Stereodifferentiation in the photochemical cycloreversion of diastereomeric methoxynaphthalene - oxetane dyads

Article abstract of DOI:10.1021/jo048708+

(Chemical Equation Presented) Intramolecular PET cycloreversion of oxetanes 1 and 2 has been achieved in acetonitrile and chloroform as solvents. Interestingly, a higher photoreactivity has been found in acetonitrile, while a significant stereodifferentiation has been found in chloroform. This stereodifferentiation can be attributed to the folded conformation which predominates in 2, with the naphthalene ring directed toward the oxetane region, allowing for the intramolecular electron transfer. Accordingly, intramolecular fluorescence quenching is also more efficient in acetonitrile, whereas stereodifferentiation is markedly higher in chloroform. Thus, a good correlation can be established between the results from steady-state irradiations and fluorescence measurements.

Full text of DOI:10.1021/jo048708+

Stereodifferentiation in the Photochemical Cycloreversion of
Diastereomeric Methoxynaphthalene-Oxetane Dyads
Rau´l Pe´rez-Ruiz,† Salvador Gil,‡ and Miguel A. Miranda*,†
Departamento de Quı´mica/Instituto de Tecnologı´a Quı´mica UPV-CSIC, Universidad Polite´cnica de
Valencia, Camino de Vera s/n, Apartado 22012, 46022 Valencia, Spain, and Departamento de Qu´ımica
Orga´nica, Universidad de Valencia, 46100 Burjassot, Valencia, Spain
mmiranda@qim.upv.es
Received July 27, 2004
Intramolecular PET cycloreversion of oxetanes 1 and 2 has been achieved in acetonitrile and
chloroform as solvents. Interestingly, a higher photoreactivity has been found in acetonitrile, while
a significant stereodifferentiation has been found in chloroform. This stereodifferentiation can be
attributed to the folded conformation which predominates in 2, with the naphthalene ring directed
toward the oxetane region, allowing for the intramolecular electron transfer. Accordingly,
intramolecular fluorescence quenching is also more efficient in acetonitrile, whereas stereodiffer-
entiation is markedly higher in chloroform. Thus, a good correlation can be established between
the results from steady-state irradiations and fluorescence measurements.
Introduction
This process is enzymatically achieved by photolyases,9
whose mode of action involves photochemical transfer of
one electron from a reduced and deprotonated flavin
(FADH-)10,11 to an oxetane. Subsequently, the oxetane
radical anion cleaves to provide one neutral pyrimidine
plus one pyrimidine radical anion.
Despite its biological interest, only a few reports have
appeared on the cycloreversion (CR) of oxetane radical
anions generated by photoinduced electron transfer
(PET).12 Cycloadducts of 1,3-dimethylthymine with benz-
aldehyde and benzophenone have been used as model
systems to study their PET reactions with a variety of
electron-donor photosensitizers.12-15 The radical anion of
A variety of DNA lesions with known mutagenic,
carcinogenic, and lethal effects can be formed by the
exposure of cells to UV-radiation.1,2 They include cyclobu-
tane pyrimidine dimers and (6-4) photoproducts.3,4 The
latter (with higher mutagenic potential) are formed
through a Paterno-Bu¨chi photoreaction between two
adjacent pyrimidines in the DNA duplex; the resulting
oxetanes rearrange to the final (6-4) photoadducts.5
Repair of the (6-4) photoproducts in DNA follows a
mechanism analogous to the reductive electron transfer
established for cyclobutane pyrimidine DNA dimers.6-8
* Phone: 963877340. Fax: 963879349.
(8) Hitomi, K.; Nakamura, H.; Kim, S.-T.; Mizukoshi, T.; Ishikawa,
T.; Iwai, S.; Todo, T. J. Biol. Chem. 2001, 276, 10103-10109.
(9) Cichon, M. K.; Arnold, S.; Carell, T. Angew. Chem., Int. Ed. Engl.
2002, 41, 767-770. Sancar, A. Chem. Rev. 2003, 103, 2203-2237.
(10) Epple, R.; Wallenborn, E.-U.; Carell, T. J. Am. Chem. Soc. 1997,
119, 7440-7451.
† Universidad Polite´cnica de Valencia.
‡ Universidad de Valencia.
(1) Taylor, J.-S. J. Chem. Educ. 1990, 67, 835-841.
(2) Taylor, J.-S. Acc. Chem. Res. 1994, 27, 76-82.
(3) Heelis, P. F.; Hartman, R. F.; Rose, S. D. Chem. Soc. Rev. 1995,
289-297.
(11) Hartman, R. F.; Rose, S. D. J. Am. Chem. Soc. 1992, 114, 3559-
3560.
(4) Smith, C. A.; Wang, M.; Jiang, N.; Che, L.; Zhao, X.; Taylor, J.-
S. Biochemistry 1996, 35, 4146-4154.
(12) Prakash, G.; Falvey, D. E. J. Am. Chem. Soc. 1995, 117, 11375-
11376.
(5) Taylor, J.-S.; Nadji, S. Tetrahedron 1991, 47, 2579-2590.
(6) Kim, S.-T.; Malhorta, K.; Smith, C. A.; Taylor, J.-S.; Sancar, A.
J. Biol. Chem. 1994, 269, 8535-8540.
(13) Joseph, A.; Prakash, G.; Falvey, D. E. J. Am. Chem. Soc. 2000,
122, 11219-11225.
(7) Kim, S.-T.; Malhotra, K.; Smith, C. A.; Taylor, J. S.; Sancar, A.
Photochem. Photobiol. 1996, 63, 292-295.
(14) Joseph, A.; Falvey, D. E. J. Am. Chem. Soc. 2001, 123, 3145-
3146.
10.1021/jo048708+ CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/19/2005
1376
J. Org. Chem. 2005, 70, 1376-1381

