Photodecarboxylation of Adamantane Amino Acids Activated by Phthalimide

Article abstract of DOI:10.1002/ejoc.201600491

Adamantane α-, β-, and δ-amino acids activated by phthalimide (i.e., 3–6) were synthesized, and their photochemical reactivities were investigated. Amino acid derivatives 3–6 underwent a photoinduced electron transfer (PET) and decarboxylation reaction sequence, most probably through a triplet excited state. The decarboxylations of the β-amino acid derivatives were succeeded by cyclization reactions that afforded complex polycyclic molecules with potential biological interest. The adamantyl radical that is produced by the photoinduced decarboxylation could be trapped by alkenes or oxygen to deliver adducts or alcohols, respectively. The photodecarboxylation process was shown to be more efficient under acetone sensitization conditions (with quantum yields, Φ = 0.02–0.5) than upon direct excitation, and the reactivity was dependent on the chain length (intramolecular distance) between the electron donor (carboxylate) and acceptor (phthalimide in the triplet excited state) of the derivative. The formation of different radicals, that is, the 1- or 2-adamantyl intermediate, probably does not affect the overall rate of the decarboxylation This current report provides a better understanding of photodecarboxylation and the rational design of molecular systems to undergo photoinduced decarboxylation and cyclization reactions.

Full text of DOI:10.1002/ejoc.201600491

DOI: 10.1002/ejoc.201600491
Full Paper
Photochemistry
Photodecarboxylation of Adamantane Amino Acids Activated
by Phthalimide
Leo Mandić,[a] Kata Mlinarić-Majerski,[a] Axel G. Griesbeck,[b] and Nikola Basarić*[a]
Abstract: Adamantane α-, ꢀ-, and δ-amino acids activated by sensitization conditions (with quantum yields, Φ = 0.02–0.5)
phthalimide (i.e., 3–6) were synthesized, and their photochemi- than upon direct excitation, and the reactivity was dependent
cal reactivities were investigated. Amino acid derivatives 3–6 on the chain length (intramolecular distance) between the elec-
underwent a photoinduced electron transfer (PET) and decarb- tron donor (carboxylate) and acceptor (phthalimide in the
oxylation reaction sequence, most probably through a triplet triplet excited state) of the derivative. The formation of
excited state. The decarboxylations of the ꢀ-amino acid deriva- different radicals, that is, the 1- or 2-adamantyl intermediate,
tives were succeeded by cyclization reactions that afforded probably does not affect the overall rate of the decarboxylation
complex polycyclic molecules with potential biological interest. This current report provides a better understanding of photo-
The adamantyl radical that is produced by the photoinduced decarboxylation and the rational design of molecular systems
decarboxylation could be trapped by alkenes or oxygen to de- to undergo photoinduced decarboxylation and cyclization reac-
liver adducts or alcohols, respectively. The photodecarboxyl- tions.
ation process was shown to be more efficient under acetone
Introduction
process which was conducted in microflow reactors.[43] Phthal-
imide in the triplet excited state can also promote the elimina-
tion of silyl groups,[44] and – from a thermodynamic point of
view – initiate all PET reactions in which the donor oxidation
Photochemically induced decarboxylation reactions have been
extensively investigated[1–3] because of their importance in or-
ganic synthesis,[4,5] the development of photocages,[6,7] and
materials science[8] as well as the fact that some nonsteroidal
anti-inflamatory drugs can induce a photoallergic response as
a result of undergoing a photodecarboxylation process.[9–12]
Photocatalytic decarboxylation reactions have recently experi-
enced a revival in organic synthesis,[13–18] and significant
progress has been made to elucidate the photodecarboxylation
reaction mechanism that leads to anionic species.[19–28]
Photodecarboxylation can be initiated by the phthalimide
chromophore in the triplet excited state, which acts as the elec-
tron acceptor in the photoinduced electron transfer (PET) reac-
tion.[29,30] The PET-promoted decarboxylation by phthalimides
has been used in macrocyclizations,[31] the cyclization of pept-
ides,[32,33] photodecaging strategies,[34] and the enantioselect-
ive synthesis of benzodiazepines.[35] Moreover, photoinduced
decarboxylation has been applied to the addition of acet-
ates,[36,37] benzyl groups,[38–40] and α-amino acids to phthalim-
ide[41] as well as to the formation of cyclic aryl ethers,[42] the
potential is <1.6
V versus saturated calomel electrode
(SCE).[30,45]
We have recently discovered an efficient approach for the
phthalimide-promoted photodecarboxylative radical addition
to alkenes.[46] First, an unnatural adamantane γ-amino acid is
activated by phthalimide. In its triplet excited state, the phthal-
imide of 1 is reduced to give a radical anion, whereupon the
carboxylate group underwent an oxidation and the irreversible
elimination of CO2 (Scheme 1). The resulting radical could then
be trapped by an electron-deficient alkene to deliver adduct
2.[46]
Herein, we describe a more general investigation of the PET-
promoted photodecarboxylation of a series of unnatural amino
acids that are activated by phthalimides (i.e., 3–6). All of the
unnatural amino acids under investigation are adamantane de-
rivatives, which are characterized by a rigid geometry and fixed
length between the electron-donor (i.e., the carboxylate) and
electron-acceptor groups (i.e., phthalimide). Specifically, phthal-
imide 3 is an α-amino acid derivative, where as 4 and 5 are
derivatives of a ꢀ-amino acid. Phthalimides 6-E and 6-Z are δ-
amino acid derivatives. The investigation was conducted to de-
termine how the length between the electron donor and ac-
ceptor influences the decarboxylation efficiency. Moreover, de-
rivatives 4 and 5 were strategically designed to investigate
whether the rate of CO2 elimination affects the overall reaction
efficiency. The elimination of CO2, in these two examples,
should deliver two different adamantyl radicals. However, there
[a] Department of Organic Chemistry and Biochemistry,
Ruđer Bošković Institute,
Bijenička cesta 54, 10000 Zagreb, Croatia
E-mail: nbasaric@irb.hr
http://www.irb.hr/eng/People/Nikola-Basaric
[b] Department of Chemistry, University of Cologne,
Greinstr. 4, 50939 Cologne, Germany
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600491.
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Scheme 1. Photochemical reaction of 1 in the presence of acrylonitrile.
is no difference in the distances between the electron donor
and acceptor. Preparative irradiations were also carried out un-
der different conditions along with the isolation of the photo-
products. The photoreactions either delivered simple decarbox-
ylation products or more complex polycyclic molecules, which
are particularly interesting because of their anticipated biologi-
cal activity.[47–51]
Scheme 2. Synthesis of phthalimide derivative of α-adamantane amino acid
(DMF = N,N-dimethylformamide).
that was hydrolyzed into oxime acid 9.[59] The hydrogenation
of 9 over PtO2 afforded amino acid 10 in high yield. Alterna-
tively, amino acid 10 was obtained by a reductive amination of
ketoacid 8H (Scheme 3).
