Optimised synthesis and photochemistry of antenna-sensitised 1-acyl-7-nitroindolines

Article abstract of DOI:10.1016/j.tet.2004.10.094

Benzophenone antenna-sensitised 1-acyl-7-nitroindolines show a significantly enhanced extent of photochemical cleavage in aqueous solution over their non-sensitised analogues and release the carboxylate derived from their 1-acyl group. The present work in

Full text of DOI:10.1016/j.tet.2004.10.094

Tetrahedron 61 (2005) 609–616
Optimised synthesis and photochemistry of antenna-sensitised
1-acyl-7-nitroindolines*
*
George Papageorgiou and John E. T. Corrie
National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK
Received 16 August 2004; revised 6 October 2004; accepted 29 October 2004
Abstract—Benzophenone antenna-sensitised 1-acyl-7-nitroindolines show a significantly enhanced extent of photochemical cleavage in
aqueous solution over their non-sensitised analogues and release the carboxylate derived from their 1-acyl group. The present work
investigates length and functional group effects in the linker between the benzophenone sensitiser and the nitroindoline and concurrently
establishes a more efficient synthetic route to an effective conjugate than previously described. An incidental finding is that a TBDMS ether is
stable during claycop-mediated nitration.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
than from simple, non-sensitised nitroindolines such as 1.
The L-glutamate conjugate 2a has recently been reported
and shown to fulfil the latter requirement.5
We have previously reported1 the efficient photochemistry
of 1-acyl-7-nitroindolines such as 1, that release a
carboxylate derived from the 1-acyl group upon irradiation
by near-UV light in neutral aqueous solution, as shown in
Scheme 1. Our interest in this photochemistry arose from a
requirement for reagents that could rapidly release neu-
roactive amino acids by flash photolysis within biological
preparations, particularly mammalian brain slices: the L-
glutamate conjugate 1 exemplifies a means to achieve one
aspect of this goal and has been used in a number of
published studies.2
Based on mechanistic studies of this photochemistry,3 we
have described triplet-sensitised antenna conjugates of
nitroindolines with substantially enhanced photosensi-
tivity,4 which is desirable as it should enable the
photorelease of higher concentrations of the amino acid
With the general approach and efficacy of the antenna-
sensitised methodology established, we wished to optimise
Scheme 1. Overall photocleavage reaction of 1.
* Parts of this work are the subject of UK Patent Application GB03/07231.
Keywords: Photolysis; Nitroindolines; Benzophenones; Triplet sensitisation; Nitration.
* Corresponding author. Tel.: C44 20 8816 2276; fax: C44 20 8906 4419; e-mail: jcorrie@nimr.mrc.ac.uk
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.10.094

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G. Papageorgiou, J. E. T. Corrie / Tetrahedron 61 (2005) 609–616
the synthetic chemistry required to assemble conjugates
such as 2 and to examine the effects on photolysis efficiency
of changes in the link between the benzophenone antenna
and the nitroindoline. In particular, two broad aspects
relevant to synthesis of the conjugates needed to be
addressed. The first was the formation of some 5-nitro
isomer during introduction of the nitro group, which
required separation either before or after linkage to the
benzophenone. The second was a convolution of the
relatively cumbersome previous synthesis4 of the sensitiser
moiety of 2 and the more fundamental matter of whether the
length of the linker had a significant effect on the
photochemistry. Here we report our studies to explore
these two issues.
4-hydroxy-5-methylindole 5 as the starting material. We
had previously prepared this compound by Leimgruber–
Batcho synthesis but the overall yield was poor.1b In the
present work we used an alternative, published method6 in
which 4-hydroxyindole was converted to its 5-(dimethyl-
aminomethyl) derivative with Me2NH–CH2O, then hydro-
genolysed over Pd–alumina. Details of the procedure in our
hands are given in the Supplementary Data. With 5
available, further elaboration to the carboxylic acid 10, as
shown in Scheme 2, followed essentially the route
previously used for the 5-nor analogue in our previous
work,4 except for the minor variant of a tert-butyl rather
than a methyl ester to protect the carboxylic acid side chain.
This allowed easy deprotection without the danger of partial
hydrolysis of the N-acetyl group during saponification of the
1
ester. In both compounds 9 and 10, the H NMR spectrum
2. Results and discussion
showed a resolved benzylic coupling (0.5 Hz) between the
5-methyl group and H-6. This was not resolved in other
similar compounds within this work, although its presence
could be inferred from the broadened line width of the
5-methyl and H-6 signals.
The obvious means to address the first of these issues was to
block the 5-position with a suitable substituent. A previous
attempt to achieve this in the unsensitised compounds such
as 1 used a 5-methyl group, as in 3, but this caused a
significant reduction in photoefficiency that we ascribed to
steric inhibition of the resonance interaction between the
methoxy group and the aromatic ring.1b However we
speculated for the sensitised compounds, where light
absorption is principally via the sensitiser, that energy
transfer might not be adversely affected by the presence of a
5-substituent and therefore set out initially to prepare the
conjugate 4.
Final assembly of the conjugate 4 was by carbodiimide
coupling of 10 with the previously described4,5 amino-
functionalised benzophenone 11, followed by TFA treat-
ment to remove the tert-butyl protecting groups. As
described for previous conjugates of this type,4 the
calculated UV–Vis spectrum of 19, obtained by adding
the molar absorption spectra of the individual chromo-
phores, gave an absorption maximum at 300 nm
(3 27,100 MK1 cmK1; see Supplementary Data). This
value was used to quantify solutions of 4. Comparative
irradiations of separate solutions of 4 and its nor-analogue
2b, with the disappearance of the starting compounds being
monitored by reverse-phase HPLC, showed that photolysis
of 4 was w16% more efficient. Furthermore, progressive
irradiation led to very clean changes in the UV–Vis spectra,
as previously reported for 2b,4 indicating that the photolysis
reaction was not altered by the additional substituent.
