P-Hydroxyphenacyl photoremovable protecting groups Robust photochemistry despite substituent diversity

Article abstract of DOI:10.1139/V10-143

A broadly based investigation of the effects of a diverse array of substituents on the photochemical rearrangement of p-hydroxyphenacyl esters has demonstrated that common substituents such as F, MeO, CN, CO₂R, CONH₂, and CH₃ have little effect on the rate and quantum efficiencies for the photo-Favorskii rearrangement and the release of the acid leaving group or on the lifetimes of the reactive triplet state. A decrease in the quantum yields across all substituents was observed for the release and rearrangement when the photolyses were carried out in buffered aqueous media at pHs that exceeded the ground-state pK_a of the chromophore where the conjugate base is the predominant form. Otherwise, substituents have only a very modest effect on the photoreaction of these robust chromophores.

Full text of DOI:10.1139/V10-143

364
p-Hydroxyphenacyl photoremovable protecting
groups — Robust photochemistry despite
substituent diversity
Richard S. Givens, Kenneth Stensrud, Peter G. Conrad II, Abraham L. Yousef,
Chamani Perera, Sanjeewa N. Senadheera, Dominik Heger, and Jakob Wirz
Abstract: A broadly based investigation of the effects of a diverse array of substituents on the photochemical rearrange-
ment of p-hydroxyphenacyl esters has demonstrated that common substituents such as F, MeO, CN, CO2R, CONH2, and
CH3 have little effect on the rate and quantum efficiencies for the photo-Favorskii rearrangement and the release of the
acid leaving group or on the lifetimes of the reactive triplet state. A decrease in the quantum yields across all substituents
was observed for the release and rearrangement when the photolyses were carried out in buffered aqueous media at pHs
that exceeded the ground-state pKa of the chromophore where the conjugate base is the predominant form. Otherwise, sub-
stituents have only a very modest effect on the photoreaction of these robust chromophores.
Key words: substituent, pH, pKa, solvent effects, photorelease.
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Resume : Une etude detaillee des effets de divers arrangements de substituants sur le rearrangement photochimique des
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esters de p-hydroxyphenacyle a demontre que les substituants communs, tel F, MeO, CN, CO2R ou CH3, n’ont que peu
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d’effet sur la vitesse et les efficacites quantiques du rearrangement de photo-Favorskii et sur la liberation du groupe acide
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partant ou sur les temps de vies de l’etat triplet reactif. On a observe que tous ces substituants provoquent des diminutions
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des rendements quantiques pour la liberation et le rearrangement quand les reactions de photolyse sont effectuees dans un
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milieu aqueux tamponne a un pH superieur a celui du pKa de l’etat fondamental du chromophore pour lequel la base
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conjuguee est la forme predominante. Autrement, les substituants n’ont qu’un effet modeste sur la photoreaction de ces
robustes chromophores.
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Mots-cles : substituant, pH, pKa, effets de solvant, liberation photochimique.
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[Traduit par la Redaction]
that have demonstrated the advantages of its rapid, efficient
release properties. An intriguing aspect of pHP photochem-
istry is the rearrangement of the chromophore that accompa-
nies the release of the substrate. Anderson and Reese,7 who
discovered the photorearrangement by photolysis of p-
hydroxyphenacyl chloride, isolated ethyl p-hydroxyphenyl-
acetate (HPAA), which they described as a ‘‘photo-Favor-
skii’’ rearrangement product and depicted the skeletal
rearrangement as passing through a putative spirocyclohexa-
2,5-dienylcyclopropyl-4,8-dione intermediate (I2) (eq. [1]).
Introduction
Among the known photoremovable protecting groups, p-
hydroxyphenacyl (pHP, 1, eq. [1]) has consistently provided
very fast (within a few nanoseconds after excitation or even
less) and efficient, high conversion release of substrates.
Moreover, the photoproducts are transparent at irradiation
wavelengths ‡280 nm, permitting essentially complete con-
version. Since developing the pHP as a ‘‘caging group’’ a
decade ago,1 pHP derivatives have found several applica-
tions in neurobiology,2 enzyme catalysis,3 and chemistry4–6
Received 1 June 2010. Accepted 22 August 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 2 March 2011.
Dedicated to Professor J. C. Scaiano, University of Ottawa, on the occasion of his 65th birthday. This article is part of a Special Issue
dedicated to Professor J. C. Scaiano.
R.S. Givens,1 K. Stensrud, P.G. Conrad II, A.L. Yousef, C. Perera, and S.N. Senadheera. Department of Chemistry, University of
Kansas Lawrence, Lawrence, KS 66045, USA.
D. Heger. Research Centre for Toxic Compounds in the Environment (RECETOX) and Department of Chemistry, Faculty of Science,
Masaryk University, Kamenice3, 625 00 Brno, Czech Republic.
J. Wirz. Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland.
1Corresponding author (e-mail: givensr@ku.edu).
Can. J. Chem. 89: 364–384 (2011)
doi:10.1139/V10-143
Published by NRC Research Press

Givens et al.
365
½1ꢀ
through comparisons of the known photochemistry of the
parent, unsubstituted pHP acetate (24),9,11 or diethyl phos-
phate (27).1,2,10,11
Our subsequent studies using a variety of leaving groups
have affirmed that the major rearrangement pathway pro-
ceeds to the p-hydroxyphenylacetic acid as long as the reac-
tions are carried out in aqueous or hydroxylic solvents.8
Recent studies have been directed toward fine-tuning the re-
quirements for the release and for the rearrangement of the
pHP group, particularly, the effects of varying the leaving
group (LG), the substituents on pHP, and the solvent.8
Details regarding the timing and multiplicity requirements
of the bond-making and bond-breaking events in the photo-
Favorskii rearrangement have been addressed in several
mechanistic and theoretical investigations.9–11 The release
rates, as well as the product distribution, are sensitive to the
presence of H2O and other hydroxylic solvent combina-
tions.8–10 An adiabatic triplet sequence has been pro-
posed10,12 that involves both the deprotonation of the
phenolic OH group and the heterolytic release of the leaving
group13 prior to formation of cyclopropanone I2 (eq. [1]). At
least one additional intermediate, an oxyallyl–phenoxy trip-
let biradical (I1), has been proposed and evidence for I1 was
recently reported.9 Still, the molecular events including the
multiplicities of the intermediates and the pathways that
form I1 and I2 and their conversions to products are not fully
established.
We report here the results of our investigations of the
substituent effect on the photochemistry, multiplicity, quan-
tum yield, reaction rate, and the effects of pKa and pH. In
addition, we have evaluated the substituent effect on the re-
arrangement including intermediates I1 and I2 suggested for
the photo-Favorskii rearrangement. A series of substituted
pHP esters (Fig. 1) were examined under a variety of condi-
tions that included changes in solvent, pH, and ionic
strength of the media. The mechanistic implications of these
results provide a clearer understanding of this most interest-
ing photorearrangement reaction.
Synthesis of substituted pHP derivatives
Schemes 1A and 1B describe the synthetic protocols em-
ployed for the substituted pHP esters in Fig. 1. The GABA
leaving group was chosen to assure good aqueous solubility.
GABA is particularly advantageous in this respect, and in
our laboratories it has served as the standard leaving group
for several previous studies of pHP release rates and effi-
ciencies. In general, the synthetic strategies for substituted
pHP chromophores vary according to the availability of the
substituted phenol, p-hydroxybenzoic acid, or p-hydroxyace-
tophenone precursor. The p-hydroxyacetophenones, when
available, are converted directly to the pHP GABA and
other leaving-group analogs by any one of a number of con-
venient transformations that activate the a-methyl group,
e.g., bromination to the a-bromomethyl ketone followed by
SN2 displacement with N-BOC protected GABA and then
deprotection with TFA, e.g., 13–23 (Scheme 1B).
The syntheses of the GABA derivatives of pHP (1), 3-
methoxy-pHP (2), and 3,5-dimethoxy-pHP (3) followed the
published protocols developed earlier for glutamates.2a,2b
The 3-carboxymethyl-pHP (10) and 3-carbamoyl-pHP (11)
derivatives were derived by the same sequence from the
commercially available methyl 5-acetylsalicylate. The 2-
and 3-cyano and all of the fluoro ketones (13d–23d), were
synthesized from the corresponding phenols by a sequence
of bromination of the phenol (13b–23b), then benzyl protec-
tion (13c–23c), followed by acylation through Stille cou-
pling with Pd(0) and 1-ethoxyvinyl stannane to furnish, in
excellent yield, the substituted p-benzyloxyacetophenone.
Debenzylation gave acetophenones 13d–23d, which were
then converted to the GABA esters 13–23.
The synthesis of new acetate derivatives 25 and 26
stemmed from a protocol uniform to the GABA esters, but
commencing with the p-hydroxyacetophenones available
from the Stille coupling route mentioned above and using
sodium acetate in lieu of GABA according to the method of
Phillips.10
The synthesis of the new diethyl phosphates 30–32 consti-
tuted an altogether separate series of steps. For the electron-
donating methoxy scaffolds 30a and 31a, Friedel–Crafts
acylation of corresponding pivalate-protected phenols with
chloroacetyl chloride, followed by chloro displacement with
sodium bromide engendered a-bromo derivatives 30d–31d.
Facile diethyl phosphate substitution of the bromoketones
Results and discussion
The array of substituted pHP derivatives was chosen to
include both electron donor and acceptor functional groups
(Fig. 1). Several of these compounds were also selected for
future synthetic alterations or attachment sites for modifica-
tions desired for various applications of pHP photochemis-
try. There were a few instances where the g-aminobutyric
acid (GABA) leaving group was not easily installed, so the
alternative acetate or diethyl phosphate leaving groups were
inserted. These leaving groups are also well-documented in
the literature of pHP photorelease chemistry and therefore
can be related to the more numerous GABA representatives
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366
Can. J. Chem. Vol. 89, 2011
Fig. 1. Substituted p-hydroxyphenacyl (pHP) esters.
was then mediated by Ag(I) followed by pivalate deprotec-
tion using ammonium acetate in aqueous methanol to fur-
nish 30 and 31.
An initial Fries rearrangement of acetyl-protected
p-hydroxybenzoic acid 32b provided the o-acetylated analog
32c, whose carboxylic acid moiety was transformed into the
a-diazo ketone through the acid chloride. After deacetylation
with ammonium acetate in aqueous methanol, reaction with
diethyl phosphoric acid gave 32.
was anticipated that the photochemistry of the two forms
might differ. In fact, the photochemistry will be shown to
be sensitive to the solution pH as well as to the pKa of the
chromophore.
The UV–vis spectra of unsubstituted p-hydroxyacetophe-
none (pHA, 1d) measured at neutral pH (phosphate buffer,
pH 7) and in basic media (pH 8) typify the significant shifts
in the absorption maxima upon ionization of these chromo-
phores (Fig. 2).4a The spectrum of pHA displays a promi-
nent p–p* absorption band with a maximum at 275 nm. In
some solvents, a weak shoulder on the red edge of the p–p*
band of the p-hydroxyketone (not shown) has been attrib-
uted to the carbonyl n–p* transition.10c The absorption band
centered at 330 nm is assigned to the conjugate base p–p*
transition. Based on the significant contributions of both
chromophores in aqueous solutions between pH 5 and 8, it
was necessary to study the photochemistry and physical
properties of substituted pHP derivatives under buffered
conditions to assure a constant ratio of the two forms of the
UV–vis spectral properties of substituted pHPs
The UV–vis spectral properties of 1–32 in H2O and in
aqueous buffer at pH 9 and the groundstate pKas of the cor-
responding p-hydroxyacetophenones are given in Table 1.
The pKas range from 3.9 to >8.0, indicating that at physio-
logical pH the chromophore will be a mixture of the conju-
gate base (phenoxide) and the neutral, un-ionized phenol. In
most cases, the un-ionized form is prevalent at pH 5.0–7,
whereas at pH 9, in all cases, the conjugate base prevails. It
Published by NRC Research Press