The first step in the preparation of ꢀ-amino acid 15 from
keto ester 8 involved a reaction with p-toluenesulfonylmethyl
isocyanide (TosMIC), according to a modification of the known
procedure,[63] to afford cyanide ester 11Et[59] (Scheme 3). The
hydrolysis into acid 11H[60] followed by a reaction with SOCl2
and NaN3 afforded carbonyl azide 12. The rearrangement of 12
by following a modification of a known procedure[64] gave an
isocyanate, which was not isolated but instead treated with
EtOH to afford carbamate 13Et (87 % yield in four steps, start-
ing from 11H). We also attempted to prepare carbamate 13Me
by using another strategy, in which acid 8H was first converted
into carbonyl azide 16. This azide then underwent the rear-
rangement into its corresponding isocyanate, and a further re-
action with CH3OH afforded carbamate 17 in good yields. How-
ever, the treatment of ketone 17 with TosMIC gave a poor yield
of the desired carbamate 13Me. Nevertheless, base hydrolysis
of either carbamate 13Et or 13Me afforded amidoamine 14,
and subsequent acid hydrolysis gave the desired amino acid 15
in good yield along with a small amount of the hydroxy acid.
Amino acids 10 and 15 were then treated with phthalic an-
hydride to yield the corresponding phthalimide derivatives 4
and 5, respectively.
Results and Discussion
Synthesis
Phthalimide derivatives 3–6 were prepared in moderate to
good yields by a condensation reaction between the corre-
sponding amino acid and phthalic anhydride and using a modi-
fication[46] of a published procedure.[52,53] α-Amino acid 7,[54]
which was synthesized according to a known procedure[55] by
starting from 2-adamantanone through a hydantoin deriva-
tive,[54] was transformed into the corresponding phthalimide 3
in moderate yield (Scheme 2).
The synthesis of the diastereomeric phthalimide derivatives
6 started from hydroxy ketone 18, which was obtained from
the oxidation of adamantanone by chromic acid.[65] Alcohol 18
The syntheses of the adamantane ꢀ-amino acids activated was converted into ketoacid 19[66] by using a Koch–Haaf reac-
by phthalimide (i.e., 4 and 5) are shown in Scheme 3. ꢀ-Amino tion, and ketone 19 was then transformed through a reductive
acids 10[56] and 15[57] were synthesized from known keto esters amination into a diastereomeric mixture of amino acids 20,[66]
or ketoacid 8[58–60] according to a modification[61] of a known which were not separated. A condensation reaction with
procedure that took place in three steps[62] from homo- phthalic anhydride gave a mixture of diastereomers 6, which
adamantanone. Keto ester 8Me was converted into an oxime were then separated and characterized (Scheme 4).
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Scheme 3. Synthesis of phthalimide derivatives of ꢀ-adamantane amino acids (i.e., 4 and 5; DME = 1,2-dimethoxyethane).
Scheme 4. Synthesis of phthalimide derivatives of δ-adamantane amino acid (i.e., 6-Z and 6-E).
Photochemical Reactivity
sions reached over the same irradiation times. This finding is in
line with the better electron-donating ability of carboxylates
(10-undecenoic acid in CH3CN in the presence of 0.5 equiv. of
On the basis of literature precedent,[46,72] the excitation of
phthalimides 3–6 in the presence of a base to deprotonate the
carboxylic group is anticipated to lead to an irreversible PET
decarboxylation and deliver simple decarboxylation or cycliza-
tion products. To probe the reactivity of phthalimides 3–6 to-
wards decarboxylation, these compounds were irradiated at
300 nm in the presence of 0.5 equiv. K2CO3 in either CH3CN/
H2O (2:1) or acetone/H2O (2:1), where acetone acts as a triplet
sensitizer. The irradiation of 3 and 4 was performed in both the
presence and absence of base. The results of these experiments
are summarized in Table 1. Regardless of the solvent in which
the irradiation was conducted, the addition of base increased
benzyltrimethylammonium hydroxide, irreversible process Eox
≈
1.38 V vs. Ag/AgCl[67]) versus carboxylic acids (10-undecenoic
acid in CH3CN, Eox ≈ 1.85 V vs. Ag/AgCl[67]).[68,69] Moreover, the
similar reaction efficiency upon sensitized excitation in acetone
versus CH3CN indicates that the decarboxylation takes place
through the triplet excited state. Namely, the triplet energy of
acetone (ET = 332 kJ mol–1)[70] is higher than that of the N-
alkylphthalimides (ET = 297 kJ mol–1)[70] to allow for an exer-
gonic energy transfer.
To isolate the photoproducts, preparative irradiation reac-
the efficiency of the photoreaction, as observed by the conver- tions of 3–6 were conducted in CH3CN/H2O (2:1) in the pres-
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Table 1. Irradiation conditions, conversions, and product yields upon photoly-
abstraction of 21, the composition of the irradiation mixture
was analyzed during the process (see Supporting Information,
Figure S1). Because 22 is formed as the major product after a
short irradiation time (1 min) and the relative ratio of 22 to 21
does not change over the first few minutes of irradiation, we
concluded that 22 is formed directly from 4.
sis of phthalimides 3–6.[a]
Solvent,
conditions
Time of
Conv. Products [% yield]
irradiation [%][b]
3
3
CH3CN/H2O, 2:1;
CH3CN/H2O, 2:1;
no K2CO3
acetone/H2O, 2:1
acetone/H2O, 2:1;
no K2CO3
CH3CN/H2O, 2:1;
CH3CN/H2O, 2:1;
no K2CO3
acetone/H2O, 2:1;
acetone/H2O, 2:1;
no K2CO3
5 min
5 min
16
4
21 (100)[b]
21 (100)[b]
3
3
5 min
5 min
33
7
21 (100)[b]
21 (100)[b]
4
4
5 min
5 min
82
17
21 (20), 22 (80)[b]
21 (76), 22 (24)[b]
4
4
5 min
5 min
68
8
21 (25), 22 (75)[b]
21 (38), 22 (62)[b]
Scheme 6. Photochemical reaction of 4.
5
5
6-E
6-E
6-Z
6-Z
CH3CN/H2O, 2:1
acetone/H2O, 2:1
CH3CN/H2O, 2:1
acetone/H2O, 2:1
CH3CN/H2O, 2:1
acetone/H2O, 2:1
5 min
5 min
1.5 min
3 min
1.5 min
3 min
71
65
2
2
18
10
23 (67), 24 (17), 25 (16)[b]
23 (71), 24 (14), 25 (15)[b]
21 (100)[b,c]
The irradiation of phthalimide 5 afforded the three products
23–25 (Scheme 7), the ratio of which depended on the irradia-
tion time. Because 23 could undergo a photochemical intramo-
lecular H-abstraction to give 24 and 25,[73,74] it was important
to investigate whether 24 was formed from the decarboxylative
cyclization of 5 or from 23 (Scheme 7). Thus, the composition
of the irradiation mixture was analyzed during the process (see
Supporting Information, Figure S2). Although 24 is not the ma-
jor product, its formation at an early irradiation time suggests
that it is formed directly from 5. Moreover, after longer irradia-
tion times when the conversion of 5 was almost complete, the
concentration of 23 increased but the content of 24 decreased
as a result of a secondary photochemical reaction, that is, the
transformation of 24 into 25. This observed dependence of the
distribution of photoproducts on the irradiation time can be
explained by a more efficient photodecarboxylation process fol-
lowed by H-abstraction, which is in line with literature prece-
dent[71] and our quantum yield measurements (see below).
Similar to the photochemistry of 3, the photolysis of either
6-E or 6-Z proceeded cleanly to give only one product, phthal-
imide 21 (Scheme 8). The reaction of 6-E, however, was less
efficient than that of 6-Z (see below).
21 (100)[b,c]
21 (100)[c]
21 (100)[c]
[a] The irradiation reactions were conducted by using a lamp with a 300 nm
output. The concentration of the phthalimide solution was 1 m (5 mg/
15 mL), whereas that of the K2CO3 solution was 0.5 m . The solutions were
M
M
purged with N2 prior to irradiation. [b] The percent conversion was deter-
mined by 1H NMR spectroscopic analysis of the crude irradiation mixture.
