experiments employed tetrahydrofuran (spectroscopic grade,
Burdick & J ackson), which was stored over sodium/benzophe-
none under high vacuum. Mass spectral analyses were per-
formed in the MIT Department of Chemistry Instrument
Facility. All manipulations were carried out under a nitrogen
atmosphere using standard Schlenk and glovebox techniques
unless otherwise noted.
Bis(N-a cetyltyr osyl m eth yl ester ) Oxa la te Ester (1). In
a 100-mL Schlenk flask was dissolved N-acetyltyrosyl methyl
ester (4.0 g, 15.6 mmol) in dry THF (30 mL). The solution was
stirred overnight over activated 4 Å molecular sieves, after which
triethylamine (2.17 mL, 15.6 mmol) was added. A solution of
oxalyl chloride (0.64 mL, 7.3 mmol) in THF (10 mL) was
prepared in a separate 250-mL Schlenk flask. Both flasks were
cooled to -78 °C, and the solution of N-acetyltyrosyl methyl ester
was transferred dropwise via cannula to the solution of oxalyl
chloride. The solution was stirred at -78 °C for 1 h and then
allowed to warm to room temperature and stirred for an
additional hour. The resulting white precipitate was collected
by filtration in air and redissolved in dichloromethane containing
a small amount of methanol. The organic layer was washed with
7.12 (dd, J
27.3 Hz, J
NH), 4.65 (dd, J
CH ), 1.45 (s, 9H, Boc), 1.42 (s, 18H, Bu), 1.36 (s, 9H, Bu).
HRFABMS (MNa ) m/z: calcd for NaC41H53NO 710.367, found
8
710.368.
(N-R-Boc-tyr osyl)(2,4,6-Tr i-ter t-bu tylp h en yl) Oxa la te Es-
ter (4). Ester 3 (1.0 g, 1.45 mmol) was dissolved in THF (50
mL) and stirred over 10% Pd/C (250 mg) for 6 h under a
hydrogen atmosphere. The Pd/C was removed by filtration, and
the solvent was evaporated to give pure 4 as a pale manila solid
1
) 14.1 Hz, J
) 12 Hz, 2H, benzyl-CH
) 14.7 Hz, J ) 7.2 Hz, 1H, CH), 3.13 (m, 2H,
2
) 8.4 Hz, 4H, ArH), 5.16 (dd, J
1
)
2
2
), 5.03 (d, J ) 8.7 Hz, 1H,
1
2
t
t
t
2
+
1
(0.8 g, 92% yield). H NMR (300 MHz, CDCl , 25 °C): δ 7.38 (s,
3
2H, ArH), 7.27 (dd, J 1 ) 15.9 Hz, J 2 ) 9 Hz, 4H, ArH), 4.99 (d,
J ) 10.2 Hz, 1H, NH), 4.63 (m, 1H, CH), 3.20 (dd, J ) 45 Hz,
1
t
t
J
2
) 6.9 Hz, 2H, CH ), 1.45 (s, 9H, Boc), 1.41 (s, 18H, Bu), 1.36
t
2
+
(s, 9H, Bu). HRFABMS (MNa ) m/z: calcd for NaC
3
4
47 8
H NO
620.320, found 620.319.
P h ysica l Mea su r em en ts. 1H NMR spectra were collected
at 25 °C in CDCl
3
at the MIT Department of Chemistry
Instrumentation Facility (DCIF). All chemical shifts are reported
using the standard δ notation in parts-per-million; positive
chemical shifts are to higher frequency from the given reference.
Nanosecond transient absorption measurements were performed
using a Nd:YAG laser with OPO running at 20 Hz to excite the
samples at 270 nm after frequency doubling from 540 nm. A
HCl (3 × 25 mL, 3.7% v/v) and dried over Na
2
SO
4
. The solution
was evaporated to dryness to yield pure 1 as a white powder
(
(
J
2
2.5 g, 65% yield). 1H NMR (500 MHz, CDCl
3
, 25 °C): δ 7.20
m, 8H, ArH), 5.96 (d, J ) 8 Hz, 2H, NH), 4.92 (dd, J ) 9.5 Hz,
) 6 Hz, 2H, CH), 3.75 (s, 6H, methyl ester), 3.17 (ddd, J
1
2
1
)
5
0-W Xe arc lamp (unpulsed) provided the probe light. A shutter
9.5 Hz, J
2
) 14 Hz, J
3
) 6 Hz, 2H, CH
2
), 2.02 (s, 3H, acetyl).