Givens et al.
367
Scheme 1. (A) Synthetic strategy for g-aminobutyric acid (GABA) derivatives 4–23. Conditions: (i) Br2, TFA, room temperature (rt) 90%–
100%. (ii) BnBr, K2CO3, CH3CN, rt, 85%–95%. (iii) Pd(PPh3)4, 1-ethoxyvinyltributyltin, PhCH3, 100 8C, 70%–92%. (iv) (a) (R’ = Bn),
phenyltrimethylammonium tribromide (PTAB), CH3OH–CH2Cl2 (1:1) rt, 90%–98%; (b) (R’ = H), CuBr2, CHCl3–EtOAc (1:1), reflux, 70%–
80%. (v) (a) (R’ = Bn), N-t-Boc GABA, K2CO3, CH3CN, rt, 40%–95%; (b) (R’ = H), N-t-Boc GABA, 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), benzene–DMF (20:1), 7 8C to rt, 55%–60%. (vi) TFA, 24 h, rt, 50%–85%. (vii) Pd–H2, EtOAc, rt, 95%–100%. (viii) TFA–CH2Cl2
(1:1), rt, 70%–100%. (B) Synthetic strategies for diethyl phosphate derivatives 30–32. Conditions (top reaction): (i) (CH3)3COCl, Et3N, 0 8C
to rt, 100%. (ii) ClCH2COCl, AlCl3, 0 8C, 20%–50%. (iii) NaBr, acetone, reflux, 90%–98%. (iv for 30) HO–P(O)(OEt)2, Ag2O, CH3CN,
60 8C, 76%. (v for 31) K+ OP(O)(OEt)2, dibenzo[18]crown-6, CH3CN, reflux, for 20%. (vi) NH4OAc, H2O–MeOH, 1:4, 50 8C, 24 h, 90%–
95%. Conditions (bottom reaction): (i) AlCl3, PhNO2, 150 8C, 98%. (ii) AcCl, TEA, CH2Cl2, rt, 70%. (iii) (a) SOCl2; (b) CH2N2, Et2O; (c)
MeOH(aq), NH4OAc, 50 8C, 21% overall. (iv) Diethyl phosphoric acid, benzene, 60 8C, 79%
chromophore. Studies were also conducted in unbuffered
aqueous media for comparison.
substituted pHP derivative: the leaving group, i.e., GABA,
diethyl phosphate, or acetate, and the substituted p-hydroxy-
phenylacetic acid (pHPAA), which is the rearrangement
product from the photo-Favorskii reaction (eq. [1]). A minor
product, the substituted p-hydroxybenzyl alcohol, is formed
Product studies
Two major photolysis products were obtained from each
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368
Can. J. Chem. Vol. 89, 2011
Table 1. The pKa and UV absorption maxima in H2O and at pH 9 for substituted p-hydroxy-
phenacyl (pHP) g-aminobutyric acid (GABA) derivatives arranged in order of ascending pKa.
a
Derivatives
23 (2,3,5,6-tetraF)
22 (2,3,5-triF)
12 (3-NO2)
13 (3-CN)
pKa
3.9
4.5
5.2
5.2
5.3
5.5
5.7
5.9
6.2
6.5
6.7
6.8
7.2
7.7
7.8
7.8
7.9
7.9
8.0
8.0
8.1
8.2
NA
NA
NA
NA
NA
lmax (nm) (log 3)b
316 (3.95)
324 (4.05)
397 (3.20),d 334 (4.14)
322 (3.46)e
lmax (nm) (log 3) at pH 9c
316 (4.10)
323 (4.01)
399 (3.22), 335 (4.20)
321 (3.44)
330 (4.14)
328 (4.24)
326 (4.12)
323 (4.07)
NA
329 (4.18)
351 (3.67), 328 (3.96)
313 (4.06)
328 (3.90), 319 (3.91)
NA
19 (3,5-F)
14 (3-CF3)
331 (4.14)
325 (4.22)f
20 (2,5-diF)
18 (2,3-diF)
11 (3-CO2NH2)
15 (3-OCF3)
16 (3-F)
278 (3.26), 326 (3.92)
272 (4.09), 328 (3.04)
324 (4.15)
273 (3.79), 328 (4.31)
274 (4.00), 335 (2.97)
274 (4.00)
21 (2,6-F)
17 (2-F)
271 (3.97)
10 (3-CO2CH3)
3, 29 (3,5-OCH3)
5 (2-OH)
2, 28 (3-OCH3)
4 (3-OH)
1, 24, 27 (parent)
7 (2-CH3)
6 (3-CH3)
8 (3,5-CH3)
26 (2-CN)
30 (2-OCH3)
31 (2,6-OCH3)
32 (3-COCH3)
9 (3-CO2H)
272 (4.09), 330 (3.94)
279, 300 (3.97), 355 (3.55)
276 (3.85)
276 (3.93), 341 (3.58)
309 (3.97), 279 (3.86)
279 (4.09), 325 (3.40)
287 (4.06)
357 (4.28)
342 (4.27)
324 (4.33)
343 (3.86), 328 (4.10)
356 (3.75), 334 (3.97)
362 (3.91), 348 (4.05)
284 (3.84)
304 (3.79), 287 (3.96)
275 (3.45), 305 (3.39)
278 (3.98), 307 (3.93)
283 (3.70)
274 (3.39), 319 (3.06)
280 (3.90), 310 (3.58)
Note: NA, not available.
aThe pKa values of the corresponding p-hydroxyacetophenones were determined by spectrophotometric
titration. Ionic strength was low (I < 0.05 mol/L) and not held constant. The ionization quotients pKac of the
p-hydroxyacetophenone analogues of 1–3 and 9–11, measured at constant ionic strength (I = 0.1 mol/L),
were ~0.05 units lower.
bWater.
cN-(2-Hydroxyethyl)piperazine-N’-ethanesulfonic acid (HEPES, 0.01 mol/L).
dThe absorption maxima (lmax (nm) (log ") for 12 was also measured at pH 5 in 0.01 mol/L ammonium
acetate: 256 (3.92).
eThe absorption maxima (lmax(nm) (log ") for 13 was also measured at pH 5 in 0.01 mol/L ammonium
acetate: 270 (sh).
fThe absorption maxima (lmax(nm) (log ") for 14 was also measured at pH 5 in 0.01 mol/L ammonium
acetate: 274 (4.14).
in <5% yield and is derived by decarbonylation of the puta-
tive spirodienedione intermediate I2, followed by hydration
of the resulting p-quinone methide.9 All of the products
were characterized by HRMS and by comparison of their
1H and 13C NMR spectral properties either with authentic
samples or related products from our previous pHP studies.
Photolyses carried out in aprotic organic solvents gener-
ally produced reduction products such as the substituted
pHA and other radical-derived products. These studies were
not pursued further, and the products were not observed in
the aqueous solution photolyzates.
Triplet-state pKa, quantum yield, and Stern–Volmer
studies
The pKa of the excited triplet state of 1d is strongly re-
duced by about 4.3 pKa units relative to pHA’s ground-state
pKa of 7.9.8 Since proton transfer plays a significant role in
the mechanism of the release of the leaving group,4,8–13
changes in the pKa of the chromophore will provide addi-
tional insight into the mechanistic features of the reaction.
Currently, there remains a controversy over the timing and
importance of the proton transfer to the solvent. One sug-
gested pathway depicts proton transfer from the chromo-
phore in concert with leaving group departure (vide infra).10
For this scenario, the enhanced acidity of the pHP triplet
would enhance the reactivity of the chromophore toward re-
lease and rearrangement. The second scenario suggests that
the phenol group is merely a spectator and does not play a
pivotal role in the release or rearrangement.10,14 Therefore,
Quantitative determination of the products was obtained
1
1
by H NMR or by LC–MS/MS. The H NMR spectra of
pHP caged GABAs in D2O were cleanly transformed into
spectra of the two major photoproducts accompanied by
complete disappearance of pHP GABA. Product studies
were routinely carried out to quantitative conversion.
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Givens et al.
369
Table 2. The effect of pKa on the quantum yields for substituted
p-hydroxyphenacyl (pHP) g-aminobutyric acid (GABA) in unbuf-
fered H2O. Entries are arranged according to decreasing disappear-
ance quantum yields.
Fig. 2. UV spectral data for p-hydroxyacetophenone (1d). The ab-
sorption spectra are of a 0.13 mmol/L solution of 1d in NaH2PO4
buffer (3% CH3CN) at pH 7 and 8 showing the shift in absorption
from the phenol to its conjugate base. (From Givens and Yousef
2005, in Dynamic Studies in Biology: Phototriggers, Photoswitches,
and Caged Biomolecules, Chap. 1.3.2.2, p. 60, reproduced with
permission, #Wiley-VCH Verlag CmbH & Co. KGaA.)
Quantum yield in H2O
a
b
c
Substituted pHP GABA
13 (3-CN)d
17 (2-F)
18 (2,3-diF)
20 (2,5-diF)
1 (parent)
14 (3-CF3)
21 (2,6-F)
16 (3-F)
8 (3,5-CH3)
6 (3-CH3)
23 (tetraF)
19 (3,5-F)
7 (2-CH3)
pKa
5.2
7.2
5.9
5.7
8.0
5.5
6.8
6.7
8.2
8.1
3.9
5.3
8.0
6.5
4.5
7.9
7.8
Fdis
Fapp
0.35
0.27
0.24
0.21
0.19
0.16
0.16
0.15
0.14
0.14
0.10
0.11
0.10
0.09
0.07
0.06
0.03
Fapp
0.39
0.26
0.22
0.20
0.16
0.14
0.15
0.15
0.13
0.13
0.10
0.10
0.10
0.07
0.06
NA
0.42
0.28
0.24
0.22
0.20
0.17
0.16
0.16
0.15
0.15
0.11
0.11
0.11
0.09
0.08
0.07
0.03
15 (3-OCF3)
22 (2,3,5-triF)
2 (3-OCH3)
3 (3,5-OCH3)
a study of the effect of pKa on the reaction efficiency and
rate of release should be valuable.
NA
The quantum yields for the disappearance of pHP esters
and for the appearance of the two major products were de-
termined by LC–MS/MS and H NMR at low conversion
(<5%) in unbuffered H2O (Table 2) and in buffered solu-
tions (Table 3) at acidic, neutral, and basic pH.
Note: NA, not available.All runs were low conversions to products
(<2%). Standard deviations are < 0.02. 3-NO2 (12), 3-OH (4), and 2-OH
(5) did not react under these conditions and are omitted. Unbuffered
18 MO Ultrapure H2O was used.
aQuantum yield for the disappearance of pHP GABA.
bQuantum yield for the appearance of GABA.
cQuantum yield for the appearance of rearranged acid.
dCH3CN–H2O (1:1).
1
The quantum yields were first determined in unbuffered
H2O (Table 2). However, these must be considered less ac-
curate because the pH changes during photolysis. The mildly
acidic phenolic p-hydroxyphenacyl chromophore (pKa * 4–
8) is converted into a series of three new acids, a bifunc-
tional p-hydroxyphenylacetic acid along with the leaving
group, which, in several cases, is the strongest acid present,
e.g., a phosphoric or sulfonic acid. The contributions of the
three newly created acids decrease the pH. Since the photo-
lysis solution for a particular chromophore is a delicate bal-
ance of the phenolic pHP ester and its conjugate base and
since their ratio is dependent on the chromophore’s pKa, the
changing pH of the media will affect that ratio and, there-
fore, the quantum yield. Furthermore, as reported earlier
and amplified in this paper, the quantum yields are dramati-
cally different for the un-ionized chromophore and its conju-
gate base. Taken together, the quantum yields in unbuffered
media are a complex function of the pKa, a changing pH,
and the relative quantum yields. Nevertheless, the quantum
yields reported in Table 2 may be considered as representa-
tive because they were derived by extrapolation to initial ef-
ficiencies at low conversions (<2%). However, they are not
corrected for the relative concentrations of the un-ionized
and conjugate base present.
The lower quantum yield may be attributable to several
factors including a decrease in intersystem crossing (isc) ef-
ficiency (FST) or in the rate constant of release relative to
that of competing nonproductive pathways. One of these
may simply be deprotonation to the triplet conjugate base
(1–).3 Results in Table 4 suggest, however, that the rates for
intersystem crossing are essentially insensitive to substitu-
tion for the un-ionized esters. Less is known for the rates of
isc for the conjugate bases.
Two substituents of interest are cyano and methoxy. The
ortho (or 2) substituted derivatives were not amenable to
our synthetic strategies and difficulty was encountered in
the attempts to synthesize the GABA derivatives, especially
in the Boc deprotection step. However, acetate and phos-
phate analogs were synthetically accessible and, therefore,
their photochemistries were examined. Disappearance quan-
tum yields were measured for 2- and 3-cyano pHP OAc and
for the four variations of the methoxy pHP diethyl phos-
phates. These are given in Table 5 along with the parent
pHP OAc and diethyl phosphates reported earlier for com-
parison.1,9–11
Table 3 reveals that in buffered media the efficiencies for
release decrease as the pH of the media increases for essen-
tially every pHP ester. The decrease in efficiency is most
pronounced as the increasing pH approaches the pKa of the
pHP ester, reaching its lowest when the chromophore is
completely converted to its conjugate base, which is graphi-
cally depicted in Fig. 3. This normally resulted in a reduc-
tion of 60%–80% in the quantum yield.
Clearly, the cyano-substituted acetates are less efficient
than the parent, which was surprising, since the GABA ana-
log of the 3-CN pHP GABA gave an enhanced efficiency
for photodeprotection. It should be noted that poorer aque-
ous solubility of acetate and other carboxylic esters requires
cosolvents such as acetonitrile or DMSO. There is mounting
evidence that the concentration of the organic cosolvent re-
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370
Can. J. Chem. Vol. 89, 2011
Table 3. Substituent effects on the quantum yields for g-aminobutyric acid (GABA) release at 300 nm in buffered CH3CN–H2O. Entries
are arranged according to decreasing disappearance quantum yields at pH 7.3.
Quantum yield
pH 5.0a,b
pH 7.3a,c
pH 8.2a,c
d
e
f
d
e
f
d
e
f
pHP GABA
13 (3-CN)
1 (Parent)
17 (2-F)
8 (3,5-CH3)
6 (3-CH3)
14 (3-CF3)
16 (3-F)
18 (2,3-diF)
23 (tetraF)
7 (2-CH3)
21 (2,6-F)
20 (2,5-diF)
22 (2,3,5-F)
15 (3-OCF3)
19 (3,5-F)
pKa
5.2
8.0
7.2
8.2
8.1
5.5
6.7
5.9
3.9
8.0
6.8
5.7
4.5
6.5
5.3
Fdis
Fapp
0.13
0.20
0.23
Fapp
0.19
0.18
0.23
Fdis
Fapp
Fapp
Fdis
Fapp
Fapp
0.21
0.21
0.24
0.33g
0.21
0.21
0.17
0.15
0.12
0.12
0.11
0.10
0.10
0.10
0.10
0.06
0.06
0.05
0.28g
0.20
0.20
0.17
0.16
0.11
0.12
0.11
0.10
0.09
0.09
0.09
0.06
0.06
0.05
0.28g
0.20
0.19
0.15
0.13
0.10
0.11
0.09
0.08
0.08
0.08
0.09
0.04
0.05
0.04
0.19h
0.09
0.06
0.11
0.08
0.08
0.02
0.05
0.09
0.07
0.04
0.02
0.02
0.02
0.02
0.09
0.06
0.11
0.08
0.08
0.02
0.05
0.09
0.07
0.04
0.02
0.02
0.02
0.02
0.06
0.03
0.09
0.06
0.24
0.15
0.16
0.08
0.23
0.14
0.16
0.07
0.14
0.15
<0.01
0.04
0.06
0.02
0.07
0.07
0.07
0.08
0.07
0.06
0.05
0.07
<0.01
<0.01
0.01
0.07
<0.01
aStandard deviations were < 0.02.
bAmmonium acetate (0.01 mol/L).
cN-(2-Hydroxyethyl)piperazine-N’-ethanesulfonic acid (HEPES, 0.01 mol/L) and LiClO4 (0.1 mol/L), pH 7.3.
dQuantum yield for the disappearance of p-hydroxyphenacyl (pHP) GABA.
eQuantum yield for the appearance of GABA.
fQuantum yield for the appearance of rearranged acid.
gAmmonium acetate (0.01 mol/L, pH 7).
hAmmonium acetate (0.01 mol/L, pH 9).
duces quantum yields and rates of release.9,11–13 This is not
consistently the case, but may be a factor in the cyano ace-
tate series.
Methoxy substitution, on the other hand, has little effect
for the 3-MeO pHP phosphates but does enhance the reac-
tivity of the 2-MeO pHP phosphates (30 and 31). These are
among the most efficient photodeprotection efficiencies we
have encountered in the phosphate series.1,3c,4a,8,9,12 It was
discovered, however, that these latter two derivatives were
less soluble and less stable to aqueous hydrolysis.
Picosecond pump–probe spectroscopy on a cross section
of the substituted pHP that included both electron-donating
and -withdrawing derivatives and fluorinated examples
showed little variation in the rate constants of triplet rise
(k1) and decay (k2) (Table 4).
Scheme 2 summarizes a series of the parallel pathways
available to the photoactivated pHP derivatives and their
conjugate bases. The products are the same for both path-
ways, but the details concerning the excited states and the
partitioning among their available decay and reaction path-
ways are different. We have a much less-detailed under-
standing of the mechanism for the reaction of the conjugate
bases than the un-ionized pHP system. However, we have
monitored the change in quantum yields at different pH and
at a longer excitation wavelength (RPR 350 nm lamps). The
values are uniformly much lower than those obtained for the
un-ionized pHP analog using RPR 300 nm lamps. The con-
jugate bases display a weak fluorescence (F < 1 ꢁ 10–4).
Otherwise, the two forms show comparable rates of triplet
formation and decay upon excitation at 350 nm. Thus, the
low product quantum yields for the conjugate bases are
most likely due to variations in the partitioning among the
competing exit channels from the triplet excited states of
the two chromophores.
Parallel studies on Stern–Volmer quenching of the ester
disappearance and product appearance by sorbate for a few
of the esters further indicated no significant differences in
triplet lifetimes. This also suggests that there is little differ-
ence in the partitioning among the triplet decay pathways,
but does not exclude the possibility of recombination of ini-
tially formed ion-triplet biradical pairs (See the Experimen-
tal section and Table 6 for details).
Heterolytic release of the leaving group generates an in-
3
termediate biradical–ion pair ([I ꢂLG]) raising the possibil-
¯
1
ity of cage return, which might contribute to the differences
in the photorelease quantum yields. Evidence for cage return
was obtained by increasing the ionic strength of the aqueous
solutions with LiClO4, NaClO4, MgCl2, etc. With several
derivatives, the quantum yields increased significantly,
some by a factor of two or more, whereas others remained
unchanged and, in a few cases, even decreased. The in-
creased ionic strength had no effect on the LFP triplet decay
rates.
Finally, the triplet energies of the pHP and its conjugate
base may be a source of the differences in reactivity. The
71.6 kcal/mol (1 cal = 4.184 J) triplet energy of pHP is di-
minished to 61.2 kcal/mol for the conjugate base.8 This
~10 kcal/mol decrease in energy may favor the barrierless
or low-barrier processes over the more energy demanding
Published by NRC Research Press

Givens et al.
371
Fig. 3. Graphic representation of the effect of pKa on the quantum yield of substituted p-hydroxyphenacyl (pHP) g-aminobutyric acid
(GABA) as a function of pH. Numerical identifications of the esters are placed adjacent to the pH 7.3 values. (A) Disappearance quantum
*
yield of substituted pHP GABA as a function of its pKa at pH 7.3 ( ) and 8.2 (*). (B) Appearance quantum yields of GABA as a function
*
of the pHP ester pKa at pH 7.3 ( ) and 8.2 (*). Data are from Table 3. The 3-CN derivative 13 is omitted because of a solvent change.
heterolytic release pathway of the photo-Favorskii rearrange-
ment.
Pathways that may account for nonproductive decay of
the triplet would include deprotonation to the conjugate
base with subsequent rapid decay to its ground state15 or
proton tautomerization to 33, the proton tautomer of pHP
originally suggested by Wan and co-workers11 (which was
later shown to be a triplet8) that, following isc to the ground
state, would rapidly tautomerize back to the pHP ester. In-
terestingly, there is a prominent, long-lived 340 nm transient
that also is observed in the LFP spectra of most pHP deriva-
tives. This band is appropriately positioned for assignment
Published by NRC Research Press