[c] The product ratio was determined by HPLC analysis.
ence of 0.5 equiv. K2CO3. Phthalimide 3 was irradiated until a
high conversion was reached to cleanly afford phthalimide 21
(Scheme 5). Product 21, which contains the phthalimide
chromophore, can then undergo secondary photochemical re-
actions. However, intramolecular H-abstractions by the phthal-
imide group generally proceed with very low quantum effi-
ciency as a result of inefficient formation of the reactive triplet
state by intersystem crossing (ISC).[71] Therefore, in CH3CN/H2O,
a much longer irradiation time is needed to transform 21 into
its photochemical products.
Scheme 5. Photochemical reaction of 3.
The irradiation of phthalimide 4 gave the simple decarbox-
ylation product 21 and cyclic product 22 in a ratio of 1:3. How-
ever, the cyclic product could, in principle, be formed from 21
Scheme 8. Photochemical reaction of 6.
The irradiation of 1 in the presence of an electron-deficient
by a photochemical reaction (Scheme 6).[72] To investigate if 22 alkene gave adduct 2 as a result of trapping the radical formed
is a product of the decarboxylative cyclization of 4 or an H- during the decarboxylation. Similarly, the irradiation of 3 and 4
Scheme 7. Photochemical reaction of 5.
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in the presence of acrylonitrile was anticipated to form the radi-
cal-trapping products. Indeed, the irradiation of 4 gave adduct
26 as the major product (41 %) along with a small amount of
the simple decarboxylation derivative 21 (Scheme 9). Product
22, the result of a cyclization reaction, was not isolated.
Scheme 11. Photochemical reaction of 4 in the presence of oxygen.
rate of kq = 2 × 109
reactive excited triplet state has a lifetime τ ≈ 400 ns. Transient
M
–1 s–1,[77] this experiment suggests that the
Scheme 9. Photochemical reaction of 4 in the presence of acrylonitrile.
The photochemical reaction of 5 with acrylonitrile gave de- absorption measurements should reveal the properties and re-
carboxylation product 23 and adduct 27 in a ratio of 1:1 activity of the triplet state involved (not within the scope of
(Scheme 10). Similar to the photolysis of 4 in the presence of this manuscript).
acrylonitrile, the cyclization products 24 and 25 were not iso-
lated. It is interesting, however, that the photolysis of 5 (com-
pared with 4) gives a higher amount of the decarboxylation
product than the acrylonitrile adduct. With theoretical[75] and
experimental data[76] supporting the higher stability of the 1-
adamantyl compared with the 2-adamantyl radical, the differ-
ence in the product distributions from the reactions of 4 and 5
can be explained by the lower selectivity of the more reactive
2-adamantyl radical.
To compare photochemical reactivities of 3–6 and investi-
gate the influence of the molecular structure on the efficiency
of the photodecarboxylation, quantum yield measurements of
the photochemical reactions in CH3CN/H2O and acetone/H2O
were conducted. The measurements were performed in the
presence of a secondary actinometer (i.e., not one of the stan-
dardized actinometers listed in ref.[78]), the photolysis of phthal-
imide 31 (Scheme 12) gave 32 and 33 (ΦR = 0.3).[79] The results
are compiled in Table 2.
Scheme 10. Photochemical reaction of 5 in the presence of acrylonitrile.
The similar efficiencies of the photochemical reactions of 3–
6 upon irradiation in acetone/H2O or CH3CN/H2O suggests that
the photoreactions take place through the triplet excited state.
To verify if the formation of the products stems from a triplet
pathway, the irradiation of 4 was conducted in the presence of
O2, a ubiquitous triplet quencher. The preparative irradiation in
the presence of O2 gave the simple decarboxylation product
21, oxidation products – peroxide 28 and alcohol 29, and the
cleavage product – phthalimide 30, which were isolated and
characterized (Scheme 11). Because of the instability of the per-
oxide, it decomposed upon chromatographic separation to give
alcohol 29. Oxidation products 28 and 29 were formed by the
O2 trapping of the radical that was produced during the de-
carboxylation process, analogous to the formation of radical-
trapping products 26 and 27.
Scheme 12. Photochemical reaction used as a secondary actinometer.
Several general trends can be seen from the measured quan-
tum yields of the photoreactions. Phthalimides undergo more
efficient reactions upon sensitized excitation by acetone than
upon direct excitation of the phthalimide chromophore at 254
or 300 nm. The difference in the efficiency is most pronounced
for α-amino acid phthalimide derivative 3 (a 15 to 35 times
more efficient reaction under sensitization conditions). For the
ꢀ- and δ-amino acid phthalimide derivatives, the sensitized re-
actions are approximately two times more efficient or have a
The irradiation of 4 was performed in N2-purged, air-satu- similar efficiency as those that undergo direct excitation of the
rated, and O2-purged solutions. Contrary to a previous report phthalimide. These results suggest that the structure of the ada-
regarding phthalimide 1,[46] the presence of O2 quenched the mantane skeleton (although not a chromophore) affects the ef-
decarboxylation reaction of 4. This dependence of the photo- ficiency of intersystem crossing (ISC) by making it less efficient
conversion on O2 concentration, which was demonstrated by for the α-amino acid compared with that of the ꢀ- and δ-deriva-
experiments conducted in N2-purged, air-saturated, and O2- tives. Moreover, a different efficiency was observed for phthal-
purged solutions, allowed for an estimation of the Stern–Volmer imide derivatives 3 and 4 upon direct irradiation at 254 or
quenching constant, KSV = 774
tion, Figure S3). Assuming that the quenching takes place at a from a higher triplet excited state, the population of which de-
2 M
–1 (see Supporting Informa- 300 nm, which may be the result of the reaction taking place
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Table 2. Quantum efficiency for photoreaction (ΦR) of 1 and 3–6.
ΦR (acetone/H2O)[a] ΦR (CH3CN/H2O)[b] ΦR (CH3CN/H2O)[c]
1
3
4
5
0.50[d]
0.11[d]
–
0.47 0.02[e]
0.35 0.03[e]
0.35 0.02[f]
0.51 0.05[e]
0.44 0.02[f]
0.50 0.04[e]
0.44 0.02[f]
0.023 0.002[e]
0.15 0.01[e]
0.026 0.002[f]
0.41 0.03[f]
0.21 0.02[f]
0.010 0.002[f]
0.25 0.01[f]
0.21 0.03[f]
6-E
6-Z
0.09 0.01[e]
0.19 0.01[e]
0.015 0.001[e]
0.11 0.01[e]
[a] Quantum efficiency was measured in acetone/H2O (2:1) in the presence
of K2CO3 (0.5 equiv.) after irradiation at 300 nm by averaging the values
obtained from three measurements. A secondary actinometer was used,
Figure 1. Anticipated radical intermediates in the photodecarboxylation of ꢀ-
amino acid derivatives.
namely, the photolysis of 31 in CH3CN/H2O (3:1) to give 32 and 33 (ΦR
=
0.30 0.03).[79] [b] Quantum efficiency was measured in CH3CN/H2O (2:1) in
the presence of K2CO3 (0.5 equiv.) upon irradiation at 254 nm by using a
termediate 36-Z or 36-E, the observed differences in the effi-
ciency of the reactions are most likely from the different rates
of the PET between the carboxylate and the phthalimide rather
than the different rates of CO2 elimination. Furthermore, the
difference in the rates of the PET step for 6-E and 6-Z indicates
that the electron transfer does not take place through the ada-
mantane sigma bonds but instead through space, being faster
for the Z isomer in which the phthalimide and carboxylate are
closer. The result may also be related to the previous report by
Griesbeck et al.,[84] in which a templating effect of K+ between
the carboxylate and the phthalimide was suggested. Namely,
K+ acts as an anchor and stabilizes the optimal geometry be-
tween the donor and acceptor.[84,85] Such a structure is not pos-
sible for the 6-E isomer.