Although the gain in photolysis efficiency was quite modest,
it contrasts markedly with the w2-fold reduction in
photoefficiency previously observed for the non-sensitised
nitroindolines when a 5-methyl substituent was added.1b
Furthermore, the presence of the 5-methyl group means that
there is no problem of unwanted isomer formation when the
nitro group is introduced. In our previous work, it had
always been possible to separate the 5 and 7-nitro isomers of
several N-acetyl compounds, but with a more complex side
chain, such as in the precursor of the glutamate conjugate
2a, it had been necessary to work with a mixture of isomers
and to rely upon separation by HPLC of the final, water-
soluble conjugate.4,5 Obviously, avoidance of the isomer
Synthesis of 4, as outlined in Scheme 2, followed similar
lines to that previously used for 2a,b4,5 but required
Scheme 2. Synthesis of precursor acid (10). Reagents and conditions: (a) NaBH3CN, AcOH, Ac2O, AcOH, D, 73%. (b) aq. NaOH, MeOH, 80%.
(c) BrCH2CO2But, K2CO3, acetone, reflux, 90%. (d) claycop, Ac2O, CCl4, 66%. (e) TFA, 75%. Yields are given for recrystallised compounds.

G. Papageorgiou, J. E. T. Corrie / Tetrahedron 61 (2005) 609–616
611
problem is synthetically preferable if, as here, it can be
achieved without compromising the photolysis efficiency.
therefore have an insufficient energy gap to allow triplet
transfer to the nitroindoline. Nevertheless, we recognise that
the situation must be more complex than this effect alone,
since the nitroindoline itself would have been expected to
show considerable direct photolysis during such prolonged
irradiation.1 However, in practical terms there seemed little
benefit in probing further into this negative result.
Experimental details relating to the synthesis of 12 are
reported in the Supplementary Data.
Encouraged by this initial success, we turned our attention
to the combined matters of the length of the linker between
the benzophenone and the nitroindoline, and establishment
of an easier synthesis of a suitably functionalised benzo-
phenone. Although it is possible that conformational
relationships between the sensitiser and the nitroindoline
could be influential, the more accessible strategy was to
investigate shortening the flexible linker. Our first approach
aimed at the conjugate 12. Note that in each of the previous
or present conjugates, the phosphate group on the side chain
of the benzophenone sensitiser was present only to promote
water solubility and is irrelevant to the photochemistry.
Synthesis of 16, a protected precursor of the sensitiser
moiety in 12, is shown in Scheme 3 and was straightforward
from known7 4-hydroxy-40-nitrobenzophenone 13.
Our second approach to the combination of linker length
and synthetic practicality was targeted on an ether-linked
conjugate, where we envisioned assembly by a Mitsunobu
coupling of a suitable u-hydroxyalkoxy nitroindoline,
exemplified by 17, with the phenolic benzophenone 18.
The expected conjugate, 19, would have the shortest
practicable linker which maintains the principal structural
features, i.e. the 4,40-dialkoxybenzophenone and the
4-alkoxyindoline, that were present in our initial successful
conjugate 2b.
Carbodiimide-mediated coupling of 10 and 16, followed by
treatment with TFA to remove the tert-butyl protecting
groups as described above for 4 gave the desired conjugate
12. Its calculated UV–Vis spectrum had lmax 300 nm
(3 30,500 MK1 cmK1; see Supplementary Data). Dis-
appointingly, irradiation of an aqueous solution of 12
resulted in no changes in the UV–Vis spectrum, even on
exposure to the light source for periods up to 8 min. In
contrast, irradiation of 2b for 7 s under the same conditions
resulted in w50% photolysis (see Figure 2 of Ref. 4). We
did not attempt to investigate the reasons for the lack of
photoreactivity in 12. However, there is some indication that
amidobenzophenones may decay to a lower-energy triplet
state than the normal n,p* benzophenone triplet,8 and may
Synthesis of 17, shown in Scheme 4, was generally
straightforward but established a useful point about stability
of the TBDMS protecting group (see below). Conversion of
Scheme 3. Synthesis of (16). Reagents and conditions: (a) BrCH2CH2OH, K2CO3, NaI, acetone, reflux, 81%. (b) Et2NP(OBut)2, 1H-tetrazole, THF, then
MCPBA, 81%. (c) H2, Pd–C, EtOH, w100%.

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G. Papageorgiou, J. E. T. Corrie / Tetrahedron 61 (2005) 609–616
(Ph3P, diisopropyl azodicarboxylate)12 proceeded in high
yield and subsequent TFA treatment to remove the tert-
butyl protecting groups gave conjugate 19. Solutions of 19
were quantified using the same molar absorbance coefficient
(3300 27,900 MK1 cmK1) calculated for 4, since the
chromophores of the two conjugates are essentially
identical. Progressive photolysis of 19 in air-saturated,
neutral aqueous solution showed a clean transition between
the initial and final UV–Vis spectra (Fig. 1), very similar to
that previously reported4 for photolysis of 2b. Comparative
300 nm irradiation of separate solutions of 2b and 19
showed that the two compounds had very similar efficiency
of photolysis (Scheme 5).
Scheme 4. Synthesis of alcohol (17). Reagents and conditions: (a) ethylene
carbonate, Et4NBr, DMF, 140 8C, 74%; (b) TBDMSCl, imidazole, CH2Cl2,
95%; (c) claycop, Ac2O, CCl4, 85%; (d) TBAF, HOAc, THF, 84%.