372
Can. J. Chem. Vol. 89, 2011
Table 4. Laser flash photolysis studies on selected deriva-
tives in unbuffered H2O.
Scheme 2. Parallel pathways for the photochemical and photophy-
sical processes of p-hydroxyphenacyl (pHP) or its conjugate base.
Rate constant
pHP GABA
1 (Parent)
k1 (1011 s–1)a,b
k2 (109 s–1)b
3.0c
1.3
2.8d
1.3
2.2
1.0
2.6
4.4
2.2
2.7
3.2
ND
2.1
3.0
3.9
3.6
2 (3-OCH3)
3 (3,5-OCH3)
10 (3-CO2CH3)
11 (3-CONH2)
9 (3-CO2H)
20 (2,5-diF)
22 (2,3,5-triF)
23 (tetraF)
0.90
3.4d
Note: pHP, p-hydroxyphenacyl; GABA, g-aminobutyric acid.
aSinglet lmax * 315 nm.
bk1 is the rate constant for triplet rise and k2 is the rate constant for
triplet decay.
cNear the tail end of the triplet decay, new absorption bands
appeared at 320, 420, and 440 nm, which have been assigned to the
oxyallyl–phenoxy biradical I1 and decayed with a rate constant of
kbirad & 1.30 Â 109 s–1.9,15
dAcetonitrile (5%) was added to the aqueous solvent to dissolve
these esters.
Table 5. Quantum yields for cyano-pHP
acetates and methoxy-pHP phosphates at
l = 300 nm.
a
Compound
Fdis
Proton loss14 or heterolysis could be the initial step or the
two could occur in concert. Two such stepwise scenarios
can be envisaged, differing by only the order of departure
of two entities. Evidence that militates against proton loss
preceding the departure of the leaving group comes from
the poor efficiencies of the conjugate bases and from our
earlier analysis of solvent kinetic isotope effects (SKIE). A
‘‘proton inventory’’ of the SKIE revealed that either two or
three protons are ‘‘in flight’’ in the release process.9,16 The
partition functions are consistent with the phenolic proton
and two solvent water protons associated with two carbonyls
during the extrusion of I1.
pHP acetatesb
24 (Unsubstituted)
25 (3-CN)
0.30
0.17
0.08
26 (2-CN)
pHP diethyl phosphatesc
27 (Unsubstituted)
28 (3-OCH3)
29 (3,5-OCH3)
30 (2-OCH3)
31 (2,6-OCH3)
32 (3-COCH3)
0.40
0.39
0.44d
0.59
0.69
0.03
The other stepwise construct would involve the departure
of the leaving group before loss of the proton, thereby gen-
erating a triplet a-keto carbenium ion. This pathway is also
unlikely based on the known photochemistry of released
derivatives such as p-methoxyphenacyl analogs, which, even
in hydroxylic solvents, do not efficiently follow a rearrange-
ment pathway for the chromophore.1b,17 This is in keeping
with the requirement that the para hydroxyl function and a
leaving group positioned a to the carbonyl be present on
the phenacyl chromophore to ensure efficient photo-Favorskii
rearrangement.
Taken together, these results are viewed as favoring the
concomitant loss of the phenolic proton and the leaving
group. Thus, the evidence is most consistent with the inter-
pretation that a concerted proton and leaving group depar-
ture occurs with the resulting extrusion of a neutral triplet
biradical I1. This process is not only a minimum energy
pathway for heterolysis, but also conserves spin.18 The oxy-
allyl–phenoxy triplet biradical I1 then relaxes to a singlet
and closes to the spirobicyclohexadienedione I2. The fate of
I2 as a groundstate intermediate mimics a typical Favorskii
Note: pHP, p-hydroxyphenacyl.
aError limits for disappearance quantum yields
( 0.03%).
bAnalysis carried out in 20% aq CH3CN or
DMSO.
cAnalysis carried out in 50% aq CH3CN.
dFdis = 0.13 at l = 350 nm.
either to the ground-state conjugate base of the pHP group
or to 33.11,14 As in the case of the conjugate base of pHP
derivatives, the tautomer 33 is an unlikely candidate for
room-temperature heterolysis.9
For the heterolytic pathway that proceeds on to the rear-
ranged product, three scenarios have also been suggested.
Published by NRC Research Press

Givens et al.
373
Table 6. Stern–Volmer studies of p-hydroxyphenacyl (pHP) g-aminobutyric acid (GABA)
derivatives in unbuffered H2O and in aqueous media at pH 7.3 buffer.
Studies with watera,b
Studies with buffer at pH 7.3b,c
Compound
KSV ((mol/L)–1)d
299
20.2
33.1
24.2
13.0
44.7
29.1
15.2
25.5
34.2
30.0
t3 (10–9s)e
40.5
2.75
4.47
3.23
1.99
6.02
4.04
2.12
3.31
4.67
4.16
KSV ((mol/L)–1)d
t3 (10–9s)e
13
14
15
16
17
18
19
20
21
22
23
30.3
29.4
26.0
19.2
39.0
32.6
51.1
8.02
36.8
14.1
4.12
4.03
3.66
2.52
5.33
4.31
6.93
1.13
4.82
1.90
aUnbuffered 18 MO Ultrapure H2O.
bStd. dev. <10%.
cN-(2-Hydroxyethyl)piperazine-N’-ethanesulfonic acid (HEPES, 0.01 mol/L) and LiClO4 (0.1 mol/L).
dSorbic acid quencher, Stern–Volmer constant.
eTriplet lifetime.
rearrangement19 by ring-opening hydrolysis via nucleophilic
attack of solvent (water) to produce rearranged p-hydroxy-
phenylacetic acid. Decarbonylation, a minor pathway open
to I2, yields p-quinone methide that hydrolyzes to p-hydroxy-
benzyl alcohol, the minor product.
causes lower quantum yields, since the conjugate base is
much less efficient at releasing the leaving group.
4-Hydroxybenzyl alcohol, a ubiquitous minor product,
serves as additional supporting evidence for the intermedi-
acy of I2 through its formation by decarbonylation and hy-
drolysis.9
The effect of structure on the photolytic reactivity of
substituted pHP GABAs
Finally, these results provide evidence that further modifi-
cation of the chromophore is possible without decreasing its
photoreactivity, enhancing the pHP physical and photochem-
ical versatility. Carboxyl, carbonyl, ether, and amide func-
tional groups can be introduced to enhance the solubility,
binding, and other physical properties without fear of losing
reactivity.
Current studies on this intriguing chromophore and on the
mechanism of the unusual photochemical rearrangement are
in progress.
There appears to be no correlation between the ground-
state pKas of the chromophores and their photochemical be-
havior in terms of their quantum yields or rate constants for
release. The pKa dictates the ratio of the un-ionized and con-
jugate base, and this ratio in turn affects the photochemical
reactivity. Therefore, the photochemical and photophysical
properties do not change with the introduction of most sub-
stituents. The most notable exceptions are the effect of 3-
nitro and the 2- and 3-hydroxyl groups, which completely
quench reactivity. Similarly unreactive were the 3-acetyl
and any appended auxochromes that could quench such as
naphthyl (not shown).
Experimental
Methods
Melting points were conducted with open-ended capillary
tubes using a noncalibrated Thomas–Hoover melting-point
apparatus. Lyophilization was performed using a Labconco
FreezeZone device set at –52 8C and 0.04 mbar (1 bar =
100 kPa). Solution pH values were determined using a
Fisher Scientific pH 510 meter calibrated with certified
Fisher buffer solutions of pH 4, 7, and 10. Products of all
reactions were assessed for purity using the following instru-
Conclusions
A unique rearrangement, the photo-Favorskii reaction, of
pHP esters combines the accelerated release of substrate li-
gands with a hypsochromic shift of the pHP chromophore.
The rearrangement occurs on the triplet manifold by a spin-
conserved extrusion of the chromophore as a triplet biradical
I1 followed by relaxation of the biradical and closure to
yield I2, the ‘‘Favorskii’’ intermediate, merging the photo-
chemical pathway with a ground-state Favorskii reaction.19
With few exceptions, substituents on the chromophore do
not influence the mechanics of the rearrangement sequence
and cause only small variations in the quantum yields and
reaction-rate constants. The release of substrate is consis-
tently more efficient for the un-ionized chromophores. Sub-
stituents do influence the chromophore pKa; electron-
donating and -withdrawing groups both lower the pKa and,
therefore, favor the conjugate base at neutral pH. This
1
mental techniques: For H, 13C, and 19F NMR, Bruker DRX
400 and DRX 500 MHz spectrometers were utilized with tri-
fluoroacetic acid as an internal standard (d = –76.55 ppm).
GC–MS analysis was done on an Agilent 6890N Network
GC system equipped with a Quattro Micromass triple quad-
rupole electron impact mass spectrometer. Exact masses
were performed on a Quattro Micromass triple quadrupole
electrospray ionization mass spectrometer. UV–vis data
were obtained on a Carey Bio100 instrument using 1.5 mL
quartz cuvettes. Fluorescence data were acquired on a Carey
Eclipse fluorescence spectrophotometer using a 10 mm
Published by NRC Research Press

374
Can. J. Chem. Vol. 89, 2011
quartz cell. IR data were obtained using a Shimadzu FT-IR
8400 S instrument with pressed potassium bromide (KBr)
pellets or a prefabricated sodium chloride (NaCl) chloroform
cell for solid-phase analysis and NaCl prefabricated plates
for oil analysis. Ground-state pKas were determined through
sample titration with increasing amounts of 0.0461 mol/L
NaOH (standardized with potassium hydrogen phthalate),
correlating pH vs [OH–] and ascertaining the half equiva-
lence point as the pKa. Product separation was achieved by
flash chromatography using EM Science silica gel and gra-
dient hexanes – ethyl acetate as eluting solvents. All reac-
tions were run under ambient conditions unless otherwise
graphic peaks. Calibration curves to obtain R values from
linear least-squares regression were determined at concentra-
tions of the reactants and products in photolyses by systematic
increases of pHP-caged GABA, free GABA, and p-hydroxy-
phenylacetic acid concentrations to determine correlations
with internal standards caffeine or 4-acetamidophenol. The
quantum efficiencies were then calculated from the ratio of
the reactant or product concentrations to the photons ab-
sorbed using the actinometer values obtains as indicated
above.
Lifetime and rate measurements
1
stated. H NMR spectroscopy was utilized to monitor the
The Stern–Volmer quenching technique20 effectively dis-
tinguished the triplet lifetimes of pHP derivatives, using po-
tassium sorbate as the quencher. Concentrations of pHP
GABA solutions ranged from 0.001 to 0.01 mol/L; these
were diluted with sorbate solutions of increasing concentra-
tion (0–0.1 mol/L) and subsequently photolyzed under previ-
ously indicated conditions to ascertain the change in
quantum efficiencies of GABA release. F0/F vs [Q], and
KSV were determined. To establish the triplet lifetime (t3),
the rate of quenching (kq) was assumed to be commensurate
progress of the photolysis of all new pHP cages. In general,
a 1–10 mmol/L sample of pHP-caged GABA was prepared
in 2 mL D2O and ~2 mL was placed in an NMR tube. This
was positioned in a photoreactor equipped with two 15 W,
˚
3000 A Rayonet lamps and a merry-go-round. Irradiation
for 30 min generally led to 100% conversion as judged by
the absence of the ester proton signals. Photoproduct identi-
fication was achieved through the spiking the samples of the
irradiated mixture with authentic samples of the resolved
GABA. Quantitative photolysis conditions for determination
of quantum yields and Stern–Volmer quenching constants (KSV)
were as follows: The lamp light output (in mEinsteins/min)
was established using the potassium ferrioxalate method.7
Milligram quantities of caged compounds and caffeine or
acetamidophenol were weighed out on a Fisher brand micro-
balance and dissolved in 4 mL of 18 MO ultrapure water
salt solutions of various concentrations, buffers with or with-
out adjusted ionic strengths, or purified organic solvents
were then added to a quartz tube and vortexed, resulting in
a homogenous solution of the caged compound and internal
standard. Concentrations of the caged compounds ranged
from 1 to 9 mmol/L. These tubes were then placed in a car-
ousel within a Rayonet photochemical reactor equipped with
with the rate of bimolecular diffusion in water (kdiff
*
7.2 ꢁ 109 s–1). The results are shown in Table 6.
Femtosecond pump–probe spectroscopy
Femtosecond transient absorption was measured with the
pump – supercontinuum probe technique using a Ti/Sa laser
system (Clark MXR CPA-2001; 775 nm, pulse energy
0.9 mJ, full width at half maximum 150 fs, and operating
frequency 426 Hz). Part of the beam was fed into a Clark
MXR NOPA. The output at 532 nm was frequency-doubled
by a b-barium borate (BBO) crystal to 266 nm and upon
compression, elicited pump pulses with an energy of 1 mJ
and <150 fs pulse width. A probe beam continuum was gen-
erated by focusing the 775 nm beam in front of a CaF2 plate
with a 2 mm path length that produced a supercontinuum
probe beam spanning a wavelength range of 270–690 nm.
The pump and probe beams were focused to a 0.2 mm spot
on the sample that was flowing in an optical cell with a
thickness of 0.4 mm. The probe beam and a reference signal
(passing the solution beside the pump beam) were spectrally
dispersed and registered with two photodiode arrays
(512 pixels). Transient absorption spectra were calculated
from the ratio of the two beams. The pump–probe cross-
correlation was <100 fs over the entire spectrum. Measure-
ments on short time scales (up to 50 ps) were corrected for
chirp using a program (SPAN) kindly provided by Professor
˚
two 3000 A, 15 W Rayonet photochemical reactor RPR3000
mercury lamps as light sources. One hundred microlitre
samples were removed at 30 s intervals up to 5 min using a
250 mL Hamilton microsyringe and diluted to 1 mL with
water using 1 mL volumetric flasks.
Quantum efficiency determinations
Quantitative analysis was achieved by HPLC–UV or
HPLC–MS/MS. The LC–MS/MS instrument was a Waters
2695 Liquid Chromatographer equipped with a Quattro Mi-
cromass triple quadrupole electrospray ionization mass spec-
trometer, outfitted with an autosampler. UV–vis detection
consisted of a Waters 2497 type with a dual wavelength de-
tector set at 220 and 240 nm. The reservoirs used were as
follows: (i) 99% water, 1% methanol, 10 mmol/L ammo-
nium formate, and 0.06% formic acid; and (ii) 99% metha-
nol, 1% water, 10 mmol/L ammonium formate, and 0.06%
formic acid. The column was a reverse-phase (C18), 4 mm
mesh Altech Altima, 50 mm in length. Injections of 100 mL
were made with an automated sampler for each run for a to-
tal of three injections per vial. A mobile phase gradient was
utilized to optimize compound separation. The flow rate was
set at 300 mL/min. Data analysis was performed by Mass
Lynx Ultima software and Microsoft Excel. Smoothing
functions were used for peak analysis of the chromato-
¨
N. Ernsting, Institut fu¨r Chemie, Humboldt Universitat zu
Berlin, Germany. To improve the signal-to-noise ratio, the
data were averaged over multiple pump–probe scans (3–
6 scans with 400 shots per temporal point).
Materials
Unless indicated, starting materials were obtained from
Sigma-Aldrich or Matrix Scientific. Before usage, solvents
were purified via simple distillation employing phosphorus
pentoxide, calcium hydride, or calcium chloride and stored
˚
in containers with microwave-activated 4 A molecular
sieves. Ultrapure (18 MO) water was used in all instances.
Published by NRC Research Press