secondary actinometer, namely, the photolysis of 31 in CH3CN/H2O (3:1; ΦR
=
0.30). The errors correspond to the values obtained from three independent
measurements. The quantum yield for the decomposition of 31 was taken
from the literature and measured by using three primary actinometers,
namely, potassium ferrioxalate (ΦR = 1.25),[70,80] KI/KIO3 (ΦR = 0.74),[70,81] and
valerophenone (ΦR = 0.65 0.03).[70,78] [c] Quantum efficiency was measured
in CH3CN/H2O (2:1) in the presence of K2CO3 (0.5 equiv.) after irradiation at
300 nm by averaging the values obtained from three measurements and
using a secondary actinometer, namely, the photolysis of 31 in CH3CN/H2O
(3:1; ΦR = 0.30). [d] Value was taken from the literature. See ref.[46]. [e] The
analysis of the irradiated solution was performed by NMR spectroscopy.
[f] The analysis of the irradiated solution was performed by HPLC.
pends on the excitation wavelength. Photochemical reactions
of phthalimides from higher excited triplet states have been
demonstrated.[74,77,82] However, it is unusual that upon excita-
tion to S2, the internal conversion into S1 competes with the
ISC. Such a pathway is plausible if the rate of the ISC is very
fast (kISC > 1011 s–1), which is known to be the case for examples
that obey El Sayed's rules and possible with carbonyl chromo-
phores that have n,π* and π,π* excited states.[83]
A comparison of the efficiency of decarboxylation upon sen-
sitization for phthalimides 4 and 5 (both ꢀ-amino acid deriva-
tives), in which the distances between the electron donor (carb-
oxylate) and the electron acceptor (phthalimide) are the same,
does not reveal significant differences. The reaction of 4, how-
ever, proceeds through 1-adamantyl radical 34, whereas 5 pro-
gresses through 2-adamantyl radical 35 as an intermediate (Fig-
ure 1). The formation of these anticipated intermediates indi-
cates that that irreversible decarboxylation step that affords the
corresponding radical is not the slowest step in the process
and suggests that the intramolecular PET process is the slowest
step.
The most interesting feature of this investigated series of
compounds is how the reaction efficiency is influenced by the
distance between the electron donor and acceptor (for the
plots of the efficiency dependence on the molecular distances,
see Figures S4 and S5 in the Supporting Information). Thus,
upon sensitization, δ-derivatives undergo a less efficent reac-
tion than α-, ꢀ-, or γ-derivatives. Moreover, diastereomer 6-Z
undergoes approximately a 10 times more efficent reaction
than that of the 6-E isomer. Because both diastereomers give
Conclusions
Unnatural adamantane α-, ꢀ-, and δ-amino acids activated by
phthalimide (i.e., 3–6) were synthesized. All derivatives 3–6 un-
derwent a PET and decarboxylation, which was accompanied
by a cyclization reaction in the case of the ꢀ-amino acid deriva-
tives. The adamantyl radical that is formed by decarboxylation
can be trapped by acrylonitrile or oxygen to give adducts or
peroxides, respectively. The efficiency of the photodecarboxyl-
ation reaction is generally higher under acetone-sensitized con-
ditions than upon direct excitation, which suggests that the
photoreaction takes place through a triplet excited state. The
efficiency of the photodecarboxylation is higher for α- and ꢀ-
than for δ-derivatives, which indicates that the rate-determin-
ing step is probably the PET between the carboxylate and
phthalimide moieties. This observation has important implica-
tions in the design of molecular systems for the synthesis of
polycyclic and macrocyclic molecules by using a photodecarb-
oxylative cyclization strategy.
Experimental Section
General Methods: The 1H and 13C NMR spectroscopic data were
recorded at room temperature at either 300 or 600 MHz. TMS was
used as an internal reference, and the chemical shifts were reported
in ppm. Melting points were measured with a Mikroheiztisch appa-
the same product and proceed through 1-adamantyl radical in- ratus. IR spectra were recorded with a spectrophotometer by using
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KBr pellets for the samples. The frequencies of the characteristic
band are reported in cm–1. HRMS data were obtained on a MALDI
TOF/TOF instrument. Irradiation experiments were performed in a
reactor that was equipped with 11 lamps and provided a 300 nm
output or a reactor that was equipped with 8 lamps at 254 or
2.18 (dd, J = 12.2, 1.8 Hz, 1 H), 2.05 (dd, J = 2.6, 3.0 Hz, 2 H), 1.56–
1.86 (m, 7 H), 1.52 (ddd, J = 13.0, 1.6, 1.3 Hz, 1 H), 1.39 (br. s, 2 H),
1.32 (ddd, J = 12.2, 2.6, 1.8 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 176.2 (s), 54.2 (d), 51.2 (t), 49.6 (s), 40.6 (t), 37.4 (t), 36.3 (t), 32.4
(t), 31.3 (d), 30.2 (d), 29.0 (d) ppm. IR (KBr): ν = 3344, 3249, 2931,
˜
300 nm (one lamp: 8 W). During the irradiation process, the irradi- 2909, 2849, 1649, 1588, 1561, 1451, 1391, 1099, 926, 743 cm–1. MS
ated solutions were continuously purged with Ar and cooled by a
tap water finger condenser. Solvents for the irradiation processes
were of HPLC purity. Chemicals were purchased from the usual
commercial sources and used as received. Solvents for chromato-
graphic separations were used as is from the supplier (p.a. or HPLC
grade) or purified by distillation (CH2Cl2). Experimental procedures
for the preparation of known intermediates are given in the Sup-
porting Information.
(ESI+): m/z = 195.
Methyl 2-Oxoadamantane-1-carbamate (17): Azide 16 (1.49 g,
6.80 mmol) was dissolved in benzene (35 mL), and the resulting
mixture was heated at reflux for 3.5 h. Anhydrous CH3OH (15 mL)
was added, and the heating was continued overnight. The solvent
was removed on a rotary evaporator, and the residue was purified
by chromatography on a silica gel column (EtOAc/hexane, 1:1) to
afford pure 17 (1.09 g, 70 %) as a colorless solid; m.p. 107–110 °C.