7 (Scheme 2) to its 2-hydroxyethyl ether 20 was readily
achieved by heating with ethylene carbonate in DMF in the
presence of catalytic tetraethylammonium bromide.9 This
procedure was much more effective in this case than
alkylation with 2-bromoethanol (K2CO3-butanone),
although the latter method was effective when the phenol
was more acidic, as with 4-hydroxybenzophenones, for
example in synthesis of 14 (Scheme 3). The next significant
step in the synthesis was introduction of the 7-nitro group,
for which the previously established use of claycop–Ac2O
as the nitrating reagent10 was our preferred option.
However, it was obviously necessary to protect the primary
alcohol to avoid side reaction(s) under the oxidative
conditions of this reaction. Protection as a silyl ether was
attractive, since its subsequent deprotection was expected to
be achievable without perturbing other parts of the molecule
but we had some concern as to whether it would survive the
conditions: previously we have reported partial loss
(presumably acid-catalysed) of a Boc group from a
(Boc)2N-moiety during claycop-mediated nitration.5 How-
ever, we were gratified to find that the TBDMS ether 21
underwent claycop nitration to give an 86% yield of purified
product 22, implying that there was no significant cleavage
of the silyl ether during the reaction. This useful observation
is particularly relevant to proposed future work with more
complex acyl groups (such as protected amino acid
residues) attached to the indoline nitrogen. Normal TBAF
deprotection (buffered with acetic acid) then gave the
required alcohol 17.
Figure 1. UV–Vis absorption spectra for an aqueous solution of 19 upon
300-nm irradiation for the cumulative time periods indicated. The arrows
indicate the direction of absorbance changes with increasing irradiation
time.
3. Conclusions
At its outset, a hoped-for outcome this work was to obtain
enhanced photolysis efficiency over that of our previous
conjugates of general structure 2. In the event, the
improvements have been modest (for 4) or absent (for 19),
while the amidobenzophenone conjugate 12 was inert. Thus
variation in length of the linker between the sensitiser and
the nitroindoline, at least over the range explored here, has
little effect on the photolysis efficiency. On the other hand,
changes in the benzophenone substituents can have major
negative consequences. The principal benefit to have
emerged is a synthesis of 19 that is more practicable in
several respects than that of the initial conjugates 2. These
improvements are (a) in the use of a 5-methyl substituent to
block nitration other than at the 7-position of the indoline,
(b) a short, effective route to preparation of the sensitiser
moiety and (c) efficient chemical coupling of the sensitiser
and the nitroindoline. An effective synthesis of conjugates
of this general type will be important if the sensitised
nitroindoline photochemistry is to become widely adopted.
Future work will be to adapt the present synthesis of 19
to an L-glutamate analogue and to perform detailed
assessment of the utility of such a conjugate (and perhaps
of the corresponding GABA and glycine analogues) in
Our second required compound was the benzophenone 18,
and a desirable goal was to establish a direct route from a
readily available benzophenone precursor. In our previous
work, the unsymmetrical benzophenone used as the
sensitiser had been prepared from mono-aryl starting
materials in order to establish different substituents on the
two rings.4 In the present work (Scheme 5), we0started from
the symmetrical dipivalate ester 23 of 4,4 -dihydroxy-
benzophenone. Controlled alkaline hydrolysis gave the
monopivalate 24 in purified yield of 73% after simple
recrystallisation of the crude hydrolysis mixture. Previous
preparations by partial esterification of 4,40-dihydroxy-
benzophenone required chromatography for isolation of the
pure monoester.11 Conversion of 24 via 25 to the phosphate
ester 26 was uneventful and final alkaline hydrolysis of the
pivalate ester gave 18 in excellent yield. The overall yield
for the five-stage sequence from commercial 4,40-dihydroxy-
benzophenone was 29%.
Mitsunobu coupling of the alcohol 17 and the phenol 18

G. Papageorgiou, J. E. T. Corrie / Tetrahedron 61 (2005) 609–616
613
Scheme 5. Synthesis of benzophenone (18). Reagents and conditions: (a) Me3CCOCl, pyridine, 88%. (b) NaOH (2 equiv), aq. THF, 73%. (c) BrCH2CH2OH,
K2CO3, NaI, butanone, reflux, 58%. (d) Et2NP(OBut)2, 1H-tetrazole, THF, then MCPBA, 72%. (e) NaOH (1 equiv), aq. MeOH, 91%.
neurophysiological applications. This chemistry and its
biological sequelae will be reported in due course.
8.1 Hz, 1H), 7.06 (d, JZ8.1 Hz, 1H), 4.08 (t, JZ8.5 Hz,
2H), 3.04 (t, JZ8.5 Hz, 2H), 2.32 (s, 3H), 2.20 (s, 3H), 2.13
(s, 3H). Anal. calcd for C13H15NO3: C, 66.94; H, 6.48; N,
6.00; found: C, 66.51; H, 6.07; N, 5.97.
4. Experimental
4.1. General
4.2.2. 1-Acetyl-5-methylindolin-4-ol (7). A solution of 6
(3.15 g, 13.5 mmol) in MeOH (95 mL) was treated with 1 M
aq. NaOH (14.85 mL, 14.85 mmol) and stirred at rt for
0.75 h, then diluted with water (100 mL) and concentrated.