Givens et al.
375
H2O–EA. The aqueous layer was collected and the water re-
moved by lyophilization to give 4-(2-(4-hydroxy-3,5-dime-
thoxyphenyl)-2-oxoethoxy)-4-oxobutan-1-amonium-2,2,2-tri-
fluoroacetate as a colorless oil (3, 100%). IR (film, cm–1):
3388, 3179, 2943, 2847, 1737, 1693, 615, 1536, 1466,
Synthesis
pHP GABAs
4-(2-(4-Hydroxyphenyl)-2-oxoethoxy)-4-oxobutan-1-
aminium-2,2,2-trifluoroacetate (1)
1
1431, 1379, 1326, 1300, 1169, 1047. H NMR (400 MHz,
The steps to synthesize 1 were described in the protocol
D2O) d: 1.94 (m, 2H), 2.58 (t, J = 7.1 Hz, 2H), 3.00 (t, J
= 7.7 Hz, 2H), 3.63 (s, 6H), 5.18 (s, 2H), 6.75 (s, 2H).
FAB MS (free amine) m/z: 342 (M +1). HRMS calcd for
C15H20NO8 (M + H): 342.1189; found: 342.1193.
by Givens et. al.5
4-(2-(4-Hydroxy-3-methoxyphenyl)-2-oxoethoxy)-4-
oxobutan-1-amonium-2,2,2-trifluoroacetate (2)
The synthesis of 2 was accomplished by using the proce-
dures of Givens et al.5 and Conrad et al.2 as reported for 1.
Under an inert atmosphere of argon, a flame-dried 100 mL
round-bottom flask was charged with N-Boc-g-aminobutyric
acid (248 mg, 1.22 mmol), 2-bromo-1-(4-hydroxy-3-methoxy-
phenyl)ethanone (300 mg, 1.22 mmol), and acetonitrile
(50 mL). Potassium carbonate (188 mg, 1.36 mmol) was
added and the mixture was stirred at room temperature (rt)
for 24 h. The solvent was removed under reduced pressure
and the remaining residue was dissolved in ethyl acetate
and washed with water (3 ꢁ 10 mL). The organic extract
was dried with MgSO4 and concentrated to afford a pink
solid. The crude product was flash chromatographed on
silica gel with 2:1, 1:1, and then 2:3 hexane – ethyl acetate.
Collection of the appropriate fractions and removal of the
solvent under reduced pressure provided 2-(4-hydroxy-3-
methoxyphenyl)-2-oxoethyl-4-[(tert-butoxycarbonyl)amino]-
butanoate as a light pink solid. Yield: 333 mg, 74%; mp 88–
89 8C. The t-Boc group was removed by treating the 2-(4-
hydroxy-3-methoxyphenyl)-2-oxoethyl-4-[(tert-butoxycarbo-
nyl)amino]butanoate (281 mg, 0.76 mmol) with cold
trifluoroacetic acid (TFA, 10 mL). The solution was stirred
at 0 8C for 1 h. The excess TFA was removed under re-
duced pressure and the remaining residue was dissolved in
water, and washed with ethyl acetate. The aqueous layer
was collected and the water removed by lyophilization to
give 4-(2-(4-hydroxy-3-methoxyphenyl)-2-oxoethoxy)-4-
oxobutan-1-amonium-2,2,2-trifluoroacetate as a tan solid (2,
545 mg, 100% yield, mp 128–130 8C). IR (KBr, cm–1):
3339, 3181, 3150, 2935, 2756, 2088, 1757, 1675, 1593,
Representative description, with 16
1-(Benzyloxy)-4-bromo-2-fluorobenzene (16b)
21
´
The general method of Frechet and co-workers was fol-
lowed. A solution of 4-bromo-2-fluorophenol (16a, 3.83 g,
20.0 mmol), benzyl bromide (2.38 mL, 20.0 mmol), and po-
tassium carbonate (6.91 g, 50.0 mmol) in CH3CN (30 mL)
was stirred at rt for 15 h. The solution was diluted with
50 mL CH2Cl2, washed with water (3 ꢁ 30 mL), dried
(magnesium sulfate), and concentrated to give 5.24 g (93%)
of 1-(benzyloxy)-4-bromo-2-fluorobenzene (16b) as a white
precipitate; mp 65–67 8C. IR (CHCl3, cm–1): 3020, 2987,
2684, 2304, 1498, 1421, 1265, 1217, 1051, 896, 738, 695.
1H NMR (400 MHz, CDCl3) d (ppm): 7.41 (m, 5H), 7.27
(dd, J = 6.4, J = 2.2 Hz, 1H), 7.26 (dt, J = 4.3, J = 2.0 Hz,
1H), 6.89 (t, J = 8.8 Hz, 1H), 5.13 (s, 2H). 13C NMR
(125 MHz, CDCl3) d (ppm): 154.23, 151.73, 146.06,
136.02, 128.88, 127.73, 120.14, 117.23, 115.86, 113.02,
71.90. 19F NMR (376 MHz, CDCl3 + 1 drop of CF3CO2H)
d (ppm): –130.78. HRMS (M+) calcd for C13H10FBrO:
279.9899; found: 279.9905.
1-(Benzyloxy)-3-fluorophenyl)ethanone (16c)
The general method of Kosugi et al.22 was used. The ex-
perimental procedure is described with that for 23 (vide in-
fra).28 White precipitate; yield: 89%, mp 79–81 8C. IR
(CHCl3, cm–1): 3053, 2987, 1679, 1610, 1514, 1498, 1421,
1265, 1217, 1052, 896, 738, 696. 1H NMR (400 MHz,
CDCl3) d (ppm): 7.74 (dd, J = 11.3, J = 2.4 Hz, 1H), 7.69
(dd, J = 9.0, J = 1.6 Hz, 1H) 7.44, (m, 5H), 7.04 (t, J =
8.4 Hz, 1H), 5.23 (s, 2H), 2.55, (s, 3H). 13C NMR
(125 MHz, CDCl3) d (ppm): 196.05, 153.66, 151.16,
135.82, 130.92, 128.95, 127.57, 125.76, 116.36, 114.22,
71.25, 26.52. 19F NMR (376 MHz, CDCl3 + 1 drop of
CF3CO2H) d (ppm): –133.02. HRMS (M + H) calcd for
C15H13FO2: 245.0978; found: 245.0951.
1
1516, 1460, 1424, 1383, 1368, 1281, 1178, 1132, 1020. H
NMR (400 MHz, D2O) d: 1.94 (p, J = 7.3 Hz, 2H), 2.60 (t,
J = 7.1 Hz, 2H), 3.00 (t, J = 7.7 Hz, 2H), 3.79 (s, 3H), 5.37
(s, 2H), 6.85 (d, J = 8.3 Hz, 1H), 7.36 (s, 1H), 7.43 (d, J =
8.4 Hz, 1H).
4-(2-(4-Hydroxy-3,5-dimethoxyphenyl)-2-oxoethoxy)-4-
oxobutan-1-amonium-2,2,2-trifluoroacetate (3)
1-(4-(Benzyloxy)-3-fluorophenyl)-2-bromoethanone (16d)
The method was adapted from Ohriand co-workers.23
The synthesis of 3 was accomplished by using the proce-
dure of Conrad et. al.2 A solution of 2-bromo-1-(4-hydroxy-
3-methoxyphenyl)ethanone2 (600 mg, 2.18 mmol/L) was
treated with 4-(tert-butoxycarbonylamino)butanoic acid (N-
t-Boc-GABA; 443 mg, 2.18 mmol/L) in 10 mL of benzene
at 7 8C to which was added 366 mg (2.41 mmol/L) of 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) in 10 mL of benzene.
The mixture was allowed to reach rt and stirred overnight.
The solvent was removed in vacuo and the crude product
purified by silica gel chromatography (522 mg, 60% yield).
The t-Boc protecting group was then removed by treatment
with TFA (10 mL) at 0 8C for 4 h. The TFA was removed
by rotary evaporation and the crude product was treated with
A
50 mL round-bottom flask was charged with 16c (700 mg,
2.86 mmol) and phenyltrimethylammonium tribromide
(PTAB) (1.07 g, 2.86 mmol). The mixture was subsequently
dissolved by the addition of 40 mL of CH2Cl2–CH3OH
(1:1). The resultant solution was stirred at rt for 6 h, at
which time full conversion was indicated by GC–MS. The
solution was transferred to a separatory funnel, diluted with
50 mL of CH2Cl2, and washed profusely with water (3 ꢁ
50 mL). Excess solvent was removed under reduced pres-
sure, affording an orange precipitate, 900 mg (2.58 mmol),
~95%.24 IR (CHCl3, cm–1): 3055, 2988, 1679, 1615, 1511,
Published by NRC Research Press

376
Can. J. Chem. Vol. 89, 2011
1498, 1417, 1267, 1219, 1050, 891, 748, 696. 1H NMR
(400 MHz, CDCl3) d (ppm): 7.77 (dd, J = 10.0, J = 2.4 Hz,
1H), 7.72 (dd, J = 7.0, J = 2.8 Hz, 1H), 7.42 (m, 5H), 7.07
(t, J = 8.4 Hz, 1H), 5.24 (s, 2H), 4.37 (s, 2H). 19F NMR
(376 MHz, CDCl3 + 1 drop of CF3CO2H) d (ppm): –133.12.
1018, 813, 742, 696. 1H NMR (400 MHz, CD3CN) d
(ppm): 5.22 (s, 2H), 7.14 (d, 1H), 7.45 (m, 5H), 7.73 (dd,
1H), 7.81 (d, 1H). 13C NMR (100 MHz, CD3CN) d (ppm):
71.9, 104.5, 112.5, 115.9, 116.2, 128.7, 129.4, 129.7, 136.7,
136.8, 138.4, 160.6. HRMS (M+) calcd for C14H10BrNO:
286.9946; found: 286.9937.
2-(4-(Benzyloxy)-3-fluorophenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (16e)
1-(4-Benzyloxy-3-cyanophenyl)ethanone (13c)
The general method of Fujita and Hiyama25 was utilized.
A solution of 1-(4-(benzyloxy)-3-fluorophenyl)-2-bromo-
ethanone (16d, 900 mg, 2.78 mmol), potassium carbonate
(1.15 mg, 8.35 mmol), and 4-(tert-butoxycarbonylamino)
butanoic acid (N-Boc–GABA, 678 mg, 3.33 mmol) in
50 mL of CH3CN was stirred for 24 h at rt. The solution
was washed with EtOAc, water, the aqueous phase dis-
carded, and the resulting phase evaporated under reduced
pressure. Flash column chromatography (hexanes–EtOAc,
5:1) afforded 16e as a white precipitate. Yield: 1.10 g
(89%); mp 97–99 8C. IR (KBr, cm–1): 3300, 3100–2800,
1742, 1701, 1600, 1517, 1437, 1400 1309, 1200, 1165, 953,
White solid. Yield: 90%; mp 120–123 8C. IR
(KBr, cm–1): 3068, 3033, 2229, 1681, 1602, 1500, 1419,
1
1355, 1275, 1137, 977, 817, 750, 630. H NMR (400 MHz,
CD3CN) d (ppm): 2.53 (s, 3H), 5.31 (s, 2H), 7.26 (d, 1H),
7.44 (m, 5H), 8.17 (dd, 1H), 8.26 (d, 1H). 13C NMR
(100 MHz, CD3CN) d (ppm): 25.45, 70.79. 101.53, 112.63,
115.27, 127.50, 128.19, 128.40, 130.15, 134.20, 134.50,
1335.27, 163.02, 194.85. HRMS (M + H) calcd for
C16H13NO2: 252.1024; found: 252.1022.
1-(4-Benzyloxy)-3-cyanophenyl)-2-bromoethanone (13d)
White solid. Yield: 91%; mp 94–96 8C. IR (CDCl3, cm–1):
688, 833, 1037, 1247, 1280, 1600, 1735, 2115, 2258, 3091.
1H NMR (400 MHz CD3CN) d (ppm): 4.63 (s, 2H), 5.35 (s,
2H) 7.16 (d, 1H), 7.46 (m, 5H), 8.25 (dd, 1H), 8.32 (d, 1H).
13C NMR (100 MHz, CD3CN) d (ppm): 32.30, 71.30,
102.26, 113.31, 115.34, 117.36, 127.28, 127.88, 128.60,
128.78, 135.10, 135.46, 135.48, 163.87, 188.96. HRMS
(M+) calcd for C16H13BrNO2: 330.0130; found: 330.0140.
1
750, 700, 668. H NMR (400 MHz, CD3CN) d (ppm): 7.80
(dd, J = 7.0, J = 2.4 Hz, 1H), 7.78 (dd, J = 7.0, J = 2.0 Hz,
1H), 7.47 (dd, J = 4.4, J = 1.8 Hz, 1H), 7.42 (m, 5H), 5.31
(s, 2H), 5.25 (s, 2H), 2.97 (t, J = 6.8 Hz, 2H), 2.45 (t, J =
7.6 Hz, 2H), 1.76 (m, 2H), 1.40 (s, 9H). 13C NMR
(125 MHz, d6-DMSO) d (ppm): 190.67, 172.51, 155.60,
152.30, 150.80, 135.82, 128.39, 128.14, 127.68, 126.93,
125.60, 115.28, 114.82, 77.45, 70.36, 66.12, 39.64, 30.63,
28.22, 24.96. 19F NMR (376 MHz, CD3CN + 1 drop of
CF3CO2H) d (ppm): –133.44. HRMS (M + Na) calcd for
C24H28FNO6Na: 468.1798; found: 468.1796.
2-(4-(Benzyloxy)-3-cyanophenyl-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (13e)
White solid. Yield: 81%; mp 119–120 8C. IR
(CHCl3, cm–1): 3456, 2252, 1743, 1706, 1600, 1502, 1271,
1
4-(2-(3-Fluoro-4-hydroxyphenyl)-2-oxoethoxy)-4-oxobutan-
1-aminium-2,2,2-trifluoroacetate (16)
The general method of Marsh and Goodman26 was fol-
lowed. To a flame-dried 50 mL round-bottom flask contain-
1166. H NMR (400 MHz, CD3CN) d (ppm): 1.39 (s, 6H),
1.78 (t, 2H), 2.20 (m, 2H), 3.10 (t, 2H), 5.31 (s, 4H), 7.29
(d, 1H), 7.40 (m, 5H), 8.17 (dd, 1H), 8.25 (d, 1H). 13C
NMR (100 MHz, CD3CN) d (ppm): 24.47, 25.46, 27.27,
30.34, 39.01, 65.62, 70.79, 70.93, 77.73, 101.89, 112.64,
112.97, 115.01, 117.00, 127.02, 127.53, 128.24, 133.88,
134.15, 135.13, 155.58, 163.57, 172.21, 189.92. HRMS
(M + H) calcd for C25H29N2O6: 452.2026; found: 453.2021.
ing
2-(4-(benzyloxy)-3-fluorophenyl)-2-oxoethyl-4-(tert-
butoxy-carbonylamino)-butanoate (16e, 850 mg, 1.91 mmol)
was added 20 mL of freshly distilled TFA. Stirring contin-
ued for 24 h at rt under ambient conditions. The solution
was evaporated under reduced pressure and washed with
EtOAc–water. The water layer was then extracted, frozen,
and then lyophilized, affording 16 as an adhesive precipitate.
Yield: 550 mg (78%). IR (KBr, cm–1): 3434, 3269, 3100–
2800, 1741, 1693, 1610, 1553, 1420, 1201, 1050, 823, 759,
719. 1H NMR (400 MHz, D2O) d (ppm): 7.70 (d, J =
9.0 Hz, 2H), 7.12 (t, J = 9.9 Hz, 1H), 5.50 (s, 2H), 3.09 (t,
J = 7.6 Hz, 2H), 2.66 (t, J = 7.0 Hz, 2H), 2.05 (m, 2H). 13C
NMR (125 MHz, MeOD) d (ppm): 190.05, 172.06, 161.60,
152.27, 151.07, 150.33, 126.03, 125.33, 117.30, 115.33,
65.95, 38.54, 30.02, 22.48. 19F NMR (376 MHz, D2O) d
(ppm): –133.58. HRMS (M+) calcd for C12H15FNO4:
256.0985; found: 256.0977.
4-(2-(3-Cyano-4-hydroxyphenyl)-2-oxoethoxy)-4-oxobutan-
1-aminium-2,2,2-trifluoroacetate (13)
Waxy solid. Yield: 50%. IR (KBr, cm–1): 3388, 2358,
2343, 2237, 1677 (broad), 1610, 1438, 1205, 1137, 842,
1
802, 723. H NMR (400 MHz, D2O) d (ppm): 1.94 (t, 2H),
2.59 (m, 2H), 3.08 (t, 2H), 5.39 (s, 2H), 7.05 (d, 1H), 8.01
(dd, 1H), 8.18 (d, 1H). 13C NMR (100 MHz, D2O) d (ppm):
21.96, 29.43, 38.54, 66.59, 99.25, 112.76, 115.10, 117.42,
119.74, 134.87, 162.83, 172.45, 192.32. HRMS (M+) calcd
for C13H15N2O4: 263.1032; found: 263.1023.
The synthetic course towards 4–9 and 12 followed the
same protocol as for 13–23, but commenced with the com-
mercially available corresponding p-hydroxyacetophenones.
Hence, only spectroscopic data for these are indicated.
The synthetic protocol for generating compounds 14 and
1527 and 17–2328 has been previously published. The synthe-
sis of 13 was analogous to that for 16 and, therefore, only
the spectroscopic data for this is described.
1-(3,4-Bis(benzyloxy)phenyl)ethanone (4d)
White solid. Yield: 98%. This compound is known. 1H
NMR (400 MHz, CDCl3) d (ppm): 7.63 (dd, J = 8.8, J =
2.3 Hz, 1H), 7.40 (m, 11H), 6.94 (dd, J = 9.2, J = 2.0 Hz,
1H), 5.25 (s, 2H), 5.22 (s, 2H), 2.52 (s, 3H).
1-Benzyloxy-4-bromo-2-cyanobenzene (13b)
White solid. Yield: 87%; mp 80–81 8C. IR (KBr, cm–1):
3068, 3035, 2229, 1591, 1487, 1454, 1287, 1132, 1132,
Published by NRC Research Press