1H NMR (600 MHz, CDCl3): δ = 6.22 (br. s, 1 H), 3.62 (s, 3 H), 2.99 (d,
J = 11.0 Hz, 1 H), 2.75 (br. s, 1 H), 2.21 (br. s, 2 H), 2.05 (ddd, J =
12.9, 2.9, 2.8 Hz, 2 H), 1.98 (d, J = 12.9 Hz, 2 H), 1.91 (d, J = 12.8 Hz,
1 H), 1.85 (dd, J = 12.8, 1.6 Hz, 1 H), 1.79 (d, J = 12.9 Hz, 2 H) ppm.
13C NMR (150 MHz, CDCl3): δ = 211.8 (s), 155.2 (s), 61.7 (s), 51.6 (q),
46.0 (d), 43.2 (t), 39.3 (t, 2 C), 34.7 (t, 2 C), 29.0 (d, 2 C) ppm. IR (KBr):
2-Cyanoadamantane-1-carboxazide (12): A round-bottomed flask
(250 mL) was charged with 11H (3.05 g, 14.9 mmol) and SOCl2
(50 mL), and the resulting mixture was heated at reflux for 1 h. The
SOCl2 was removed under reduced pressure, and the residue was
dissolved in acetone (30 mL) and then cooled to 0 °C. A solution of
NaN3 (1.49 g, 22.9 mmol) in acetone (25 mL) and H2O (15 mL) was
added dropwise to the reaction mixture, as the temperature was
kept at 0 °C. After the addition was complete, the mixture contin-
ued to stir at 0 °C for 30 min. H2O (50 mL) was added, and the
mixture was extracted with Et2O (4 × 75 mL). The combine organic
extracts were washed with a saturated aqueous NaHCO3 solution
(50 mL) and brine (25 mL), dried with anhydrous MgSO4, and then
filtered. The solvent was removed on a rotary evaporator to afford
ν = 3398, 2930, 2858, 1722, 1502, 1216, 1055, 777, 530 cm–1. MS
˜
(ESI+): m/z = 246.
Methyl 2-Cyanoadamantane-1-carbamate (13Me): A round-bot-
tomed flask (100 mL) under N2 was charged with 17 (225 mg,
1.01 mmol), TosMIC (253 mg, 1.30 mmol), DME (6 mL), and anhy-
drous EtOH (0.11 mL, 1.88 mmol). The reaction mixture was cooled
to 0 °C by using an ice bath, and tBuOK (350 mg, 3.12 mmol) was
added in two portions over 10 min. The reaction mixture was stirred
for 1 h, whereupon it was allowed to warm to room temp. The
stirring was continued overnight, as the mixture was heated to
45 °C. After cooling to room temp., Et2O (20 mL) was added, and
the reaction mixture was filtered through a sintered glass funnel.
The filter cake was washed with Et2O (80 mL), and the solvent of
the combined filtrates was removed on a rotary evaporator. The
residue was purified by chromatography on a silica gel column
(Et2O/hexane, 1:1) to afford pure 13Me (6 mg, 3 %) as colorless
1
pure 12 (3.33 g, 97 %) as colorless crystals; m.p. 70–76 °C. H NMR
(600 MHz, CDCl3): δ = 3.16 (br. s, 1 H), 2.32 (br. s, 1 H), 2.15 (ddd,
J = 13.3, 2.5, 2.1 Hz, 1 H), 2.05–2.13 (m, 3 H), 2.02 (ddd, J = 13.3,
2.5, 2.1 Hz, 1 H), 1.93 (ddd, J = 12.6, 2.8, 2.1 Hz, 1 H), 1.88 (ddd, J =
13.3, 2.8, 2.1 Hz, 1 H), 1.79 (ddd, J = 12.6, 2.8, 2.1 Hz, 1 H), 1.69–
1.77 (m, 4 H) ppm. 13C NMR (150 MHz, CDCl3): δ = 182.3 (s), 120.3
(s), 44.8 (s), 39.5 (t), 38.5 (d), 35.7 (t), 35.6 (t), 34.1 (t), 32.2 (t), 31.1
(d), 27.0 (d), 26.8 (d) ppm. IR (KBr): ν = 2934, 2862, 2253, 2142, 1699,
˜
1629, 1452, 1199, 988, 703 cm–1. MS (ESI+): m/z = 203.
1
crystals; m.p. 155–158 °C. H NMR (600 MHz, CDCl3): δ = 4.88 (br. s,
Ethyl 2-Cyanoadamantane-1-carbamate (13Et): A round-bot-
tomed flask (100 mL) was charged with 12 (3.33 g, 14.5 mmol) and
anhydrous benzene (50 mL), and the resulting mixture was heated
at reflux for 2 h. Anhydrous EtOH (50 mL) was added, and the heat-
ing was continued over 1 d. The solvent was removed on a rotary
evaporator, and the residue was purified by chromatography on a
silica gel column (Et2O/hexane, 3:7) to afford pure 13Et (3.23 g,
88 % over two steps from 11H) as a colorless solid; m.p. 113–116 °C.
1H NMR (300 MHz, CDCl3): δ = 4.73 (br. s, 1 H), 4.08 (q, J = 7.1 Hz,
2 H), 3.90 (br. s, 1 H), 2.64 (d, J = 12.2 Hz, 1 H), 2.36 (br. s, 1 H),
2.04–2.20 (m, 3 H), 2.01 (ddd, J = 12.2, 1.5, 2.7 Hz, 1 H), 1.59–1.82
(m, 7 H), 1.24 (t, J = 7.1 Hz, 3 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 154.7 (s), 120.5 (s), 60.8 (t), 51.0 (s), 40.6 (d), 40.5 (t), 39.3 (t),
36.0 (t), 35.3 (t), 32.4 (t), 32.2 (d), 28.7 (d), 28.5 (d), 14.6 (q) ppm. IR
1 H), 3.89 (br. s, 1 H), 3.64 (s, 3 H), 2.63 (d, J = 11.7 Hz, 1 H), 2.36
(dd, J = 2.6, 2.8 Hz, 1 H), 2.15 (dd, J = 2.6, 2.8 Hz, 1 H), 2.11 (dd, J =
2.6, 2.8 Hz, 1 H), 2.09 (ddd, J = 13.0, 2.3, 2.1 Hz, 1 H), 2.02 (ddd, J =
12.2, 2.8, 1.6 Hz, 1 H), 1.77 (ddd, J = 13.0, 2.8, 2.1 Hz, 1 H), 1.67–
1.75 (m, 5 H), 1.65 (ddd, J = 12.2, 2.1, 1.6 Hz, 1 H) ppm. 13C NMR
(150 MHz, CDCl3): δ = 155.0 (s), 120.4 (s), 51.8 (q), 51.0 (s), 40.5 (d),
40.4 (t), 39.2 (t), 35.9 (t), 35.2 (t), 32.3 (t), 32.1 (d), 28.7 (d), 28.4
(d) ppm. IR (KBr): ν = 3342, 3043, 2916, 2859, 2243, 1715, 1529,
˜
1277, 1269, 1236, 1070, 613 cm–1. MS (ESI+): m/z = 257.
General Procedure for the Syntheses of the Phthalimide Deriva-
tives: A round-bottomed flask (50 mL) was charged with the amino
acid derivative (10 mmol), phthalic anhydride (20 mmol), and DMF
(4 mL), and the reaction mixture was heated at reflux for 2 d. The
solvent was removed on a rotary evaporator. The resulting residue
was suspended in CH3CN (50 mL), and the unreacted amino acid
was removed by filtration through a sintered glass funnel. The filter
cake was washed with CH3CN (100 mL) and acetone (100 mL), and
the combined organic filtrates were concentrated on a rotary evap-
orator. The product was purified by chromatography on a silica gel
column (10 % EtOAc in CH2Cl2).
(KBr): ν = 3347, 2997, 2983, 2932, 2919, 2859, 2241, 1705, 1524,
˜
1455, 1281, 1234, 1061, 782, 600 cm–1. MS (ESI+): m/z = 249.