The residue was acidified to pH 3 with dilute HCl and the
precipitated solid was filtered off, washed with water and
dried. The filtrate was washed with EtOAc and the organic
phase was washed with saturated aq. NaHCO3 and brine,
dried and evaporated to give additional solid. The combined
solid was recrystallised (MeOH–EtOAc) to give 7 as white
1H NMR spectra were determined on Varian Unityplus 500
or JEOL FX90Q spectrometers in CDCl3 solution with TMS
as internal reference, unless otherwise specified Elemental
analyses were carried out by MEDAC Ltd, Surrey, UK.
Merck 9385 silica gel was used for flash chromatography.
Analytical HPLC was performed on a 250!4 mm Merck
Lichrospher RP8 column at 1.5 mL minK1 flow rate.
Preparative HPLC was carried out on a 2!30 cm column
(Waters C18 packing, Cat. No. 20594) at 2 mL minK1 flow
rate. Details of mobile phases are given at relevant points in
the text. Detection for analytical and preparative work was
at 254 nm. Organic solvents were dried over anhydrous
Na2SO4 and evaporated under reduced pressure. Hexane
solvent (bp 40–60 8C) was redistilled before use. Photolysis
experiments were performed in a Rayonet RPR-100
photochemical reactor fitted with 16!300 nm lamps.
1
crystals (2.06 g, 80%), mp 242 8C; H NMR (500 MHz,
CDCl3CDMSO-d6) d 8.04 (s, 1H), 7.57 (d, JZ8.1 Hz, 1H),
6.88 (d, JZ8.1 Hz, 1H), 4.05 (t, JZ8.5 Hz, 2H), 3.13 (t, JZ
8.5 Hz, 2H), 2.91 (s, 3H), 2.19 (s, 3H). Anal. calcd for
C11H13NO2: C, 69.09; H, 6.85; N, 7.32; found: C, 68.93; H,
6.97; N, 7.29.
4.2.3. tert-Butyl (1-acetyl-5-methylindolin-4-yloxy)acetate
(8). A suspension of anhydrous K2CO3 (1.24 g, 9 mmol) in
acetone (60 mL) was treated with 7 (1.15 g, 6 mmol). After
15 min, tert-butyl bromoacetate (2.34 g, 12 mmol) was
added and the mixture was heated under reflux for 4 h. The
solid was filtered off, washed with acetone and the filtrate
was evaporated. The residue was dissolved in EtOAc
(50 mL), washed with brine, dried and evaporated to give 8
as white crystals (1.64 g, 90%), mp 95–96 8C (EtOAc–
4.2. Synthesis of 4-{2-[(1-acetyl-5-methyl-7-nitroindolin-
4-oxy)acetamido]ethoxy}-40-[2-(dihydroxyphosphoryloxy)
ethoxy]benzophenone (4)
4.2.1. 4-Acetoxy-1-acetyl-5-methylindoline (5). NaBH3CN
(3.58 g, 57 mmol) was added portionwise over 0.5 h to a
solution of 5-methylindol-4-ol6 (2.80 g, 19 mmol) in acetic
acid (90 mL), keeping the temperature at w15 8C by
intermittent cooling. The mixture was then stirred at rt for
1 h and water (3 mL) was added and the solvent was
evaporated. The residue was dissolved in EtOAc (50 mL)
and washed with saturated aq. NaHCO3 and brine, dried and
evaporated to give 5-methylindolin-4-ol as pale foam
1
hexanes); H NMR (500 MHz) d 7.87 (d, JZ8.1 Hz, 1H),
7.00 (d, JZ8.1 Hz, 1H), 4.38 (s, 2H), 4.05 (t, JZ8.4 Hz,
2H), 3.24 (t, JZ8.4 Hz, 2H), 2.26 (s, 3H), 2.20 (s, 3H), 1.50
(s, 9H). Anal. calcd for C17H23NO4: C, 66.86; H, 7.59; N,
4.59; found: C, 67.07; H, 7.64; N, 4.60.
4.2.4. tert-Butyl (1-acetyl-5-methyl-7-nitroindolin-4-
yloxy)acetate (9). Claycop (3.2 g; prepared as described)13
was added to a solution of 8 (1.53 g, 5 mmol) in a mixture of
CCl4 (40 mL) and acetic anhydride (20 mL) and the mixture
was stirred at rt for 4 h. The solid was filtered off, washed
with CCl4 and the filtrate was evaporated. The residue was
dissolved in EtOAc and washed with saturated aq. NaHCO3
and brine, dried and evaporated to give 9 as yellow needles
(1.15 g, 66%), mp 97.5–98.5 8C (Et2O–hexanes with
1
(2.83 g, 100%); H NMR (90 MHz) d 6.78 (d, JZ8 Hz,
1H), 6.20 (d, JZ8 Hz, 1H), 3.98 (s, 2H, exchanges with
D2O), 3.54 (t, JZ8 Hz, 2H), 2.93 (t, JZ8 Hz, 2H), 2.14 (s,
3H). The crude indoline was dissolved in a mixture of acetic
acid (20 mL) and acetic anhydride (20 mL) and heated
under reflux for 1 h. The resulting dark solution was diluted
with water (5 mL) and the solvents were evaporated. The
residue was dissolved in EtOAc (100 mL) and washed with
saturated aq. NaHCO3 and brine, dried and evaporated to
give 6 as pale fawn crystals (3.22 g, 73%), mp 103–104 8C
(EtOAc–hexanes); 1H NMR (500 MHz) d 7.98 (d, JZ
1
charcoal); H NMR (500 MHz) d 7.54 (q, JZ0.5 Hz, 1H),
4.47 (s, 2H), 4.22 (t, JZ8.0 Hz, 2H), 3.24 (t, JZ8.0 Hz,
2H), 2.31 (d, JZ0.5 Hz, 3H), 2.24 (s, 3H), 1.49 (s, 9H).