Givens et al.
377
1-(3,4-Bis(benzyloxy)phenyl)-2-bromoethanone (4e)
2-(2,4-Bis(benzyloxy)phenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (5f)
1
Oil. Yield: 96%7. H NMR (400 MHz, CDCl3) d (ppm):
Adhesive precipitate. Yield: 48%. IR (KBr, cm–1): 3310,
3100–2800, 1743, 1700, 1658, 1601, 1436, 1400, 1200,
1165, 955, 745, 742, 700, 697, 665, 651. 1H NMR
(400 MHz, d6-DMSO) d (ppm): 7.93 (t, J = 8.8 Hz, 1H),
7.43 (m, 10H), 7.14 (dd, J = 8.7, J = 2.2 Hz, 1H), 6.88 (t,
J = 5.7 Hz, 1H), 6.62 (d, J = 4.7 Hz, 1H), 5.37 (s, 2H), 5.29
(s, 1H), 5.21 (s, 1H), 5.10 (s, 2H), 2.94 (t, J = 6.6 Hz, 2H),
2.37 (t, J = 7.5 Hz, 2H), 1.76 (m, 2H), 1.37 (s, 9H). 13C
NMR (125 MHz, d6-DMSO) d (ppm): 190.50, 172.15,
164.65, 159.78, 155.58, 135.99, 132.04, 128.35, 127.51,
118.10, 107.59, 103.90, 100.33, 77.43, 70.72, 69.19, 39.95,
30.68, 28.22, 24.88. HRMS (M + H) calcd for C31H36NO7:
534.2492; found: 534.2504.
7.68 (s, 1H), 7.41 (m, 11H), 6.96 (d, 1H), 5.24 (s, 2H), 5.22
(s, 2H), 4.46 (s, 2H).
2-(3,4-Bis(benzyloxy)phenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (4f)
White precipitate. Yield: 90%; mp 112–114 8C. IR
(KBr, cm–1): 3310, 3100–2800, 1743, 1700, 1658, 1601,
1436, 1400, 1200, 1165, 955, 745, 742, 700, 697, 665, 651.
1H NMR (400 MHz, d6-DMSO) d (ppm): 7.62 (dd, J = 7.2,
J = 2.4 Hz, 1H), 7.56 (d, J = 1.8 Hz, 1H), 7.38 (m, 10H),
7.19 (d, J = 8.1 Hz, 1H), 6.92 (t, J = 7.2 Hz, 1H), 5.40 (s,
2H), 5.27 (s, 2H), 5.21 (s, 2H), 2.97 (t, J = 6.8 Hz, 2H),
2.40 (t, J = 7.7 Hz, 2H), 1.70 (m, 2H), 1.38 (s, 9H). 13C
NMR (125 MHz, d6-DMSO) d (ppm): 197.09, 171.80,
155.60, 152.85, 147.98, 136.85, 128.41, 127.94, 127.58,
126.84, 122.64, 113.04, 112.51, 77.47, 70.00, 66.07, 39.61,
30.67, 28.22, 24.54. HRMS (M + H) calcd for C31H36NO7:
534.2492; found: 534.2484.
2-(2,4-Dihydroxyphenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (5g)
Adhesive precipitate. Yield: 97%. H NMR (400 MHz,
1
d6-DMSO) d (ppm): 11.43, (s, 1H), 11.34 (s, 1H), 7.88 (s,
1H), 7.70 (d, 1H), 6.87 (t, 1H), 6.38 (d, 1H), 5.28 (s, 1H),
2.96 (t, 2H), 2.40 (t, 2H), 1.67 (m, 2H), 1.38 (s, 9H).
2-(2,4-Dihydroxyphenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (4g)
4-(2-(2,4-Dihydroxyphenyl)-2-oxoethoxy)-4-oxobutan-1-
aminium-2,2,2-trifluoroacetate (5)
An adapted method of Theodorakis and co-workers29 was
utilized. To a stoppered round-bottom flask containing a sol-
ution of 4f (1.20 g, 2.24 mmol) and 120 mg of 10% w/w Pd/
C in 25 mL of EtOAc was added H2 via a needle from a re-
inforced balloon. The reaction was deemed complete after
4 h by the absence of the reagent band on TLC (hexanes–
EtOAc, 1:1). Excess solvent was removed in vacuo to afford
White precipitate. Yield: 85%; mp 136–138 8C. IR
(KBr, cm–1): 3432, 3274, 3100–2800, 1739, 1691, 1602,
1520, 1421, 1377, 1309, 1298, 1201, 1178, 1128, 1055,
1
1027, 1008, 823, 762, 717, 613. H NMR (400 MHz, D2O)
d (ppm): 7.97 (d, J = 6.8 Hz, 1H), 7.72 (d, J = 8.8 Hz, 1H),
6.53 (d, J = 7.6v, 1H), 5.48 (s, 1H), 5.43 (s, 1H), 3.14 (t, J =
7.6 Hz, 2H), 2.72 (t, J = 7.0 Hz, 2H), 2.08 (m, 2H). 13C
NMR (125 MHz, D2O) d (ppm): 193.77, 174.15, 162.75,
151.02, 144.22, 125.92, 122.70, 119.65, 117.37, 115.58,
66.60, 38.51, 30.28, 22.00. HRMS (M+) calcd for
C12H16NO5: 254.1028; found: 254.1038.
1
an adhesive precipitate. Yield: 98%. H NMR (400 MHz,
d6-DMSO) d (ppm): 7.32 (d, J = 7.2 Hz, 2H), 6.93 (s, 1H),
6.89 (t, J = 6.9 Hz, 1H), 5.39 (s, 2H), 2.98 (t, J = 6.8 Hz,
2H), 2.38 (t, J = 7.9 Hz, 2H), 1.68 (m, 2H), 1.40 (s, 9H).
13C NMR (125 MHz, d6-DMSO) d (ppm): 197.34, 171.97,
155.66, 155.52, 136.92, 123.01, 113.09, 112.39, 77.55,
70.13, 39.56, 30.65, 28.19, 24.53.
1-(4-(Benzyloxy)-3-methyl-phenyl)ethanone (6d)
White precipitate. Yield: 95%. This compound is known.
1H NMR (400 MHz, CDCl3) d (ppm): 7.82 (s 1H), 7.80 (s,
1H), 7.40 (m, 5H), 6.90 (d, J = 8.8 Hz, 1H), 5.17 (s, 2H),
2.56 (s, 3H), 2.33 (s, 3H).
4-(2-(3,4-Dihydroxyphenyl)-2-oxoethoxy)-4-oxobutan-1-
aminium-2,2,2-trifluoroacetate (4)
White precipitate. Yield: 92%; mp 56–58 8C. IR
(KBr, cm–1): 3432, 3274, 3100–2800, 1739, 1691, 1602,
1520, 1421, 1377, 1309, 1298, 1201, 1178, 1128, 1055,
1-(4-(Benzyloxy)-3-methylphenyl)-2-bromoethanone (6e)
Oil. Yield: 97%.7 1H NMR (400 MHz, CDCl3) d (ppm):
7.82 (d, 2H), 7.40 (m, 5H), 6.88 (d, J = 8.8 Hz, 1H), 5.19
(s, 2H), 4.33 (s, 2H), 2.33 (s, 3H).
1
1027, 1008, 823, 762, 717, 613. H NMR (400 MHz, D2O)
d (ppm): 7.35 (s, 1H), 7.33 (s, 1H), 6.91 (d, J = 7.1 Hz, 1H),
5.36 (s, 2H), 3.07 (t, J = 6.8 Hz, 2H), 2.65 (t, J = 7.8 Hz,
2H), 2.00 (m, 2H). 13C NMR (125 MHz, D2O) d (ppm):
194.29, 174.13, 163.06, 150.88, 144.11, 125.88, 122.69,
119.74, 117.42, 115.10, 66.56, 38.30, 30.23, 21.95. HRMS
(M+) calcd for C12H16NO5: 254.1028; found: 254.1032.
2-(4-(Benzyloxy)-3-methylphenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (6f)
White precipitate. Yield: 94%; mp 105–106 8C. IR
(KBr, cm–1): 3307, 3100–2800, 1742, 1701, 1659, 1606,
1437, 1407, 1209, 1167, 955, 743, 700, 667. 1H NMR
(400 MHz, d6-DMSO) d (ppm): 7.82 (d, J = 9.1 Hz, 2H),
7.40 (m, 5H), 7.14 (d, J = 8.1 Hz, 1H), 6.87 (t, J = 7.4 Hz,
1H), 5.39 (s, 2H), 5.25 (s, 2H), 2.96 (t, J = 6.8 Hz, 2H),
2.42 (t, J = 7.6 Hz, 2H), 2.24 (s, 3H), 1.70 (m, 2H), 1.38 (s,
9H). 13C NMR (125 MHz, d6-DMSO) d (ppm): 191.14,
172.20, 161.40, 155.60, 136.67, 131.00, 130.10, 129.26,
128.50, 127.34, 126.45, 125.07, 111.62, 77.45, 69.55, 66.03,
39.33, 30.68, 28.18, 24.98, 16.04. HRMS (M + Na) calcd
for C25H31NO6Na: 464.2049; found: 464.2017.
1-(2,4-Bis(benzyloxy)phenyl)ethanone (5d)
White precipitate. Yield: 97%. This compound is known.
1H NMR (400 MHz, CDCl3) d (ppm): 7.86 (t, J = 7.6, 1H),
7.39 (m, 11H), 6.64 (dd, J = 8.7, J = 2.3 Hz, 1H), 5.12 (s,
2H), 5.10 (s, 2H), 2.57 (s, 3H).
1-(2,4-Bis(benzyloxy)phenyl)-2-bromoethanone (5e)
Waxy precipitate. Yield: 94%.7 1H NMR (400 MHz,
CDCl3) d (ppm): 7.78 (s, 1H), 7.35 (m, 10H), 6.60 (d, 2H),
5.12 (s, 2H), 5.09 (s, 2H), 4.51 (s, 2H).
Published by NRC Research Press

378
Can. J. Chem. Vol. 89, 2011
2-(4-Hydroxy-3-methylphenyl)-2-oxoethyl-4-(tert-
4-(2-(4-Hydroxy-2-methylphenyl)-2-oxoethoxy)-4-
oxobutan-1-aminium-2,2,2-trifluoroacetate (7)
butoxycarbonylamino)butanoate (6g)
Adhesive precipitate. Yield: 98%. H NMR (400 MHz,
1
White precipitate. Yield: 95%; mp 110–112 8C. IR
(KBr, cm–1): 3460, 3300, 3100–2800, 1740, 1699, 1662,
1606, 1437, 1407, 1209, 1167, 955, 743, 700, 667. 1H
NMR (400 MHz, d6-DMSO) d (ppm): 7.86 (s, 3H), 7.80 (d,
J = 9.0 Hz, 1H), 6.74 (t, J = 6.2 Hz, 1H), 5.28 (s, 2H), 2.96
(t, J = 6.7 Hz, 2H), 2.54 (t, J = 7.6 Hz, 2H), 2.40 (s, 3H),
1.87 (m, 2H). 13C NMR (125 MHz, d6-DMSO) d (ppm):
192.71, 171.72, 161.19, 158.64, 141.88, 131.90, 124.55,
118.81, 117.56, 112.58, 67.02, 37.84, 30.14, 22.54, 20.34.
HRMS (M+) calcd for C13H18NO4: 252.1236; found:
252.1227.
d6-DMSO) d (ppm): 10.76 (s, 1H), 7.86 (d, J = 9.2 Hz, 2H),
7.66 (d, J = 8.0 Hz, 1H), 6.89 (t, 1H), 5.38 (s, 2H) 2.96 (t,
J = 6.9 Hz, 2H), 2.42 (t, J = 7.8 Hz, 2H), 2.24 (s, 3H), 1.70
(m, 2H), 1.38 (s, 9H).
4-(2-(4-Hydroxy-3-methylphenyl)-2-oxoethoxy)-4-
oxobutan-1-aminium-2,2,2-trifluoroacetate (6)
White precipitate. Yield: 89%; mp 118–120 8C. IR
(KBr, cm–1): 3432, 3274, 3100–2800, 1739, 1691, 1602,
1520, 1421, 1377, 1309, 1298, 1201, 1178, 1128, 1055,
1
1027, 1008, 823, 762, 717, 613. H NMR (400 MHz, d6-
1-(4-(Benzyloxy)-3,5-dimethylphenyl)ethanone (8d)
Oil. Yield: 95%. This compound is known. GC–MS (EI):
One peak; retention time, 8.38 min (initial temp 50 8C,
ramp 25 8C/min until 300 8C). MS m/z: 254.0.
DMSO) d (ppm): 7.86 (s, 3H), 7.72 (d, J = 6.2 Hz, 1H),
7.67 (dd, J = 9.0, J = 1.8 Hz, 1H), 5.39 (s, 1H), 2.88 (t, J =
7.0 Hz, 2H), 2.56 (t, J = 7.6 Hz, 2H), 2.16 (s, 3H), 1.87 (m,
2H). 13C NMR (125 MHz, d6-DMSO) d (ppm): 190.67,
171.72, 160.98, 158.27, 130.76, 127.76, 125.10, 120.30,
117.94, 114.76, 66.11, 38.09, 30.15, 22.10, 15.80. HRMS
(M+) calcd for C13H18NO4: 252.1230; found: 252.1212.
1-(4-(Benzyloxy)-3,5-dimethylphenyl)-2-bromoethanone
(8e)
White precipitate. Yield: 95%. This compound is known.
GC–MS (EI): One peak; retention time, 9.87 min (initial
temp 50 8C, ramp 25 8C/min until 300 8C). MS m/z: 331.9,
333.9.
1-(4-(Benzyloxy)2-methylphenyl)ethanone (7d)
White precipitate. Yield: 93%. This compound is known.
GC–MS (EI): One peak; retention time, 8.24 min (initial
temp 50 8C, ramp 25 8C/min until 300 8C). MS m/z: 240.0.
2-(4-(Benzyloxy)-3,5-dimethylphenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (8f)
1-(4-(Benzyloxy)-2-methylphenyl)-2-bromoethanone (7e)
White precipitate. Yield: 92%. This compound is known.
GC–MS (EI): One peak; retention time, 9.77 min (initial
temp 50 8C, ramp 25 8C/min until 300 8C). MS m/z: 317.9,
319.9.
Adhesive precipitate. Yield: 96%. IR (KBr cm–1): 3307,
3100–2800, 1742, 1701, 1659, 1606, 1437, 1407, 1209,
1
1167, 955, 743, 700, 667. H NMR (400 MHz, d6-DMSO) d
(ppm): 7.70 (d, J = 8.6 Hz, 1H), 7.50–7.40 (m, 5H), 6.89 (t,
J = 7.2 Hz, 1H), 5.42 (s, 2H), 4.88 (s, 2H), 2.97 (t, J =
6.9 Hz, 2H), 2.43 (t, J = 7.8 Hz, 2H), 2.29 (s, 6H), 1.68 (m,
2H), 1.36 (s, 9H). 13C NMR (125 MHz, d6-DMSO) d (ppm):
191.84, 172.19, 160.04, 155.60, 137.06, 131.49, 129.59,
128.68, 128.36, 128.13, 127.85, 77.45, 73.52, 66.20, 38.97,
30.50, 28.18, 24.97, 16.20. HRMS (M + Na) calcd for
C26H33NO6Na: 478.2206; found: 478.2191; (M + H) calcd
for C26H34NO6: 456.2386; found: 456.2388.
2-(4-(Benzyloxy)-2-methylphenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (7f)
Adhesive precipitate.Yield: 91%. IR (KBr, cm–1): 3307,
3100–2800, 1742, 1701, 1659, 1606, 1437, 1407, 1209,
1
1167, 955, 743, 700, 667. H NMR (400 MHz, d6-DMSO) d
(ppm): 7.86 (d, J = 9.1 Hz, 1H), 7.47–7.40 (m, 5H), 6.97 (t,
J = 6.5 Hz, 2H), 6.87 (t, J = 7.4 Hz, 1H), 5.28 (s, 2H), 5.19
(s, 2H), 2.97 (t, J = 6.7 Hz, 2H), 2.42 (m, 5H), 1.68 (m,
2H), 1.37 (s, 9H). 13C NMR (125 MHz, d6-DMSO) d
(ppm): 193.63, 172.21, 161.05, 155.60, 141.48, 136.40,
132.04, 128.47, 127.42, 125.06, 118.30, 111.85, 107.74,
77.45, 69.30, 67.05, 38.83, 30.68, 28.22, 24.54, 21.11.
HRMS (M + Na) calcd for C25H31NO6Na: 464.2049; found:
464.2040.
2-(4-Hydroxy-3,5-dimethylphenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (8g)
Adhesive precipitate. Yield: 99%. IR (KBr, cm–1): 3548,
3301, 3100–2800, 1739, 1702, 1661, 1606, 1437, 1407,
1
1209, 1167, 955, 743, 700, 667. H NMR (400 MHz, d6-
DMSO) d (ppm): 9.40 (s, 1H), 7.59 (d, J = 8.6 Hz, 2H),
6.89 (t, J = 7.2 Hz, 1H), 5.36 (s, 2H), 2.98 (t, J = 7.0 Hz,
2H), 2.55 (t, J = 7.5 Hz, 2H), 2.20 (s, 6H), 1.92 (m, 2H).
13C NMR (125 MHz, d6-DMSO) d (ppm): 190.91, 172.20,
159.67, 155.59, 128.60, 125.25, 123.74, 77.44, 65.91, 38.96,
30.90, 28.18, 24.98, 16.52. HRMS (M + Na) calcd for
C19H27NO6Na: 388.1736; found: 388.1735.
2-(4-Hydroxy-2-methylphenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (7g)
Adhesive precipitate. Yield: 98%. IR (KBr, cm–1): 3460,
3300, 3100–2800, 1740, 1699, 1662, 1606, 1437, 1407,
1
1209, 1167, 955, 743, 700, 667. H NMR (400 MHz, d6-
DMSO) d (ppm): 10.30 (d, 1H), 7.78 (d, J = 9.1 Hz, 1H),
6.89 (t, J = 7.5 Hz, 1H), 6.71 (t, J = 6.3 Hz, 1H), 5.26 (s,
2H), 2.98 (t, J = 6.7 Hz, 2H), 2.41 (m, 5H), 1.69 (m, 2H),
1.39 (s, 9H). 13C NMR (125 MHz, d6-DMSO) d (ppm):
192.91, 172.21, 160.96, 155.60, 141.83, 131.87, 125.29,
118.74, 112.51, 77.45, 66.86, 38.97, 30.46, 28.22, 24.95,
21.68. HRMS (M + Na) calcd for C18H25NO6Na: 374.1580;
found: 374.1577.
4-(2-(4-Hydroxy-3,5-dimethylphenyl)-2-oxoethoxy)-4-
oxobutan-1-aminium-2,2,2-trifluoroacetate (8)
White precipitate. Yield: 92%; mp 125–127 8C. IR
(KBr, cm–1): 3550, 3301, 3100–2800, 1739, 1702, 1661,
1606, 1437, 1407, 1209, 1167, 955, 743, 700, 667. 1H
NMR (400 MHz, d6-DMSO) d (ppm): 9.36 (s, 1H), 7.86 (s,
3H), 7.62 (d, J = 8.6 Hz, 2H), 5.36 (s, 2H), 2.98 (t, J =
7.0 Hz, 2H), 2.43 (m, 5H), 2.22 (s, 6H), 1.69 (m, 2H), 1.37
Published by NRC Research Press