1-Aminoadamantane-2-carboxamide (14):
A round-bottomed
flask (250 mL) was charged with 13 (720 mg, 2.90 mmol), a satu-
rated solution of NaOH (75 mL), and EtOH (25 mL), and the resulting
mixture was heated at 110–115 °C (oil bath) for 72 h. After cooling,
H2O (50 mL) was added, and the mixture was extracted with Et2O
(4 × 100 mL) and then CH2Cl2 (4 × 100 mL). The combined extracts
were dried with anhydrous MgSO4 and filtered, and the solvent was
2-Phthalimidoadamantane-2-carboxylic Acid (3): The reaction of
2-aminoadamantane-2-carboxylic acid (1.49 g, 7.63 mmol) and
removed on a rotary evaporator to afford pure 14 (238 mg, 42 %) phthalic anhydride (2.24 g, 15.12 mmol) followed by purification
as a colorless solid; m.p. 186–190 °C. 1H NMR (300 MHz, CDCl3): δ =
9.06 (br. s, 1 H), 5.56 (br. s, 1 H), 2.66 (br. s, 1 H), 2.26 (br. s, 1 H),
gave 3 (780 mg, 31 %) as colorless crystals; m.p. 297–299 °C. 1H
NMR [300 MHz, dimethyl sulfoxide-d6 ([D6]DMSO)]: δ = 12.87 (br. s,
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Full Paper
1 H), 7.80–7.89 (m, 4 H), 3.63 (br. s, 2 H), 2.10 (d, J = 12.2 Hz, 2 H),
1.60–1.92 (m, 10 H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 171.8
(s), 169.0 (s, 2 C), 134.7 (d, 2 C), 131.1 (s, 2 C), 122.8 (d, 2 C), 71.2
(s), 36.9 (t), 33.8 (t, 2 C), 33.5 (t, 2 C), 29.6 (d, 2 C), 25.7 (d), 25.6
(Z)-4-Phthalimidoadamantane-1-carboxylic Acid (6-Z): Colorless
crystals (30 mg, 6 %); m.p. 210–212 °C. 1H NMR (300 MHz, [D6]-
acetone): δ = 7.82 (br. s, 4 H), 4.29 (br. s, 1 H), 2.77 (br. s, 2 H), 2.51
(d, J = 13.3 Hz, 2 H), 1.88–2.10 (m, 7 H), 1.84 (d, J = 13.3 Hz, 2
H) ppm. The signal from the H atom of COOH was not observed
because of the presence of H2O in acetone. 13C NMR (75 MHz,
[D6]acetone): δ = 178.6 (s), 170.0 (s, 2 C), 134.9 (d, 2 C), 133.0 (s, 2
C), 123.4 (d, 2 C), 60.6 (d), 39.9 (t), 38.0 (t, 2 C), 35.2 (t, 2 C), 31.7 (d,
2 C), 28.4 (d) ppm. One quarternary C atom was not observed. IR
(d) ppm. IR (KBr): ν = 3437, 2923, 2857, 1774, 1723, 1706, 1624,
˜
1365, 1303, 723 cm–1. MS (ESI–): m/z = 324. HRMS (MALDI-TOF):
calcd. for C19H19NO4Na [M + Na]+ 348.1206; found 348.1206.
2-Phthalimidoadamantane-1-carboxylic Acid (4): By following
the general procedure for the synthesis of the phthalimides, the
reaction of amino acid 10 (1.75 g, 8.96 mmol) and phthalic anhy-
dride (1.72 g, 11.61 mmol) following by purification on a silica gel
column (25 % EtOAc in CH2Cl2) gave 4 (1.18 g, 41 %) as colorless
(KBr): ν = 3449, 2919, 2856, 1702, 1686, 1379, 1115, 714 cm–1
.
˜
General Procedure for Analytical Irradiation Reactions: A quartz
test tube was filled with a solution of phthalimide 3–6 (5 mg,
0.015 mmol) and K2CO3 (1.1 mg, 0.008 mmol) in either CH3CN/H2O
(2:1) or acetone/H2O (2:1; 15 mL). The resulting solution was purged
with N2 for 30 min, sealed, and irradiated in a reactor at 300 nm
with 8 lamps (1 lamp, 8 W) over 1–5 min. The samples of the irradi-
ated solutions were analyzed by HPLC at different time intervals. At
the end of the irradiation process, the solvent was removed on a
rotary evaporator, and the residue was analyzed by NMR spectro-
scopy.
1
crystals; m.p. 203–206 °C. H NMR (300 MHz, CDCl3): δ = 7.73–7.81
(m, 2 H), 7.63–7.71 (m, 2 H), 4.66 (s, 1 H), 3.06 (d, J = 11.0 Hz, 1 H),
2.31 (br. s, 1 H), 1.74–2.14 (m, 10 H), 1.59 (d, J = 12.9 Hz, 1 H) ppm.
The H atom of the carboxylic acid was not observed. 13C NMR
(75 MHz, CDCl3): δ = 179.8 (s), 169.2 (s, 2 C), 133.9 (d, 2 C), 132.1 (s,
2 C), 123.2 (d, 2 C), 59.4 (d), 43.7 (t), 43.6 (s), 37.7 (t), 37.1 (t), 34.9
(t), 33.3 (d), 30.7 (t), 28.1 (d), 26.9 (d) ppm. IR (KBr): ν = 3438, 2922,
˜
2856, 1772, 1713, 1625, 1372, 1316, 1061, 720 cm–1. MS (ESI+): m/z =
326. MS (ESI–): m/z
C
= 324. HRMS (MALDI-TOF): calcd. for
General Procedure for Preparative Irradiation Reactions: A
quartz vessel was filled with a solution of phthalimide 3 or 4
(100 mg, 0.31 mmol) and K2CO3 (21.2 mg, 0.15 mmol) in CH3CN
(150 mL) and H2O (50 mL). The resulting solution was purged with
Ar (30 min) and then irradiated in a reactor at 300 nm with 11 lamps
(1 lamp, 8 W) over 0.5–2 h. The irradiated solution was continuously
purged with Ar and cooled by a tap water finger condenser. The
solvent from a sample of the irradiated solution (approximately
10 mL) was evaporated on a rotary evaporator, and the residue was
analyzed by NMR spectroscopy. To the remaining irradiated mixture
was added H2O (150 mL), and the mixture was extracted with
CH2Cl2 (5 × 100 mL). The combined extracts were dried with anhy-
drous MgSO4 and filtered, and the solvent was removed on a rotary
evaporator. The residue was purified by chromatography on a silica
gel column (CH2Cl2 and CH2Cl2/EtOAc/CH3OH, 17:2:1) followed by
preparative TLC on silica gel (EtOAc/CH2Cl2, 1:5).
19H19NO4Na [M + Na]+ 348.1206; found 348.1201.
1-Phthalimidoadamantane-2-carboxylic Acid (5): By following
the general procedure for the synthesis of the phthalimides, the
reaction of amino acid 15 (480 mg, 2.46 mmol) and phthalic anhy-
dride (1.00 g, 6.75 mmol) followed by purification on a silica gel
column (10 % EtOAc in CH2Cl2) gave 5 (250 mg, 31 %) as colorless
1
crystals; m.p. 271–274 °C. H NMR (600 MHz, [D6]DMSO): δ = 12.12
(br. s, 1 H), 7.74–7.84 (m, 4 H), 3.89 (br. s, 1 H), 2.95 (d, J = 12.4 Hz,
1 H), 2.69 (d, J = 12.8 Hz, 1 H), 2.65 (d, J = 12.1 Hz, 1 H), 2.45 (br. s,
1 H), 2.10 (d, J = 12.1 Hz, 2 H), 1.99 (dd, J = 12.1, 2.4 Hz, 1 H), 1.77–
1.83 (m, 3 H), 1.73 (d, J = 12.3 Hz, 1 H), 1.66 (d, J = 12.3 Hz, 1 H),
1.54 (d, J = 12.8 Hz, 1 H) ppm. 13C NMR (150 MHz, [D6]DMSO): δ =
173.7 (s), 169.1 (s, 2 C), 134.4 (d, 2 C), 131.2 (s, 2 C), 122.5 (d, 2 C),
59.6 (s), 49.0 (d), 41.0 (t), 36.6 (t), 35.6 (t), 35.5 (t), 32.0 (d), 31.8 (t),
28.7 (d), 28.5 (d) ppm. IR (KBr): ν = 3448, 2927, 2911, 2852, 1765,
˜
1702, 1374, 1300, 1080, 717 cm–1. MS (ESI–): m/z = 324. MS (ESI+):
m/z = 308, 280. HRMS (MALDI-TOF): calcd. for C19H19NO4Na [M +
Na]+ 348.1206; found 348.1194.