614
G. Papageorgiou, J. E. T. Corrie / Tetrahedron 61 (2005) 609–616
Anal. calcd for C17H22N2O6: C, 58.28; H, 6.33; N, 7.99;
found: C, 58.20; H, 6.33; N, 7.96.
4.3. Synthesis of 4-[2-(1-acetyl-5-methyl-7-nitroindolin-
4-oxy)ethoxy]-40-[2-(dihydroxyphosphoryloxy)ethoxy]
benzophenone (19)
4.2.5. (1-Acetyl-5-methyl-7-nitroindolin-4-yloxy)acetic
acid (10). A solution of 9 (1.05 g, 3 mmol) in TFA
(10 mL) was stirred at rt for 1 h, concentrated and re-
evaporated from toluene (2!10 mL) to give 10 as light
brown crystals (0.66 g, 75%), mp 188–190 8C (EtOAc);
UV: lmax (EtOH)/nm 250 (3/MK1 cmK1 25,400), 287
(9300) 329sh (4000); lmax [EtOH–25 mM Na phosphate,
pH 7.0 (1:40)]/nm 247 (3/MK1 cmK1 19,700), 338 (3600);
1H NMR (500 MHz, CDCl3CDMSO-d6) d 7.51 (q, JZ
0.5 Hz, 1H), 4.57 (s, 2H), 4.23 (t, JZ8.0 Hz, 2H), 3.27 (t,
JZ8.0 Hz, 2H), 2.33 (d, JZ0.5 Hz, 3H), 2.24 (s, 3H). Anal.
calcd for C13H14N2O6: C, 53.06; H, 4.80; N, 9.52; found: C,
53.16; H, 4.79; N, 9.40.
4.3.1. 4-Hydroxy-400-(trimethylacetoxy)benzophenone
(24). A solution of 4,4 -bis(trimethylacetoxy)benzophenone
23 (7.65 g, 20 mmol; see Supplementary Data) in THF
(40 mL) was diluted with MeOH (360 mL) and rapidly
mixed with 1 M aq. NaOH (40 mL, 40 mmol). The solution
was stirred at rt for 1.5 min, then neutralised with 1 M aq.
citric acid (40 mL, 40 mmol) and concentrated. The residue
was diluted with water and washed with EtOAc and the
combined organic phases were washed with brine, dried and
evaporated to give 24 as white crystals (4.38 g, 73%), mp
171–173 8C (EtOAc), mp 171–173 8C (lit.11a 171–173 8C).
4.3.2. 4-(2-Hydroxyethoxy)-40-(trimethylacetoxy)benzo-
phenone (25). To a solution of 24 (3.88 g, 13 mmol) in
butanone (260 mL) was added anhydrous K2CO3 (3.59 g,
26 mmol), NaI (1.3 g) and 2-bromoethanol (8.12 g,
65 mmol), and the mixture was heated under reflux. The
progress of the reaction was followed by TLC [EtOAc–
hexanes (1:1)]. Further amounts of 2-bromoethanol
(8.12 g), NaI (1.3 g) and anhydrous K2CO3 (3.59 g,) were
added after each of 2 h and 5 h and reflux was continued for
a total of 7 h. The solid was filtered off, washed with acetone
and the filtrate was evaporated. The residue was taken up in
EtOAc (80 mL) and washed with water and brine, dried and
evaporated. Flash chromatography [EtOAc–hexanes (1:1)]
gave two fractions. The unreacted starting phenol 24
(0.64 g, 16%) eluted first, followed by 25 as white crystals
(2.69 g, 60%), mp 115–116 8C (EtOAc–hexanes); 1H NMR
(500 MHz) d 7.82 (dt, JZ8.6, 2.0 Hz, 2H), 7.80 (dt, JZ8.6,
2.0 Hz, 2H), 7.18 (dt, JZ8.6, 2.2 Hz, 2H), 6.99 (dt, JZ8.6,
2.2 Hz, 2H), 4.18 (t, JZ4.4 Hz, 2H), 4.03 (dt, JZ4.4,
6.2 Hz, 2H), 2.00 (t, JZ6.2 Hz, 1H, exchanges with D2O),
1.38 (s, 9H). Anal. calcd for C20H22O5 ¼H2O: C, 69.25; H,
6.54. Found: C, 69.51; H, 6.33; HRMS (ESI): calcd for
(C20H22O5CH)C: 343.1540. Found: 343.1552.
4.2.6. 4-{2-(1-Acetyl-5-methyl-7-nitroindolin-4-yloxy)acet-
amido]ethoxy]-40-[2-(dihydroxyphosphoryloxy)ethoxy]-
benzophenone (4). A solution of 4-(2-azidoethoxy)-40-
{2-[di(tert-butoxyphosphoryloxy]ethoxy}benzophenone4
(312 mg, 0.6 mmol) was reduced with Ph3P in moist THF as
previously described,5 to give the crude amine 11, which
was dissolved in CHCl3 (30 mL), dried and evaporated. The
residue was then dissolved in dry MeCN (20 mL) and
treated with the acid 10 (206 mg, 0.7 mmol) and 1-(3-
dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride
(161 mg, 0.84 mmol). The mixture was stirred at rt under
nitrogen for 18 h, then evaporated and the residue was
dissolved in EtOAc and washed successively with 0.5 M aq.
HCl, saturated aq. NaHCO3 and brine, dried and evaporated.