Givens et al.
379
(s, 9H). 13C NMR (125 MHz, d6-DMSO) d (ppm): 189.74,
170.61, 157.71, 156.71, 127.72, 124.97, 123.15, 116.95,
65.02, 37.10, 28.88, 21.48, 15.55. HRMS (M+) calcd for
C14H20NO4: 266.1392; found: 266.1389.
8.1 Hz), 5.46 (s, 2H), 3.12 (t, J = 7.2 Hz, 2H), 2.68 (t, J =
7.0 Hz, 2H), 2.06 (m, 2H). 13C NMR (125 MHz, D2O) d
(ppm): 193.77, 174.11, 171.62, 165.14, 163.06, 134.87,
131.89, 119.72, 117.89, 115.09, 113.41, 66.60, 38.55, 30.45,
21.97. HRMS (M+) calcd for C13H16NO6: 282.0978; found:
282.0967.
Benzyl 5-acetyl-2-(benzyloxy)benzoate (9d)
White precipitate. Yield: 93%; mp 98–100 8C. IR
(KBr, cm–1): 3325, 3100–2800, 1726, 1697, 1676, 1598,
1500, 1452, 1370, 1269, 1230, 1176, 1060, 1030, 1008,
1-(4-(Benzyloxy)-3-nitrophenyl)ethanone (12d)
White precipitate. Yield; 96%. This compound is known.
1H NMR (400 MHz, CDCl3) d (ppm): 8.45 (dd, J = 9.2, J =
2.4 Hz, 1H), 8.14 (dd, J = 9.2, J = 1.8 Hz, 1H), 7.42 (m,
6H), 5.34 (s, 2H), 2.60 (s, 2H).
1
821, 757, 721, 698. H NMR (400 MHz, CDCl3) d (ppm):
8.46 (d, J = 2.1 Hz, 1H), 8.08 (dd, J = 9.0, J = 2.4 Hz,
1H), 7.44–7.33 (m, 10H), 7.09 (d, J = 8.8 Hz, 2H), 5.38 (s,
2H), 5.25 (s, 2H), 2.58 (s, 3H). 13C NMR (125 MHz,
CDCl3) d (ppm): 195.42, 165.04, 160.91, 135.11, 134.24,
133.04, 130.28, 129.16, 128.22, 127.81, 119.35, 117.36,
112.49, 70.08, 68.86, 25.74. HRMS (M + Na) calcd for
C23H20O4Na: 383.1259; found: 383.1262.
1-(4-(Benzyloxy)-3-nitrophenyl)-2-bromoethanone (12e)
Oil. Yield: 97%.7 1H NMR (400 MHz, CDCl3) d (ppm):
8.34 (s, 1H), 8.00 (d, 1H), 7.28 (m, 6H), 5.20 (s, 2H), 4.23
(s, 2H).
Benzyl 2-(benzyloxy)-5-(2-bromoacetyl)benzoate (9e)
Oil. Yield: 95%. IR (KBr, cm–1): 3325, 3100–2800, 1726,
1697, 1676, 1598, 1500, 1452, 1370, 1269, 1230, 1176,
2-(4-(Benzyloxy)-3-nitrophenyl)-2-oxoethyl-4-(tert-
butoxycarbonylamino)butanoate (12f)
1
1060, 1030, 1008, 821, 757, 721, 698. H NMR (400 MHz,
White precipitate. Yield: 67%; mp 144–146 8C. IR
(KBr, cm–1): 3310, 3100–2800, 1740, 1700, 1655, 1603,
1433, 1400, 1204, 1170, 955, 745, 697, 665. 1H NMR
(400 MHz, CD3CN) d (ppm): 8.37 (dd, J = 7.2, J = 2.6 Hz,
1H), 8.12 (d, J = 1.5 Hz, 1H), 7.43 (m, 6H), 6.85 (t, J =
7.2 Hz, 1H), 5.34 (s, 2H), 3.09 (t, J = 6.8 Hz, 2H), 2.46 (t,
J = 7.6 Hz, 2H), 1.77 (m, 2H), 1.40 (s, 9H). 13C NMR
(125 MHz, CD3CN) d (ppm): 195.22, 173.42, 160.65,
158.77, 139.23, 136.54, 135.40, 133.98, 131.78, 128.36,
125.89, 119.24, 116.82, 66.72, 38.44, 30.51, 29.11, 21.88.
HRMS (M + Na) calcd for C24H28N2O8Na: 495.1743; found:
495.1755
CDCl3) d (ppm): 8.50 (d, J = 2.2 Hz, 1H), 8.10 (dd, J = 9.0,
J = 2.4 Hz, 1H), 7.44–7.33 (m, 10H), 7.10 (d, J = 8.9 Hz,
2H), 5.38 (s, 2H), 5.25 (s, 2H), 4.41 (s, 2H). 13C NMR
(125 MHz, CDCl3) d (ppm): 189.39, 165.32, 162.18,
135.68, 135.48, 134.52, 133.49, 128.74, 128.26, 127.11,
126.47, 120.87, 113.44, 70.86, 67.21, 30.48. HRMS (M +
Na) calcd for C23H19BrO4Na: 461.0365; found: 461.0372.
Benzyl-2-(benzyloxy)-5-(2-(4-(tert-
butoxycarbonylamino)butanoyloxy)acetyl)benzoate (9f)
White precipitate. Yield: 75%; mp 75–77 8C. IR
(KBr, cm–1): 3271, 3100–2800, 1733, 1701, 1697, 1600,
1577, 1500, 1454, 1415, 1365, 1269, 1251, 1215, 1166,
1056, 1028, 1008, 919, 875, 821, 758, 700. 1H NMR
(400 MHz, d6-DMSO) d (ppm): 8.26 (d, J = 2.4 Hz, 1H),
8.12 (dd, J = 9.0, J = 2.3 Hz, 1H), 7.44–7.33 (m, 11H),
6.89 (t, J = 5.5 Hz, 2H), 5.45 (s, 2H), 5.33 (s, 2H), 2.97 (t,
J = 7.2 Hz, 2H), 2.42 (t, J = 7.0 Hz, 2H), 1.68 (m, 2H), 1.38
(s, 9H). 13C NMR (125 MHz, CDCl3) d (ppm): 190.85,
172.13, 165.28, 161.01, 155.90, 135.82, 133.51, 130.81,
128.42, 128.04, 127.89, 127.33, 126.27, 113.95, 77.78,
70.10, 64.96, 59.73, 39.30, 30.64, 28.22, 25.24. HRMS
(M + Na) calcd for C32H35NO8Na: 584.2260; found:
584.2239.
4-(2-(4-Hydroxy-3-nitrophenyl)-2-oxoethoxy)-4-oxobutan-
1-aminium-2,2,2-trifluoroacetate (12)
White precipitate. Yield: 78%; mp 76–78 8C. IR
(KBr, cm–1): 3441, 3265, 3100–2800, 1740, 1690, 1605,
1520, 1430, 1377, 1309, 1252, 1225, 1199, 1166, 1055,
1
820, 760. H NMR (400 MHz, D2O) d (ppm): 8.72 (d, J =
2.2 Hz, 1H), 8.20 (d, J = 9.2 Hz, 1H), 7.32 (d, J = 8.8 Hz,
1H), 5.56 (s, 2H), 3.14 (t, J = 7.8 Hz, 2H), 2.72 (t, J =
9.2 Hz, 2H), 2.08 (m, 2H). 13C NMR (125 MHz, D2O) d
(ppm): 193.16, 174.10, 163.13, 157.81, 135.75, 134.33,
126.61, 120.50, 117.44, 115.12, 66.69, 38.54, 30.23, 21.96.
HRMS (M+) calcd for C12H15N2O6: 283.0925; found:
283.0914.
The procedures for constructing 10 and 11 were the same
as those described by Conrad et. al.;2a,2c therefore, only
spectral data for these are indicated.
5-(2-(4-(tert-Butoxycarbonylamino)butanoyloxy)acetyl)-2-
hydroxybenzoic acid (9g)
Adhesive precipitate. Yield: 96%. H NMR (400 MHz,
1
d6-DMSO) d (ppm): 8.37 (d, J = 2.4 Hz, 1H), 8.08 (dd, J =
9.2, J = 2.3 Hz, 1H), 7.10 (d, J = 8.9 Hz, 2H) 6.90 (t, J =
5.5 Hz, 2H), 5.43 (s, 2H), 2.98 (t, J = 7.2 Hz, 2H), 2.44 (t,
J = 7.0 Hz, 2H), 1.68 (m, 2H), 1.36 (s, 9H).
5-Bromoacetyl-2-hydroxybenzoic acid methyl ester (10b)
White solid. Yield: 73%; mp 91–92 8C. IR (CHCl3, cm–1):
3030, 3010, 2957, 1675, 1589, 1491, 1444, 1355, 1309,
4-(2-(3-Carboxy-4-hydroxyphenyl)-2-oxoethoxy)-4-
oxobutan-1-aminium-2,2,2-trifluoroacetate (9)
1
1089, 965. H NMR (400 MHz, CDCl3) d (ppm): 4.01 (s, 3
Adhesive precipitate. Yield: 94%. IR (KBr, cm–1): 3440,
3274, 3100–2800, 1733, 1741, 1692, 1649, 1631, 1596,
1500, 1450, 1416, 1299, 1201, 1184, 1124, 1053, 1006,
823, 800, 760, 707, 680. 1H NMR (400 MHz, D2O) d
(ppm): 8.36 (s, 1H), 8.00 (d, J = 8.9 Hz, 1H), 7.08 (d, J =
H), 4.41 (s, 2 H), 7.05 (d, J = 8.8 Hz, 1H), 8.09 (dd, J =
8.8 Hz, 2.2, 1H), 8.50 (d, J = 2.2 Hz, 1H). 13C NMR
(74.5 MHz, CDCl3) d (ppm): 30.4, 52.8, 112.3, 118.4,
125.6, 132.2, 133.1, 136.0, 165.8, 169.9, 189.3. HRMS
(M + H) calcd for C10H10BrO4: 272.9762; found: 272.9759.
Published by NRC Research Press