Irradiation of Phthalimide 3: Phthalimide 3 (100 mg, 0.31 mmol)
was dissolved in CH3CN/H2O (3:1, 200 mL) in the presence of K2CO3
(21.2 mg, 0.15 mmol), and the mixture was irradiated in a reactor
by using 11 lamps for 30 min. After workup, the evaporation of the
solvent and purification by chromatography afforded 21 (50 mg,
58 %). The NMR spectroscopic data of N-(2-adamantyl)phthalimide
(21) are in accord with those in the literature.[72]
4-Phthalimidoadamantane-1-carboxylic Acid (6): By following
the general procedure for the synthesis of the phthalimides, the
reaction of amino acid 10 (320 mg, 1.64 mmol) and phthalic anhy-
dride (1.36 g, 9.18 mmol) following by purification on a silica gel
column (15 % EtOAc in CH2Cl2) gave a diastereomeric mixture of 6-
E and 6-Z (270 mg, 50 %; according to the integration of the signals
in the 1H NMR, E/Z, 4:1) as colorless crystals. The pure diastereomers
were isolated by MPLC (Licroprep 40–63 μm, RP-8, 310–35) using
CH3OH/H2O [55:45 + 0.1 % trifluoroacetic acid (TFA), V ≈ 1.5 L) as
the eluent. The samples were analysed by HPLC analysis [C18 col-
umn, CH3OH/H2O (57.5:42.5 + 0.1 % TFA)].
Irradiation of Phthalimide 4: Phthalimide 4 (100 mg, 0.31 mmol)
was dissolved in CH3CN/H2O (2:1, 150 mL) in the presence of K2CO3
(21.2 mg, 0.15 mmol), and the mixture was irradiated in a reactor
by using 11 lamps for 30 h. After workup, the evaporation of the
solvent and purification by chromatography afforded 21 (11 mg,
13 %) and 22 (51 mg, 59 %). The NMR spectroscopic data of prod-
ucts 21 and 22 are in accord with those in the literature.[72]
(E)-4-Phthalimidoadamantane-1-carboxylic Acid (6-E): Colorless
crystals (80 mg, 15 %); m.p. 207–208 °C. 1H NMR (300 MHz,
[D6]DMSO): δ = 12.12 (br. s, 1 H), 7.82 (br. s, 4 H), 4.19 (br. s, 1 H),
2.68 (br. s, 2 H), 2.18 (d, J = 12.9 Hz, 2 H), 1.88–2.04 (m, 5 H), 1.84
(br. s, 2 H), 1.56 (d, J = 12.9 Hz, 2 H) ppm. 13C NMR (75 MHz,
[D6]DMSO): δ = 177.9 (s), 169.0 (s, 2 C), 134.3 (d, 2 C), 131.5 (s, 2 C),
122.7 (d, 2 C), 59.7 (d), 39.2 (t), 38.7 (t, 2 C), 31.4 (t, 2 C), 29.6 (d, 2
C), 26.1 (d) ppm. The signal of one quarternary C atom overlapped
Irradiation of Phthalimide 5: Phthalimide 5 (5 mg, 0.015 mmol)
in CH3CN/H2O (2:1, 15 mL) in the presence of K2CO3 (1.1 mg,
0.008 mmol) was irradiated in a reactor by using 8 lamps for 5 min.
After the evaporation of the solvent, compounds 23–25 were iden-
tified in the crude mixture by comparison with authentic samples.
The NMR spectroscopic data of the products are in accord with
those in the literature.[72]
with that of DMSO. IR (KBr): ν = 3449, 2933, 2863, 1775, 1723, 1708,
Irradiation of Phthalimide 6-E and 6-Z: Phthalimide 6-E or 6-Z
(5 mg, 0.015 mmol) in CH3CN/H2O (2:1, 15 mL) in the presence of
K2CO3 (1.1 mg, 0.008 mmol) was irradiated in a reactor by using 8
lamps for 3 min. After the evaporation of the solvent, compound
˜
1695, 1365, 1315, 1083, 714 cm–1. MS (ESI–): m/z = 324. MS (ESI+):
m/z = 348. HRMS (MALDI-TOF, mixture of 6-E and 6-Z): calcd. for
C19H19NO4Na [M + Na]+ 348.1206; found 348.1222.
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Full Paper
21 was identified in the crude mixture by comparison with an au-
H), 1.73 (d, J = 12.3 Hz, 1 H), 1.58 (d, J = 13.0 Hz, 2 H) ppm. 13C
thentic sample. The NMR spectroscopic data of product 21 are in NMR (75 MHz, CDCl3): δ = 170.4 (s, 2 C), 134.2 (d, 2 C), 132.0 (s, 2
accord with those in the literature.[72]
C), 123.4 (d, 2 C), 81.3 (s), 59.4 (d), 42.2 (t), 39.1 (t), 37.7 (t), 36.0 (d),
35.3 (t), 32.2 (t), 30.8 (d), 30.2 (d) ppm. HRMS (MALDI-TOF): calcd.
for C18H18NO4Na [M – H + Na]+ 335.1134; found 335.1131.
Irradiation of Phthalimide 4 or 5 in the Presence of Acrylo-
nitrile: A solution of 4 or 5 (20 mg, 0.06 mmol) in acetone/H2O
(2:1, 15 mL) and K2CO3 (4.4 mg, 0.031 mmol) was divided into 4
quartz test tubes. To each test tube, acrylonitrile (1 mL) was added.
The solutions were purged with N2 for 30 min, sealed, and then
irradiated by using 8 lamps for 1 h. After irradiation, the solvent
was removed on a rotary evaporator, and the residue was purified
by chromatography on a preparatory TLC plate (EtOAc/Et2O/hexane,
1:1:8). The irradiation of 4 afforded products 21 (0.3 mg, 2 %) and
26 (8.5 mg, 41 %). The irradiation of 5 afforded products 23 (4.0 mg,
23 %) and 27 (4.7 mg, 23 %).
N-[2-(1-Hydroxyadamantyl)]phthalimide (29): Colorless crystals,
1
m.p. 162–164 °C. H NMR (300 MHz, CDCl3): δ = 7.78–7.86 (m, 2 H),
7.67–7.76 (m, 2 H), 4.28 (br. s, 1 H), 3.15 (br. s, 1 H), 2.76 (dd, J =
12.5, 3.0 Hz, 1 H), 2.51 (br. s, 1 H), 2.20 [ddd (dt), J = 11.3, 3.0 Hz, 2
H], 1.87–2.04 (m, 4 H), 1.80 (d, J = 12.5 Hz, 1 H), 1.67–1.76 (m, 3 H),
1.52 (d, J = 11.3 Hz, 1 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 170.3
(s, 2 C), 134.2 (d, 2 C), 132.0 (s, 2 C), 123.3 (d, 2 C), 69.0 (s), 66.3 (d),
46.9 (t), 41.1 (t), 37.4 (t), 36.6 (t), 35.0 (d), 31.0 (t), 30.6 (d), 29.7
(d) ppm. IR (KBr): ν = 3497, 2919, 2852, 1764, 1701, 1454, 1377,
˜
1318, 1131, 1095, 1060, 893, 714 cm–1. HRMS (MALDI-TOF): calcd.
for C18H18NO2 [M – OH]+ 280.1343; found 280.1337.