Flash chromatography [CHCl3–MeOH (95:5)] gave the di-
tert-butyl ester of 4 (393 mg, 85%) as a yellow viscous oil
which was used in the next step without further purification;
1H NMR (90 MHz) d 7.79 (d, JZ8 Hz, 4H), 7.42 (s, 1H),
6.98 (d, JZ8 Hz, 4H), 4.42 (s, 2H), 4.04–4.36 (m, 8H),
3.76–3.98 (m, 2H), 3.12 (t, JZ8 Hz, 2H), 2.23 (s, 3H), 2.18
(s, 3H), 1.50 (s, 18H). This ester (393 mg, 0.51 mmol) was
dissolved in TFA (10 mL), stirred at rt for 1 h and
concentrated in vacuo. The residue was dissolved in water
(65 mL) and adjusted to pH 7.08 with 1 M aq. NaOH. The
solution was washed with ether and analysed by reverse-
phase HPLC [mobile phase 25 mM Na phosphate, pH 6.0—
MeCN (100:45 v/v), tR 4.4 min. The solution was
lyophilised, dissolved in 25 mM Na phosphate, pH 6.0
(110 mL) and pumped onto the preparative HPLC column.
The column was first washed with 25 mM Na phosphate, pH
6.0 for 2 h, then with water for 2 h and finally the product
was eluted with water–MeOH (4:1 v/v). Fractions contain-
ing the product were analysed by HPLC as above, combined
and concentrated in vacuo. The residue was dissolved in
water, filtered through a 0.2 mm membrane, lyophilised and
the remaining yellow powder was dissolved in water
(10.5 mL) and quantified by UV–Vis spectroscopy at
300 nm (see above) to give a solution of 4 (NaC salt)
4.3.3. 4-{2-[Di(tert-butoxy)phosphoryloxy]ethoxy}-40-
(trimethylacetoxy)benzophenone (26). A solution of 25
(2.40 g, 7 mmol) in dry THF (50 mL) was treated under
nitrogen with 1H-tetrazole (1.96 g, 28 mmol) and di-tert-
butyl N,N-diethylphosphoramidite (93% purity; 3.75 g,
14 mmol) and the mixture was stirred at rt overnight. The
solution was cooled to 0 8C and treated dropwise with a
solution of m-chloroperbenzoic acid (55% peracid; 6.59 g,
21 mmol) in CH2Cl2 (50 mL). The solution was stirred at
4 8C for 1 h, diluted with CH2Cl2 (100 mL), washed with
10% aq. Na2S2O5 and the organic phase was washed with
saturated aq. NaHCO3 and brine, dried and evaporated.
Flash chromatography [EtOAc–hexanes (4:1)] gave 26 as
white crystals (3.18 g, 85%), mp 80–81 8C (Et2O–hexanes);
1H NMR (500 MHz) d 7.81 (dt, JZ8.6, 2.0 Hz, 2H), 7.80
(dt, JZ8.6, 2.0 Hz, 2H), 7.18 (dt, JZ8.6, 2.0 Hz, 2H), 6.97
(dt, JZ8.6, 2.0 Hz, 2H), 4.31–4.34 (m, 2H), 4.25–4.28 (t,
JZ5 Hz, 2H), 1.50 (s, 18H), 1.38 (s, 9H). Anal. calcd for
C28H39O8P: C, 62.91; H, 7.35; found: C, 63.00; H, 7.38.
1
(27.5 mM, 289 mmol, 48% from 11); H NMR (500 MHz,
D2O, acetone ref.) d 7.53 (d, JZ8.5 Hz, 4H), 7.10 (s, 1H),
6.97 (d, JZ8.5 Hz, 2H), 6.85 (d, JZ8.5 Hz, 2H), 4.41 (s,
2H), 4.21–4.24 (m, 2H), 4.12–4.19 (m, 4H), 3.91 (t, JZ
7.7 Hz, 2H), 3.68–3.76 (m, 2H), 2.92 (t, JZ7.4 Hz, 2H),
2.10 (s, 3H), 1.93 (s, 3H); LRMS (ESI) calcd for
(C30H30N3O12PCH)K: 656.2; found: 656.4.
4.3.4. 4-{2-[Di(tert-butoxy)phosphoryloxy]ethoxy}-40-
hydroxybenzophenone (18). A solution of 26 (2.94 g,
5.5 mmol) in MeOH (220 mL) was treated with water

G. Papageorgiou, J. E. T. Corrie / Tetrahedron 61 (2005) 609–616
615
(16.5 mL) and 1 M aq. NaOH (5.5 mL, 5.5 mmol) and
stirred at rt for 40 min, then neutralised with 1 M aq. citric
acid (5.5 mL, 5.5 mmol) and concentrated. The residue was
diluted with water and washed with EtOAc and the
combined organic phases were washed with brine, dried
and evaporated to give 18 as white crystals (2.24 g, 90%),
acetic acid (0.45 g, 7.4 mmol) in THF (37 mL) was treated
at 0 8C with TBAF (1 M in THF; 7.4 mL, 7.4 mmol) and the
mixture was stirred at rt overnight. The solvent was
evaporated and the residue was taken up in EtOAc
(40 mL) and washed with saturated aq. NaHCO3 and
brine, dried and evaporated to give 17 as yellow crystals
1
1
mp 104–106 8C (EtOAc–hexanes); H NMR (500 MHz) d
(0.87 g, 84%), mp 171–173 8C (EtOAc–MeOH); H NMR
7.73 (dt, JZ8.6, 2.0 Hz, 2H), 7.69 (dt, JZ8.6, 2.0 Hz, 2H),
6.95 (dt, JZ8.6, 2.0 Hz, 2H), 6.91 (dt, JZ8.6, 2.0 Hz, 2H),
4.31–4.34 (m, 2H), 4.23 (t, JZ4.7 Hz, 2H), 1.51 (s, 18H).
Anal. calcd for C23H31O9P: C, 61.33; H, 6.94; found: C,
61.13; H, 7.05.