380
Can. J. Chem. Vol. 89, 2011
5-[4-tert-Butoxycarbonylaminobutyryloxy)acetyl]-2-
hydroxybenzoic acid methyl ester (10c)
118.2, 125.2, 130.4, 134.1, 163.2, 171.1, 174.4, 193.7.
HRMS (M + H) calcd for C13H17N2O5: 281.1137; found:
281.1114.
The same protocol used for 13–23 was utilized for the
synthesis of 24 and 25 up to the formation of benzyl-
protected a-bromoacetophenones 13–23a.
White solid. Yield: 46%; mp 113–114 8C. IR
(CHCl3, cm–1): 3447, 3023, 2981, 2953, 1745, 1695, 1681,
1
1588, 1504, 1444, 1368, 1163, 1089, 962. H (400 MHz,
CDCl3) d (ppm): 1.44 (s, 9 H), 1.91 (q, J = 7.0 Hz, 2 H),
2.54 (t, J = 7.3 Hz, 2 H), 3.23 (q, J = 6.4 Hz, 2 H), 4.00 (s,
3 H), 5.32 (s, 2 H), 7.07 (d, J = 8.8 Hz, 1 H), 8.04 (dd, J =
8.8 Hz, 2.2, 1 H), 8.44 (d, J = 2.2 Hz, 1 H). 13C NMR
(74.5 MHz, CDCl3) d (ppm), 25.5, 28.6, 31.3, 53.0, 65.8,
112.5, 118.7, 126.0, 131.0, 135.1, 166.0, 170.0, 172.9,
190.2. HRMS (M + H) calcd for C19H26NO8: 396.1650;
found: 396.1648.
2-(4-Hydroxyphenyl)-2-oxoethyl acetate (24)
The sequence towards the synthesis of 24 conformed to
that of Wan and co-workers.30
2-(Acetyloxy)-1-[3-cyano-4-
(phenylmethoxy)phenyl]ethanone (26f)
A solution of NaOAc (175 mg, 2.13 mmol) and 25e
(470 mg, 1.42 mmol) in acetone was stirred overnight. The
resulting solution was evaporated to dryness, 15 mL of
water was added, and the mixture was extracted with ethyl
acetate (3ꢁ 15 mL). The ethyl acetate layer was dried with
anhyd MgSO4. The solvent was removed and the resulting
solid was further purified by column chromatography
(hexanes – ethyl acetate, 1:1) to give 2-(acetyloxy)-1-[3-
cyano-4-(phenylmethoxy)phenyl]ethanone (25f) as a white
solid (250 mg, 57%); mp 122–124 8C. IR (KBr, cm–1):
4-(2-(4-Hydroxy-3-(methoxycarbonyl)phenyl)-2-oxoethoxy)-
4-oxobutan-1-aminium-2,2,2-trifluoroacetate (10)
Clear oil. Yield: 100%. IR (CH2Cl2, cm–1) 3695, 3440,
3040, 2967, 1739, 1648, 1601, 1526, 1492, 1454, 1359,
1
1228, 1203, 773, 671. H NMR (400 MHz, D2O) d (ppm):
2.04 (q, J = 7.0 Hz, 2H), 2.70 (t, J = 7.1 Hz, 2H), 3.10 (t,
J = 7.3 Hz, 2H), 3.96 (s, 3H), 5.47 (s, 2H), 7.03 (d, J =
8.73 Hz, 1H), 8.02 (dd, J = 8.8 Hz, 2.2, 1H), 8.36 (d, J =
8.8 Hz, 2.2, 1H). 13C NMR (74.5 MHz, D2O) d (ppm):
22.1, 30.4, 38.7, 53.0, 66.5, 112.7, 117.8, 125.1, 131.3,
134.9, 164.3, 169.4, 174.0, 193.2. HRMS (M+) calcd for
C14H18NO6: 296.1134; found: 296.1132.
1
2225, 1735, 1701, 1595, 1321, 1222, 1078, 989, 761. H
NMR (400 MHz, CD3Cl) d (ppm): 2.20 (s, 3H), 5.17 (s,
2H), 5.27 (s, 2H), 7.21 (m, 1H), 7.37 (m, 6H), 7.87 (d, 1H).
13C NMR (100 MHz, CD3CN) d (ppm): 19.3, 65.8, 70.3,
112.2, 118.0, 118.2, 122.2, 127.4, 127.6, 128.1, 128.3,
128.4, 128.4, 131.5, 135.5, 161.7, 169.9, 190.0. HRMS
(M + Na) calcd for C18H15NO4Na: 332.0899; found:
332.0901.
5-Bromoacetyl-2-hydroxybenzamide (11b)
White solid. Yield: 29%; mp 185 8C (dec.). IR
(film, cm–1): 3417, 3364, 3305, 3280, 1671, 1650, 1625,
1
1587, 1487, 1427, 1363, 1286, 1228, 837, 592. H NMR
(400 MHz, CDCl3) d (ppm): 4.72 (s, 2 H), 7.03 (d, J =
8.2 Hz, 1H), 7.45 (bs, 1H), 8.12 (dd, J = 8.8 Hz, 2.20, 1H),
8.38 (bs, 1H), 8.56, (d, J = 2.0 Hz, 1H). 13C NMR
(74.5 MHz, CDCl3) d (ppm): 32.7, 114.6, 119.2, 126.2,
130.5, 135.8, 167.9, 173.4, 190.2. HRMS (M + H) calcd for
C9H9BrNO3 (M + H): 257.9766; found: 257.9783.
2-(3-Cyano-4-hydroxyphenyl)-2-oxoethyl acetate (26)
An adapted method from the synthesis of 4g was em-
ployed. White solid. Yield: 80%; mp 158–160 8C. IR
(KBr cm–1): 3203, 2227, 1741, 1672, 1604, 1560, 1321,
1220, 1122, 1074, 896, 837, 740. 1H NMR (400 MHz
(CD3)2CO) d (ppm): 2.13 (s, 3H), 5.37 (s, 2H), 7.23 (m,
1H), 7.25 (s, 1H), 8.10 (d, 1H). 13C NMR (100 MHz,
(CD3)2CO): 19.5, 65.9, 113.0, 117.2, 118.1, 122.5, 128.0,
132.1, 161.5, 169.6, 189.6. HRMS (M + Na) calcd for
C11H9NO4Na: 242.0429; found: 242.0417.
4-tert-Butoxycarbonylaminobutyric acid 2-(3-carbamoyl-4-
hydroxyphenyl)-2-oxoethyl ester (11c)
White solid. Yield: 59%; mp 138–139 8C. IR
(CHCl3, cm–1): 3535, 3462, 3414, 3028, 2975, 2931, 1750,
1699, 1663, 1611, 1508, 1417, 1368, 1166. 1H NMR
(400 MHz, acetone-d6) d (ppm): 1.4 (s, 9H), 1.85 (q, J =
7.2 Hz, 2H), 2.50 (t, J = 7.4 Hz, 2H), 3.17 (q, J = 6.6 Hz,
2H), 5.42 (s, 2H), 6.05 (bs, 1H), 7.02 (d, J = 8.7 Hz, 1H),
7.55 (bs, 1H), 8.07 (dd, J = 8.7, 1.7 Hz, 1H), 8.37 (bs, 1H),
8.51 (d, J = 1.8 Hz, 1H). 13C NMR (75.4 MHz, acetone-d6)
d (ppm): 26.3, 28.7, 31.7, 40.4, 66.7, 78.6, 114.5, 119.2,
126.2, 129.4, 134.6, 156.8, 167.7, 173.2, 173.4, 191.2.
HRMS (M + H) calcd for C18H25N2O7: 381.1662; found:
381.1645.
2-(Acetyloxy)-1-[2-cyano-4-
(phenylmethoxy)phenyl]ethanone (25f)
The same procedure used for 26f was followed. White
solid. Yield: 90%; mp 92–94 8C. IR (KBr, cm–1): 2231,
1
1741, 1691, 1604, 1377, 1284, 1224, 1085, 823, 738. H
NMR (400 MHz, CDCl3) d (ppm): 2.22 (s, 3H), 5.24 (s,
2H), 5.30 (s, 2H), 7.08 (d, 2H), 8.05 (m, 5H), 8.16 (d, 2H).
13C NMR (100 MHz, CDCl3) d (ppm): 20.6, 65.6, 71.2,
113.2, 115.1, 127.0, 127.4, 128.7, 128.9, 134.1, 134.2,
134.6, 163.8, 170.4, 189.3. HRMS (M + Na) calcd for
C17H16NO4Na: 332.0899; found: 332.0916.
4-(2-(3-Carbamoyl-4-hydroxyphenyl)-2-oxoethoxy)-4-
oxobutan-1-aminium-2,2,2-trifluoroacetate (11)
Clear oil. Yield: 100%. IR (CHCl3, cm–1): 3415, 3219,
1728, 1692, 1665, 1629, 1497, 1432, 1377, 1238, 1200,
1144. 1H NMR (400 MHz, D2O) d (ppm): 1.92 (q, J =
7.16 Hz, 2H), 2.61 (t, J = 7.4 Hz, 2H), 3.00 (q, J = 6.6 Hz,
2H), 5.40 (s, 2H), 6.94 (d, J = 8.7 Hz, 1H), 7.90 (dd, J =
8.72, 1.72 Hz, 1 H), 8.32 (d, J = 1.88 Hz, 1 H). 13C NMR
(75.4 MHz, D2O) d (ppm): 22.4, 30.6, 38.9, 66.9, 115.6,
2-(2-Cyano-4-hydroxyphenyl)-2-oxoethyl acetate (25)
White solid. Yield: 90%; mp 185–186 8C. IR
(KBr, cm–1): 3150, 2237, 1739, 1666, 1585, 1508, 1421,
1379, 1240, 1124, 1083, 941, 835. 1H NMR (400 MHz
(CD3)2CO) d (ppm): 2.14 (s, 3H), 5.42 (s, 2H), 7.19 (d, 1H)
8.12 (dd, 1H), 8.29 (d, 1H). 13C NMR (100 MHz,
(CD3)2CO): 19.5, 65.8, 100.0, 115.3, 116.8, 129.9, 133.8,
Published by NRC Research Press

Givens et al.
381
134.0, 163.1, 169.6, 189.7. HRMS (M + H) calcd for
C11H8NO4: 218.0453: found: 218.
chloroacetyl)-3-methoxyphenyl pivalate (30c) as green-white
crystalline solid (2.32 g, 50%), mp 58–61 8C. IR (KBr,
cm–1): 2980–2854, 1757, 1686, 1606, 1420, 1271, 1186,
1
pHP Diethyl phosphates
1117, 897, 791. H NMR (500 MHz, CD3COCD3) d (ppm):
7.85–7.84 (1H, d, J = 8.6 Hz), 7.01–7.00 (1H, d, J =
2.0 Hz), 6.86–6.83 (1H, d, J = 8.6 Hz), 4.90 (2H, s), 4.01
(3H, s), 1.34 (9H, s). 13C NMR (125 MHz, CD3COCD3) d
(ppm): 190.4, 176.7, 161.3, 157.6, 132.6, 123.4, 115.4,
107.2, 56.8, 51.8, 39.8, 27.3. HRMS (M + H) calcd for
C14H18O4Cl: 285.0894; found: 285.0912.
Diethyl (2-(4-hydroxyphenyl)-2-oxoethyl) phosphate (27)
The synthesis of 27 was achieved using the protocol of
Givens and Park.1
Diethyl (2-(4-hydroxy-3-methoxyphenyl)-2-oxoethyl)
phosphate (28)
The synthesis of 28 was achieved using the same protocol
4-(2-Diethoxyphosphoryloxyacetyl)-3-methoxyphenyl
pivalate (30e)
used for 27.1
The general methods of Finkelstein34 and Yoh et al.35
were utilized with modifications. A 50 mL round-bottom
flask was charged with 4-(2-chloroacetyl)-3-methoxyphenyl
pivalate (30c, 500 mg, 1.8 mmol) and NaBr (1.08 g,
10.5 mmol). Acetone (20 mL) was added and the resulting
mixture was refluxed at 60 8C for 24 h. The reaction mix-
ture was then concentrated and the resulting residue was
suspended in ethyl acetate (20 mL) and washed sequentially
with brine (2 ꢁ 20 mL) and water (20 mL). The ethyl ace-
tate layer was separated, dried over anhyd MgSO4, filtered,
and the supernatant concentrated under reduced pressure to
afford 4-(2-bromoacetyl)-3-methoxyphenyl pivalate (30d) as
a white solid (95% conversion to the a-bromo ketone analog
Diethyl (2-(4-hydroxy-3,5-dimethoxyphenyl)-2-oxoethyl)
phosphate (29)
The procedure for synthesizing 29 has been previously
published.25
3-Methoxyphenyl pivalate (30b)
The general method of Gorobets et al.31 was followed
with modifications. To a cooled (0 8C), stirred solution of
3-methoxyphenol (30a, 2.0 g, 16.1 mmol) in dry CH2Cl2
(10 mL), freshly distilled triethyl amine (3.350 mL,
24.2 mmol) and catalytic amount of 4-dimethylaminopyri-
dine (20 mg, 0.16 mmol) were added under Ar. After
10 min, pivaloyl chloride (2.98 mL, 24.2 mmol) was added
dropwise to the reaction mixture. The reaction mixture was
allowed to stir for 30 min at 0 8C, brought to rt, and then
stirred overnight. At that point, water (10 mL) and ethyl
acetate (30 mL) were added to the reaction mixture and the
organic phase was washed sequentially with 10% HCl (3 ꢁ
25 mL), water (2 ꢁ 25 mL), saturated NaHCO3 solution
(3 ꢁ 25 mL) and brine (2 ꢁ 25 mL. The organic phase was
separated, dried over anhyd MgSO4, filtered, and the super-
natant concentrated under reduced pressure to produce 3-
methoxyphenyl pivalate (30b) as thick yellow oil (3.335 g,
100%). IR (Teflon film, cm–1) 2972–2837, 1753, 1606,
1
according to H NMR). This compound was used directly in
the next step without purification. To a stirred solution of 4-
(2-bromoacetyl)-3-methoxyphenyl pivalate (30d, 548 mg,
1.66 mmol) in CH3CN (10 mL), diethyl phosphoric acid
(641 mg, 4.16 mmol) and Ag2O (772 mg, 3.33 mmol) were
added. The resulting mixture was stirred at 60 8C and the
progress of the reaction was monitored by TLC. After 16 h,
the black-colored suspension was filtered through a plug of
Celite, the filtrate concentrated, and EtOAc (20 mL) was
added. The organic layer was washed sequentially with satu-
rated NaHCO3 (40 mL) and water (20 mL) and dried over
anhyd MgSO4. The solvent was evaporated to afford the
crude product as a yellow-brown liquid that was chromato-
graphed on silica gel using a gradient solvent system
(EtOAc–hexanes (1:2), EtOAc–hexanes (1:1), EtOAc–
hexanes (2:1), EtOAc–hexanes (3:1), EtOAc–MeOH (19:1))
to produce the 4-(2-(diethoxyphosphoryloxy)acetyl)-2-
methoxyphenyl pivalate (30e) as a yellow-brown solid
(468 mg, 76%), mp 80–84 8C. IR (KBr, cm–1): 2982–2852
(br), 1747, 1686, 1605, 1261, 1124, 1028, 854, 798. 1H
NMR (500 MHz, CD3COCD3) d (ppm): 7.93–7.91 (1H, d, J =
8.6 Hz), 7.01 (1H, d, J = 2.0 Hz), 6.86–6.84 (1H, d, J =
8.5 Hz), 5.18–5.16 (2H, d, J = 10.8 Hz), 4.18–4.12 (4H, m),
4.00 (3H, s), 1.34 (9H, s), 1.32–1.29 (6H, m). 13C NMR
(125 MHz, CDCl3) d (ppm): 193.0, 192.9, 176.7, 161.7,
157.7, 132.4, 122.8, 115.4, 107.1, 73.0, 64.4, 64.3, 56.8,
39.8, 27.3, 16.5. 31P NMR (162 MHz, CD3COCD3) d
(ppm): –0.24. HRMS (M + H) calcd for C19H26O8PNa:
425.1341; found: 425.1323.
1
1593, 1490, 1140, 1115, 1043, 766. H NMR (500 MHz,
CD3COCD3) d (ppm): 7.31–7.28 (1H, t, J = 8.1 Hz), 6.82–
6.82 (1H, d, J = 2.3 Hz), 6.81–6.80 (1H, d, J = 2.4 Hz),
6.69–6.66 (1H, m), 3.79 (3H, s), 1.33 (9H, s). 13C NMR
(125 MHz, CD3COCD3) d (ppm): 177.8, 161.6, 153.4,
130.6, 114.7, 112.1, 108.7, 55.8, 39.6, 27.4. HRMS (M +
1
H) calcd for C12H17O3: 209.1178; found: 209.1182. The H
NMR data were similar to the literature values.32
4-(2-Chloroacetyl)-3-methoxyphenyl pivalate (30c)
The general method of Gonzalez-Gomez et al.33 was fol-
lowed with modifications. To a cooled (0 8C) stirred solu-
tion of AlCl3 (4.73 g, 35.4 mmol) in chloroacetyl chloride
(10 mL) under Ar was added 3-methoxyphenyl pivalate
(30b, 3.355 g, 16.1 mmol). The reaction mixture was al-
lowed to stir for 3 h at 0 8C under Ar. Then, water (20 mL)
was added cautiously to quench the reaction and the result-
ing solution was extracted to EtOAc (4 ꢁ 25 mL). The or-
ganic phase was washed sequentially with saturated
NaHCO3 solution (3 ꢁ 25 mL) and brine (2 ꢁ 25 mL). The
organic phase was separated, dried over anhyd MgSO4, fil-
tered, and the supernatant concentrated under reduced pres-
sure to afford the crude product that was chromatographed
on silica gel (EtOAc–hexanes–CH2Cl2, 1:2:2) to give 4-(2-
Diethyl (2-(4-hydroxy-2-methoxyphenyl)-2-oxoethyl)
phosphate (30)
NH4OAc (1.435 g, 18.61 mmol) was added to a stirred
solution of 4-(2-(diethoxyphosphoryloxy)acetyl)-2-methoxy-
phenyl pivalate (30e, 468 mg, 1.16 mmol) in aq MeOH
Published by NRC Research Press