10 529.182, found
was used to block the probe light. The signal was dispersed and
detected using an intensified gated CCD camera (ICCD). The
timing of the ICCD, probe light shutter, and laser were
controlled using two delay generators. Series of four spectra were
taken: I (pump on/probe on), I (pump on/probe off), I (pump
F 0
off/probe on), and I0F (pump off/probe off). Transient spectra
+
HRFABMS (MH ) m/z: calcd for C26
29.181.
N-Acetyltyr osyl m eth yl ester )(2,4,6-tr i-ter t-bu tylp h e-
n yl) Oxa la te Ester (2). N-Acetyl tyrosyl methyl ester (1.5 g,
.9 mmol) was dissolved in THF (30 mL) containing pyridine
0.475 mL, 5.9 mmol). The solution was transferred via cannula
to a solution of 2,4,6-tri-tert-butylphenyl oxalyl chloride (2.8 g,
.0 mmol) in THF (20 mL) and stirred overnight at room
H
29
N
2
O
5
(
5
(
corrected for fluorescence and laser signals were calculated from
these spectra: ∆OD ) -log[(I
0
F
- I0F)/(I - I )]. Spectra reported
8
are the average of 250 of the four-spectra sequences. Instrument
control and data analysis were performed using software written
in LabView. Samples for photochemical measurements were
contained within a cell equipped with a solvent reservoir and a
temperature. Pyridine hydrochloride was removed by filtration,
and the filtrate was dried to a yellow oil that was triturated
with hexanes. The resulting white precipitate was collected and
redissolved in dichloromethane. Ether was added to precipitate
unreacted tyrosine. The remaining solution was collected,
2
-mm clear fused-quartz cell.
washed with saturated aqueous Na
over Na SO . The solvent was removed to deliver pure 2 as a
white solid (1.37 g, 42% yield). H NMR (500 MHz, CDCl
C): δ 7.38 (s, 2H, ArH), 7.21 (dd, J ) 23 Hz, J ) 9 Hz, 4H,
) 13.5 Hz, J
Hz, 1H, CH), 3.76 (s, 3H, methyl ester), 3.18 (ddd, J ) 30 Hz,
) 14 Hz, J ) 5.5 Hz, 2H, CH ), 2.03 (s, 3H, Acetyl), 1.41 (s,
2
CO
3
(1 × 50 mL), and dried
Ack n ow led gm en t . M.C.Y.C. kindly thanks the
2
4
1
National Science Foundation and the MIT/Merck Foun-
dation for predoctoral fellowships. Many thanks to
J ustin M. Hodgkiss and Dr. Niels H. Damrauer for their
efforts in attempting picosecond transient absorption
spectroscopy. We would also like to thank Dr. Martin
J aeger and Professor J ames R. Norris at the University
of Chicago for their aid in performing the spin-trapping
experiments. This work was supported by the National
Institutes of Health GM 47274. The Department of
Chemistry Instrument Facility is supported by National
Science Foundation 9729592.
3
, 25
°
1
2
ArH), 5.96 (d, J ) 7.5 Hz, 1H, NH), 4.92 (dd, J
1
2
)
6
J
1
1
2
3
2
t
t
+
8H, Bu), 1.35 (s, 9H, Bu). HRFABMS (MH ) m/z: calcd for
32 7
C H44NO 554.312, found 554.312.
(
N-R-Boc-tyr osyl ben zyl ester )(2,4,6-Tr i-ter t-bu tylp h e-
n yl) Oxa la te Ester (3). N-R-Boc-tyrosyl benzyl ester (2.0 g, 7.4
mmol) was dissolved in THF (15 mL) containing triethylamine
(1.4 mL, 10 mmol). The solution was transferred dropwise via
cannula into a solution of 2,4,6-tri-tert-butylphenyl oxalyl chlo-
ride (2.8 g, 8.0 mmol) in THF (20 mL), and the resulting solution
was stirred overnight at room temperature. The solvent was
removed in vacuo, and purification by flash column chromatog-
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
all compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
raphy (silica gel, 98/2 dichloromethane/methanol) afforded pure
1
3
as a white powder (2.1 g, 41% yield). H NMR (300 MHz,
CDCl
3
, 25 °C): δ 7.39 (s, 2H, ArH), 7.36 (m, 5H, benzyl-ArH),
J O025569S
6
822 J . Org. Chem., Vol. 67, No. 19, 2002