(500 MHz; CDCl3CDMSO-d6) d 7.50 (s, 1H), 4.30 (t, JZ
5.5 Hz, 1H, exchanges with D2O), 4.23 (t, JZ8 Hz, 2H),
4.07 (t, JZ4.8 Hz, 2H), 3.85–3.88 (m, 2H), 3.26 (t, JZ
8 Hz, 2H), 2.30 (s, 3H), 2.24 (s, 3H). Anal. calcd for
C13H16N2O5: C, 55.71; H, 5.75; N, 9.99; found: C, 55.67; H,
5.78; N, 9.83.
4.3.5. 1-Acetyl-4-(2-hydroxyethoxy)-5-methylindoline
(20). A solution of 7 (1.53 g, 8 mmol) and ethylene
carbonate (1.41 g, 16 mmol) in dry DMF (40 mL) was
treated with tetraethylammonium bromide (0.17 g,
0.8 mmol) and the mixture was heated at 140 8C for 20 h.
The solvent was then evaporated under reduced pressure
and the residue, dissolved in a mixture of EtOAc (50 mL)
and MeOH (10 mL), was washed with 1 M aq. NaOH and
brine, dried and evaporated to give 20 as white crystals
(1.39 g, 74%), mp 112–113 8C (EtOAc–hexanes); 1H NMR
(500 MHz) d 7.87 (d, JZ8.2 Hz, 1H), 7.01 (d, JZ8.2 Hz,
1H), 4.04 (t, JZ8.4 Hz, 2H), 3.97 (t, JZ4.4 Hz, 2H), 3.90–
3.93 (m, 2H), 3.19 (t, JZ8.4 Hz, 2H), 2.24 (s, 3H), 2.20 (s,
3H). Anal. calcd for C13H17NO3: C, 66.36; H, 7.28; N, 5.95;
found: C, 66.38; H, 7.38; N, 6.03.
4.3.9. 4-[2-(1-Acetyl-5-methyl-7-nitroindolin-4-oxy)-
ethoxy]-40-[2-(dihydroxy-phosphoryloxy)ethoxy]benzo-
phenone (19). A solution of 17 (0.21 g, 0.75 mmol) in dry
THF (14 mL) was treated with 18 (0.38 g, 0.85 mmol) and
triphenylphosphine (0.24 g, 0.9 mmol) and cooled to 0 8C
under nitrogen. Diisopropyl azodicarboxylate (95% purity;
0.19 g, 0.9 mmol) was added and the mixture was stirred at
rt under nitrogen for 24 h. The solvent was evaporated, the
residue was dissolved in EtOAc (30 mL) and washed with
0.5 M aq. NaOH, 0.5 M aq. HCl and brine, dried and
evaporated. Flash chromatography [EtOAc, then EtOAc–
MeOH (95:5)] followed by trituration with (CHCl3–
hexanes) gave the di-tert-butyl ester of 23 as a pale foam
(0.47 g, 88%) which was used in the next step without
further purification; 1H NMR (500 MHz) d 7.80 (dt, JZ9.0,
2.0 Hz, 2H), 7.78 (dt, JZ9.0, 2.0 Hz, 2H), 7.57 (s, 1H), 6.98
(dt, JZ9.0, 2.0 Hz, 2H), 6.97 (dt, JZ9.0, 2.0 Hz, 2H), 4.35
(s, 4H), 4.31–4.35 (m, 2H), 4.27 (t, JZ4.9 Hz, 2H), 4.22 (t,
JZ8 Hz, 2H), 3.22 (t, JZ8 Hz, 2H), 2.31 (s, 3H), 2.25 (s,
3H), 1.50 (s, 18H). This ester (0.46 g, 0.65 mmol) was
dissolved in TFA (10 mL), stirred at rt for 1 h and
concentrated in vacuo. The residue was dissolved in water
(72 mL) and adjusted to pH 7.1 with 1 M aq. NaOH. The
solution was washed with ether and analysed by reverse-
phase HPLC [mobile phase 25 mM Na phosphate, pH 6.0—
MeCN (100:55 v/v)], tR 4.6 min. The solution was
lyophilised, dissolved in 25 mM Na phosphate, pH 6.0
(100 mL) and pumped onto the preparative HPLC column.
The column was first washed with 25 mM Na phosphate, pH
6.0 for 1 h, then with water for 2 h and product was eluted
with water–MeOH (3:2 v/v). Fractions containing the
product were analysed as above, combined and concentrated
in vacuo. The residue was dissolved in water, passed
through a 0.2 mm membrane filter, lyophilised and the
yellow powder obtained was dissolved in water (15 mL),
quantified by UV–Vis spectroscopy at 300 nm (see above),
to give 19 (NaC salt) (22.1 mM, 331 mmol, 51%); 1H NMR
(500 MHz, D2O, acetone ref.) d 7.57 (d, JZ8.5 Hz, 2H),
7.54 (d, JZ8.5 Hz, 2H), 7.24 (s, 1H), 7.00 (d, JZ8.9 Hz,
2H), 6.82 (d, JZ8.5 Hz, 2H), 4.22–4.28 (m, 6H), 4.12–4.17
(m, 2H), 4.08 (t, JZ7.5 Hz, 2H), 3.01 (t, JZ7.5 Hz, 2H),
2.17 (s, 3H), 2.01 (s, 3H); LRMS (ESI) calcd for
(C28H27N2O11PCH)K: 599.1; found: 599.2.