382
Can. J. Chem. Vol. 89, 2011
(H2O–MeOH, 1:4, 20 mL). The resulting mixture was
warmed at 50 8C and the progress of the reaction was moni-
tored by TLC. After 36 h, the reaction mixture was concen-
trated and the residue was extracted with EtOAc (5 ꢁ
10 mL). Combined organic layers were dried over anhyd
MgSO4 and the solvent was evaporated to afford diethyl 2-
(4-hydroxy-3-methoxyphenyl)-2-oxoethyl phosphate (30) as
a dark yellow solid that was further purified on silica gel us-
ing a gradient solvent system (EtOAc–hexanes (3:1),
EtOAc–MeOH (19:1)) to isolate the pure compound as a
white crystalline solid (340 mg, 92%), mp 95–98 8C. IR
(KBr, cm–1): 3134, 1672, 1602, 1473, 1383, 1337, 1231,
crude diethyl 2-(4-hydroxy-2,6-dimethoxyphenyl)-2-oxoethyl
phosphate as a dark yellow thick oil. Several attempts were
made to purify the product on silica gel, but they were un-
successful. Therefore, the crude product was used in the de-
protection step without further purification. NH4OAc
(456 mg, 5.92 mmol) was added to a stirred solution of the
crude phosphate from the previous step (160 mg) in aq
MeOH (H2O–MeOH, 1:4, 10 mL). The resulting mixture
was warmed at 50 8C and the progress of the reaction moni-
tored by TLC. After 36 h, the reaction mixture was concen-
trated and the residue was extracted with EtOAc (5 ꢁ
10 mL). The combined organic layers were dried over anhyd
MgSO4, and the solvent was evaporated to afford diethyl 2-
(4-hydroxy-2,6-dimethoxyphenyl)-2-oxoethyl phosphate (31)
as a yellow-brown thick oil that was further purified by
preparative TLC (hexanes–isopropanol–chloroform, 1:1:11)
to produce the pure product as a thick yellow oil (10 mg, over-
all 2%). UV–vis [H2O–CH3CN (1:1)] lmax (3 (mol/L)–1 cm–1):
283 (5013). IR (Teflon film, cm–1): 2959–2852, 1738, 1593,
1462, 1342, 1261, 1211, 1153, 1028, 800. 1H NMR
(400 MHz, CD3COCD3) d (ppm): 9.04 (1H, s), 6.18 (1H, s),
4.81–4.79 (2H, d, J = 9.3 Hz), 4.13–4.05 (2H, m), 3.75 (3H,
s), 1.30–1.27 (6H, m). 13C NMR (125 MHz, CD3COCD3) d
ppm 196, 162.4, 160.2, 108.8, 92.9, 72.2, 64.4, 56.2, 16.5. 31P
NMR (162 MHz, CD3COCD3) d (ppm): –0.75. MS (ESI (–))
m/z calcd for (C14H21O8P–H)–: 347.0896; found: 347.0887.
1
1032, 984, 849. H NMR (400 MHz, CD3COCD3) d (ppm):
9.40 (1H, s) 7.83–7.81 (2H, d, J = 8.8 Hz), 6.56 (2H, m),
5.12–5.09 (2H, d, J = 11.2 Hz), 4.20–4.13 (4H, m), 3.94
(3H, s), 1.33–1.29 (6H, m). 13C NMR (125 MHz,
CD3COCD3) d (ppm): 191.5, 164.97, 162.9, 133.4, 117.3,
109.4, 99.7, 73.1, 73.0, 64.4, 56.1, 16.5. 31P NMR
(162 MHz, CD3COCD3) d (ppm): –0.37. HRMS (M – H)
calcd for C13H18O7P: 317.0790; found: 317.0795.
The same protocol was used for the synthesis of 31b and
31d except for the final step for 31 and only spectral data
are conveyed for these derivatives.
3,5-Dimethoxyphenyl pivalate (31b)
Oil. Yield: 100%. IR (Teflon film, cm–1): 2968–2839,
1751, 1618, 1475, 1130, 1063, 895, 835, 681. 1H NMR
(500 MHz, CD3COCD3) d (ppm): 6.38–6.37 (1H, t, J =
2.3 Hz), 6.29 (1H, s), 6.28 (1H, s), 3.77 (6H, s), 1.32 (9H,
s). 13C NMR (125 MHz, CD3COCD3) d (ppm): 176.9,
162.2, 154.0, 101.8, 98.4, 55.9, 39.6, 27.4. HRMS (M +
Na) calcd for C13H18O4Na: 261.1103; found: 261.1100.
3-Acetyl-4-hydroxybenzoic acid (32b)
The general method of Nagano and Matsumura38 was fol-
lowed with modifications. Aluminum chloride (2.96 g,
22.2 mmol) was added into a flame-dried three-neck round-
bottom flask containing a solution of 4-hydroxybenzoic acid
(32a; 1.0 g, 5.55 mmol) in nitrobenzene (10 mL) under Ar.
The reaction mixture was kept stirring vigorously at 150 8C
for 14 h in the three-neck flask fitted with a CaCl2 tube. On
cooling, some crushed ice and 10% HCl (50 mL) were
added to the flask, and the separated nitrobenzene was re-
moved by distillation. The aqueous phase was extracted
with EtOAc (3 ꢁ 25 mL) and washed with saturated
NaHCO3 solution (3 ꢁ 25 mL). The alkaline aqueous phase
was acidified with 10% HCl and extracted again with
EtOAc (3 ꢁ 25 mL). The combined later organic layers
were dried over anhyd MgSO4, filtered, and the supernatant
concentrated under reduced pressure to afford 3-acetyl-4-
hydroxybenzoic acid (32b) as a pink colored solid (980 mg,
98%), mp 227–230 8C. IR (KBr, cm–1): 3433, 1684, 1643,
1610, 1578, 1420, 1325, 1296, 1215, 1122, 943, 825, 771.
1H NMR (400 MHz, CD3COCD3) d (ppm): 12.71 (1H, s),
8.56 (1H, s), 8.17–8.15 (1H, d, J = 8.0 Hz), 7.05–7.03 (1H,
d, J = 8.0 Hz), 2.77 (3H, s). 13C NMR (125 MHz,
CD3COCD3) d (ppm): 166.7, 166.6, 138.1, 134.6, 122.4,
120.2, 119.1, 119.0, 27.0. HRMS (M – H) calcd for
C9H7O4: 179.0344; found: 179.0323
4-(2-Bromoacetyl)-3,5-dimethoxyphenyl pivalate (31d)
White solid. Yield: 20%; mp 125–128 8C. IR (KBr,
cm–1): 3018–2841, 1749, 1732, 1597, 1464, 1412, 1221,
1
1128, 997, 892. H NMR (500 MHz, CD3COCD3) d (ppm):
6.53 (2H, s), 4.59 (2H, s), 3.81 (6H, s), 1.33 (9H, s). 13C
NMR (125 MHz, CD3COCD3) d (ppm): 193.9, 176.8,
158.9, 155.6, 114.9, 99.7, 56.8, 51.0, 39.7, 27.4. HRMS
(M + Na) calcd for C15H19O5BrNa: 381.0314; found:
381.0312.
Diethyl (2-(4-hydroxy-2,6-dimethoxyphenyl)-2-oxoethyl)
phosphate (31)
The general methods of Knopik et al.36 and Das et al.37
were followed with some modifications. The potassium salt
of diethyl phosphoric acid (535 mg, 2.78 mmol) and di-
benzo-[18]-crown-6 (50 mg, 0.14 mmol) were added to a
stirred solution of 4-(2-bromoacetyl)-3,5-dimethoxyphenyl
pivalate (31d, 500 mg, 1.40 mmol) in CH3CN (10 mL). The
resulting mixture was stirred at 80 8C and the progress of
the reaction was monitored by TLC. After 16 h, the reaction
mixture was concentrated and EtOAc (20 mL) was added.
The organic layer was washed sequentially with saturated
NaHCO3 (40 mL) and water (20 mL) and dried over anhyd
MgSO4. The solvent was evaporated to afford the crude
product as a yellow-white solid that was chromatographed
on silica gel using a gradient solvent system (EtOAc–
hexanes (1:2), EtOAc–hexanes (1:1), EtOAc–hexanes (2:1),
EtOAc–hexanes (3:1), EtOAc–MeOH (19:1)) to isolate the
4-Acetoxy-3-acetylbenzoic acid (32c)
Triethyl amine (1.465 mL, 10.55 mmol) followed by ace-
tyl chloride dropwise (564 mL, 7.91 mmol) was added to a
stirred solution of 32b (950 mg, 5.27 mmol) in dry CH2Cl2
(15 mL) under Ar. The reaction mixture was allowed to stir
for 45 min at rt. At that point, ethyl acetate (40 mL) was
added to the reaction mixture, and the organic phase was
Published by NRC Research Press

Givens et al.
383
2-(3-Acetyl-4-hydroxyphenyl)-2-oxoethyl diethyl phosphate
(32)
washed with 10% HCl (3 ꢁ 25 mL) followed by saturated
NaHCO3 solution (3 ꢁ 25 mL). The aqueous fractions from
the NaHCO3 extraction were combined, acidified with 10%
HCl, and extracted into EtOAc (3 ꢁ 25 mL). The EtOAc
fractions were combined, dried over anhyd MgSO4, filtered,
and the supernatant concentrated under reduced pressure to
afford the 4-acetoxy-3-acetylbenzoic acid (32c) as a pink
solid (820 mg, 70%), mp 125–145 8C (dec.). IR (KBr,
cm–1) 3078–2552 (br), 1767, 1689, 1647, 1429, 1296, 1192,
To a stirred solution of 32d (240 mg, 1.18 mmol) in ben-
zene (10 mL), diethyl phosphoric acid (362 mg, 2.35 mmol)
in benzene (5 mL) was added dropwise. The resulting mix-
ture was stirred at 60 8C and the progress of the reaction
was monitored by the TLC. After 24 h, EtOAc (20 mL)
was added to the reaction mixture. The organic layer was
washed sequentially with saturated NaHCO3 (40 mL) and
water (20 mL) and then dried over anhyd MgSO4. The sol-
vent was evaporated to afford the crude product as a brown-
ish yellow oil that was chromatographed on silica gel (with
a gradient solvent system: EtOAc–hexanes (1:2), EtOAc–
hexanes (1:1), EtOAc–hexanes (2:1), EtOAc–hexanes (3:1))
to produce 2-(3-acetyl-4-hydroxyphenyl)-2-oxoethyl diethyl
phosphate (32) as a thick brown oil (306 mg, 79% yield).
IR (Teflon film, cm–1): 3443, 2985–2872, 1703, 1643, 1597,
1
918, 735; H NMR (400 MHz, CD3COCD3) d (ppm): 10.44
(1H, s), 8.48 (1H, d, J = 1.8 Hz), 8.23–8.21 (1H, d, J =
8.4 Hz), 7.35–7.33 (1H, d, J = 8.4 Hz), 2.60 (3H, s), 2.33
(3H, s). 13C NMR (125 MHz, CD3COCD3) d (ppm): 197.3,
169.4, 166.4, 153.5, 138.1, 135.1, 132.3, 129.3, 125.4,
119.1, 27.0, 21.2. HRMS (M – H) calcd for C11H9O5:
221.0450; found: 221.0440.
1
1369, 1265, 1209, 1153, 1030, 820. H NMR (400 MHz,
1-(3-Acetyl-4-hydroxyphenyl)-2-diazoethanone (32d)
CD3COCD3) d (ppm): 12.81 (1H, s), 8.59–8.58 (1H, d, J =
2.2 Hz), 8.17–8.14 (1H, d, J = 8.8 Hz), 7.08–7.06 (1H, d,
J = 8.8 Hz), 5.42–5.39 (2H, d, J = 10.6 Hz), 4.18–4.13 (4H,
m), 2.79 (3H, s), 1.33–1.30 (6H, m). 13C NMR (125 MHz,
CD3COCD3) d (ppm): 191.6, 167.2, 136.5, 133.3, 126.8,
120.2, 119.4, 119.3, 69.5, 64.6, 27.2, 16.5, 16.4. 31P NMR
(162 MHz, CD3COCD3) d (ppm): –0.14. HRMS (M – H)
calcd for C14H18O7P: 329.0790; found: 329.0772.
A flame-dried 50 mL round-bottom flask was charged
with 32c (830 mg, 3.74 mmol) and freshly distilled SOCl2
(20 mL) was added under Ar. The resulting mixture was re-
fluxed at 80 8C for 8 h and concentrated under reduced pres-
sure to afford 4-acetoxy-3-acetylbenzoyl chloride as
colorless oil. This acid chloride was used in next step with-
out further purification. To a solution of KOH (3.0 g,
53.5 mmol) in 2-methoxyethanol (17 mL) and water
(5 mL), diazold (5.0 g, 23.3 mmol) in diethyl ether (75 mL)
was added carefully. The resulting mixture was gently
heated to reflux temperatures (40–45 8C). The ether layer
was distilled into the collection flask in an ice bath at 0 8C.
The ether distillate was sequentially dried over KOH pellets
for 1 h at 0 8C and with Na pieces for 1 h at 0 8C. Then a
solution of acid chloride (899 mg, 3.74 mmol) in dry ether
(10 mL) was added dropwise to the distillate (diazomethane)
with vigorous stirring at –5 8C. The reaction mixture was al-
lowed to come to 0 8C and stirred overnight. The solvent
was evaporated, and the yellowish orange thick oil was
used in the deptrotection step without purification. To a
stirred solution of crude 2-acetyl-4-(2-diazoacetyl)phenyl
acetate (340 mg, 1.38 mmol) in aq MeOH (H2O–MeOH,
1:4, 20 mL), NH4OAc (852 mg, 11.05 mmol) was added.
The resulting mixture was warmed at 50 8C and the progress
of the reaction was monitored by TLC. After 8 h, the reac-
tion mixture was concentrated and the residue was extracted
with acetone (5 ꢁ 10 mL). The combined organic layers
were dried over anhyd MgSO4 and the solvent was evapo-
rated to afford 1-(3-acetyl-4-hydroxyphenyl)-2-diazo-
ethanone (32d) as a yellowish brown oil that was further
purified on silica gel (hexanes–EtOAc, 1:1) to generate the
pure compound as a yellow oil (254 mg, 21% overall yield).
IR (Teflon film, cm–1): 3364, 2112, 1761, 1690, 1614, 1418,
Acknowledgements
This work was supported by National Institutes of Health
(NIH) grants GM069663 (R.S.G.) and R01 GM72910
(R.S.G.), grants MSM0021622413 and CZ.1.05/2.1.00/
01.0001 (European Union, CETOCOEN) administered by the
Ministry of Education, Youth and Sports of the Czech Repub-
lic (D.H.), and the Swiss National Science Foundation (J.W.).
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1
1362, 1196, 1150, 1011, 914, 847, 733. H NMR (400 MHz,
CD3COCD3) d (ppm): 12.68 (1H, s), 8.40 (1H, d, J =
2.2 Hz), 8.04–8.01 (1H, d, J = 8.8 Hz), 7.03–7.01 (1H, d,
J = 8.8 Hz), 6.69 (1H, s), 2.76 (3H, s). 13C NMR
(125 MHz, CD3COCD3) d (ppm): 197.5, 185.0, 169.5,
153.2, 135.3, 132.2, 129.6, 125.4, 54.9, 21.2. HRMS (M –
H) calcd for C10H7N2O3: 203.0457; found: 203.0452.
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384
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