N-{2-[1-(2-Cyanoethyl)]adamantyl}phthalimide (26): Colorless oil.
1H NMR (600 MHz, CDCl3): δ = 7.82–7.87 (m, 2 H), 7.72–7.77 (m, 2
H), 4.43 (br. s, 1 H), 2.88 (ddd, J = 12.6, 3.0, 2.7 Hz, 1 H), 2.38 (d, J =
12.6 Hz, 1 H), 2.28 (ddd, J = 10.2, 7.0, 6.3 Hz, 1 H), 2.20 (ddd, J =
10.2, 6.3, 6.1 Hz, 1 H), 2.15 (br. s, 1 H), 2.11 (br. s, 1 H), 2.00 (br. s, 1
H), 1.96 [ddd (dt), J = 12.6, 2.6 Hz, 1 H], 1.88 [ddd (dt), J = 12.6,
2.6 Hz, 1 H], 1.79 (d, J = 12.1 Hz, 1 H), 1.75 (d, J = 12.1 Hz, 1 H),
1.68–1.73 (m, 2 H), 1.50–1.64 (m, 3 H), 1.64 (d, J = 12.3 Hz, 1 H) ppm.
13C NMR (75 MHz, CDCl3): δ = 169.8 (s, 2 C), 134.3 (d, 2 C), 132.1 (s,
2 C), 123.5 (d, 2 C), 120.5 (s), 60.4 (d), 43.4 (t), 39.4 (t), 37.9 (t), 37.3
(t), 36.1 (s), 35.5 (d), 35.1 (t), 31.5 (t), 28.9 (d), 27.8 (d), 11.0 (t) ppm.
HRMS (MALDI-TOF): calcd. for C21H22N2O2Na [M + Na]+ 357.1573;
found 357.1560.
Quantum Yield of Photodecarboxylation: The quantum yield of
photodecomposition was determined by using a secondary acti-
nometer, that is, the photolysis of 31 in CH3CN/H2O (3:1) to give 32
and 33 (ΦR = 0.30 0.03).[79] Quartz test tubes were filled with a
solution (CH3CN/H2O, 2:1 or acetone/H2O, 2:1) of phthalimide 2–6
(1 mM) that contained K2CO3 (0.5 mM). The solutions were purged
with N2 for 30 min and then irradiated at the same time in a reactor
with 8 lamps at 254 or 300 nm for 90 s (for the acetone solutions,
the irradiation was performed at 300 nm for 90 s). The compositions
of the irradiated solutions were analyzed by HPLC. After the irradia-
tion was complete, the solvent was removed on a rotary evaporator,
and the residue was analyzed by NMR spectroscopy. Measurements
were done in triplicate, and the mean value was reported. The
quantum yield of the secondary actinometer upon irradiation at
254 nm was additionally verified by using three actinometers dur-
ing the same experiment: ferrioxalate (Φ254 = 1.25),[70,80] KI/KIO3
(Φ254 = 0.74),[70,81] and valerophenone (Φ254 = 0.65 0.03),[70,78]
as described previously.[86] Results are compiled in the Supporting
Information.
N-{1-[2-(2-Cyanoethyl)]adamantyl}phthalimide (27): Colorless oil.
1H NMR (300 MHz, CDCl3): δ = 7.74–7.80 (m, 2 H), 7.67–7.73 (m, 2
H), 3.03 (d, J = 10.6 Hz, 1 H), 2.94 (d, J = 11.6 Hz, 1 H), 2.73 (d, J =
13.3 Hz, 1 H), 2.61 (d, J = 13.3 Hz, 1 H), 2.32 (ddd, J = 10.6, 7.1,
6.1 Hz, 1 H), 2.21 (ddd, J = 10.6, 7.1, 6.1 Hz, 1 H), 2.11–2.20 (m, 2
H), 1.78–2.05 (m, 6 H), 1.61–1.77 (m, 3 H), 1.48–1.54 (m, 1 H) ppm.
13C NMR (75 MHz CDCl3): δ = 170.1 (s, 2 C), 134.1 (d, 2 C), 122.9 (d,
2 C), 120.0 (s), 63.5 (s), 43.9 (d), 41.6 (t), 37.6 (t), 36.9 (t), 35.3 (t),
30.7 (d), 30.1 (t), 29.8 (d), 29.4 (d), 24.3 (t), 15.6 (t) ppm. The signal
of the phthalimide quarternary C atom was not observed. HRMS
(MALDI-TOF): calcd. for C21H22N2O2Na [M + Na]+ 357.1573; found
357.1574.
Supporting Information (see footnote on the first page of this
article): Experimental procedures of known reaction intermediates,
details of irradiation experiments, determination of photodecarbox-
ylation quantum yield, and 1H and 13C NMR spectra of all prepared
compounds.
Irradiation of Phthalimide 4 in the Presence of Oxygen: A glass
vessel was filled with a solution of phthalimide 4 (100 mg,
0.31 mmol) and K2CO3 (23 mg, 0.16 mmol) in CH3CN/H2O (3:1,
200 mL). The solution was purged with O2 for 30 min and then
irradiated in a reactor at 300 nm by using 10 lamps. The irradiated
solution was continuously purged with O2 and cooled by a tap
water finger condenser. The solvent from a sample of the irradiated
solution (approximately 10 mL) was removed on a rotary evapora-
tor, and the residue was analyzed by NMR spectroscopy. To the
remaining irradiated mixture was added H2O (150 mL), and the mix-
ture was extracted with CH2Cl2 (5 × 100 mL). The combined extracts
were dried with anhydrous Na2SO4 and filtered, and the solvent
was removed on a rotary evaporator. The residue was purified by
chromatography on a silica gel column (CH2Cl2 and CH2Cl2/EtOAc/
CH3OH, 17:2:1) followed by preparative TLC on silica gel (EtOAc/
CH2Cl2, 1:5) to afford 21 (4 mg, 4 %), 28 (13 mg, 14 %), 29 (9 mg,
10 %), and 30 (45 mg, 62 %).
Acknowledgments
These materials describe work financed by the Croatian Foun-
dation for Science (HRZZ) (IP-2014-09-6312).
Keywords: Photochemistry · Electron transfer · Polycycles ·
Nitrogen heterocycles · Amino acids
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Photochemistry
L. Mandić, K. Mlinarić-Majerski,
A. G. Griesbeck, N. Basarić* ......... 1–12
Photodecarboxylation of Adamant-
ane Amino Acids Activated by
Phthalimide
Adamantane α-, ꢀ-, and δ-amino acids transfer and decarboxylation reaction
activated by phthalimide were synthe- sequence, which was shown to pro-
sized, and their photochemical reactiv- ceed more efficiently under acetone
ities were investigated. Derivatives 3– sensitization (quantum yields, Φ =
6 underwent a photoinduced electron 0.02–0.5) than upon direct excitation.
DOI: 10.1002/ejoc.201600491
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