4.3.6. 1-Acetyl-4-[2-(tert-butyldimethylsilyloxy)ethoxy]-
5-methylindoline (21). To a solution of 20 (1.29 g,
5.5 mmol) in dry CH2Cl2 (55 mL) was added imidazole
(0.56 g, 8.25 mmol) and tert-butyldimethylsilyl chloride
(0.99 g, 6.6 mmol) and the mixture was stirred at rt under
nitrogen overnight. The precipitated white solid was filtered
off, washed with CH2Cl2 and the filtrate was washed with
0.5 M aq. HCl, saturated aq. NaHCO3 and brine, dried and
evaporated to give 21 as white crystals (1.82 g, 95%), mp
101–102 8C (Et2O–hexanes); 1H NMR (500 MHz) d 7.84 (d,
JZ8.1 Hz, 1H), 6.99 (d, JZ8.1 Hz, 1H), 4.05 (t, JZ8.4 Hz,
2H), 3.89–3.97 (m, 4H), 3.20 (t, JZ8.4 Hz, 2H), 2.24 (s,
3H), 2.20 (s, 3H), 0.91 (s, 9H), 0.10 (s, 6H). Anal. calcd for
C19H31NO3Si: C, 65.29; H, 8.94; N, 4.01; found: C, 65.27;
H, 9.09; N, 3.96.
4.3.7. 1-Acetyl-4-[2-(tert-butyldimethylsilyloxy)ethoxy]-
5-methyl-7-nitroindoline (22). A solution of 21 (1.75 g,
5 mmol) in a mixture of acetic anhydride (15 mL) and CCl4
(30 mL) was treated with claycop (3.20 g) and the mixture
was stirred at rt for 3 h. The solid was filtered off, washed
with CCl4 and the filtrate was evaporated. The residue was
dissolved in EtOAc (50 mL) and washed with saturated aq.
NaHCO3 and brine, dried and evaporated. Flash chroma-
tography [EtOAc–hexanes (1:1)] gave 22 as yellow crystals
(1.51 g, 86%), mp 79–80 8C (Et2O–hexanes); 1H NMR
(500 MHz) d 7.54 (s, 1H), 4.21 (t, JZ8 Hz, 2H), 4.02 (t, JZ
4.7 Hz, 2H), 3.91 (t, JZ4.7 Hz, 2H), 3.21 (t, JZ8 Hz, 2H),
2.28 (s, 3H), 2.23 (s, 3H), 0.90 (s, 9H), 0.09 (s, 6H). Anal.
calcd for C19H30N2O5Si ¼H2O: C, 57.19; H, 7.70; N, 7.02;
found: C, 57.18; H, 7.79; N, 7.14.
4.4. Photolysis experiments
4.4.1. Comparative irradiation of (2b) and (4). Separate
solutions of 2b and 4 (each 0.3 mM in 25 mM Na
phosphate, pH 7.0 with 5 mM dithiothreitol) were
4.3.8. 1-Acetyl-4-(2-hydroxyethoxy)-5-methyl-7-nitro-
indoline (17). A solution of 22 (1.46 g, 3.7 mmol) and

616
G. Papageorgiou, J. E. T. Corrie / Tetrahedron 61 (2005) 609–616
simultaneously irradiated in 1 mm path length cells. The
solutions were analysed by reverse-phase HPLC with
mobile phases of 25 mM Na phosphate, pH 6.0—MeCN
(100:40 v/v) for 2b, tR 4.6 min and 25 mM Na phosphate,
pH 6.0—MeCN (100:45 v/v) for 4, tR 5.6 min. The extent of
photolysis of each solution was determined by comparison
of peak heights with those of unphotolysed controls. After
5 s irradiation, conversions for 2b and 4 were 39.0% and
45.1%, respectively, indicating that photolysis of 4 was
w1.16-fold more efficient than that for 2b.
found, in the online version, at doi:10.1016/j.tet.2004.10.
094
References and notes
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(0.22 mM in 25 mM Na phosphate pH 7.0) was irradiated in
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monitored by UV–Vis spectroscopy. No change in the
spectrum was observed throughout the irradiation time
course.
4.4.1.2. Progressive photolysis of (19). A solution of 19
(0.23 mM in 25 mM Na phosphate, pH 7.0) was irradiated
in a 1 mm path length cell for increasing times in the range
of 0–35 s. The extent of photolysis was monitored by
UV–Vis spectroscopy. Conversion was w50% after 5.5 s
and the spectral evolution was complete after 30 s.
4.4.1.3. Relative photolysis efficiencies of (19) and
(2b). Separate solutions of 2b and 19 (each 0.3 mM in
25 mM Na phosphate, pH 7.0 with 5 mM dithiothreitol)
were simultaneously irradiated in 1 mm path length cells.
The solutions were analysed by reverse-phase HPLC with
mobile phases 25 mM Na phosphate, pH 6.0—MeCN
(100:40 v/v) for 2b, tR 4.0 min and 25 mM Na phosphate,
pH 6.0—MeCN (100:55 v/v) for 19, tR 5.1 min. The extent
of photolysis of each solution was determined by compari-
son of peak areas with those of unphotolysed controls and
quantification was by measurement of peak heights. After
5 s irradiation, conversions for 2b and 19 were 49.0% and
49.3%, respectively, indicating that the two compounds
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Acknowledgements
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We are grateful to Dr. Peter Wan for helpful discussions and
to Dr. Ranjit Munasinghe for recording NMR spectra. We
thank the MRC Biomedical NMR Centre for access to
facilities and the EPSRC Mass Spectrometry Centre for
electrospray mass spectrometric data. This work was
supported in part by NIH Grant HL 19242-27.
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Supplementary data
´
13. Laszlo, P.; Cornelis, A. Aldrichem. Acta 1988, 21, 97–103.
Supplementary data associated with this article can be