off-white solid; yield: 50 mg
(77%).
HRMS-ESI: m/z [M + H]+ calcd for C21H21N2O4: 365.1496; found:
365.1493.
Anal. Calcd for C21H20N2O4: C, 69.22; H, 5.53; N, 7.69. Found: C, 68.97;
H, 5.59; N, 7.58.
Funding Information
This work was supported by the Swiss National Science Foundation
Mp 225 °C (possible phase changes at 153 °C, color change at 200 °C);
(grant 200021_165551).
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0
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5
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Rf = 0.25 (silica gel; CH2Cl2–Et2O, 95:5).
IR (neat): 2934, 1720, 1605, 1532, 1417, 1356, 1176, 1068, 963, 833
Acknowledgment
cm–1
.
1H NMR (400 MHz, CDCl3): δ = 7.30 (d, J = 8.7 Hz, 1 H, H-3), 6.30 (dd,
J = 8.7, 2.3 Hz, 1 H, H-2), 6.21 (d, J = 2.3 Hz, 1 H, H-1), 6.20 (t, J = 1.1
Hz, 1 H, H-4), 5.29 (d, J = 1.1 Hz, 2 H, H-5), 4.01 (t, J = 7.4 Hz, 4 H, H-6),
3.10 (s, 3 H, H-8), 2.45 (quint, J = 7.4 Hz, 2 H, H-7).
13C NMR (100 MHz, CDCl3): δ = 161.2, 156.1, 154.1, 147.2, 124.4,
108.5, 108.2, 106.6, 97.2, 65.7, 51.7, 38.4, 16.5.
We thank Mr. Matthias Schneider for preliminary experiments.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1591742.
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HRMS-ESI: m/z [M + H]+ calcd for C14H16NO5S: 310.0744; found:
310.0742.
References
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© Georg Thieme Verlag Stuttgart · New York — Synthesis 2017, 49, A–G