Photochemical Cycloreversion of MN-Oxetane Dyads
CHART 1
naproxen (NPX) with racemic 2-(p-cyanophenyl)-4-hy-
droxymethyl-3-phenyloxetane, synthesized in turn by
Paterno-Bu¨chi photocycloaddition of p-cyanobenzalde-
hyde and trans-cinnamyl alcohol. The resulting mixture
was separated by high performance liquid chromatrog-
raphy (HPLC) leading to the pure diastereomers 1 and
2.
The structural assignment was made on the basis of
1H and 13C NMR spectroscopy, using NOE data to clarify
the encumbered aromatic region. This information was
checked against the MOPAC (AM1) calculations, which
allowed us to obtain the optimized folded and extended
geometries for the two compounds, 1 and 2 (see Support-
ing Information, pages S18 and S19). NOESY experi-
ments (Figures 1 and 2) were performed to establish the
relative stereochemistry of the chiral centers; their
results were consistent with a 2R, 3S, 4S configuration
for isomer 1 and the opposite one (2S, 3R, 4R) for 2. Some
conclusions were drawn from NOESY data: In compound
1, a clear interaction was observed between the methyl
group (δ ) 1.58 ppm) and H3 (δ ) 3.58 ppm). This was
absent in compound 2, where the methyl protons inter-
acted instead with the methylene group (δ ) 4.39 ppm)
and with the ortho protons of the phenyl substituent at
C3. Besides, the methoxy group of 2 presented a small
but clear interaction with the p-cyanophenyl protons that
could not be observed for 1.
A closer inspection of the aromatic region (see Figures
1b and 2b) showed a clear correlation between the
naproxen protons and the p-cyanophenyl group in isomer
2. A less clear situation resulted for 1, where the chemical
shifts of the relevant protons were too close to observe a
hypothetical NOE effect.
To ensure that similar intensity NOE effects were
being compared for 1 and 2, a 50:50 mixture of both
diastereomers in CDCl3 was prepared. From its NOESY
spectra, essentially the same conclusions were drawn
(whenever possible). This allowed us to validate the
observed NOE effects and to conclude that they were
intramolecular in nature.
Taking into account the whole ensemble of NOE data
and the optimized geometries according to MOPAC
(AM1) calculations, it was concluded that a folded
conformation predominates in 2, with the naphthalene
ring directed toward the oxetane region; by contrast, an
extended conformation with the naphthalene ring point-
ing to the external region appears to be preferred in the
case of 1. This is in good agreement with the number
and strength of the interactions found between the
aromatic protons of the naphthalene ring and those of
the p-cyanostilbene group, which were more important
for 2 (see above).
Photosensitized Cycloreversion of 1 and 2. The
two diastereomeric oxetanes 1 and 2 were separately
irradiated at λmax ) 300 nm in acetonitrile, under
nitrogen. As outlined in Scheme 1, the main photoprod-
ucts were cis-p-cyanostilbene (3b) and the naproxen ester
of 2-hydroxyacetaldehyde (4). This result is analogous to
that of the previously reported intermolecular process,16
where cleavage of the O-C2 and C3-C4 bonds was also
observed.
the carbonyl fragment has been detected by means of
laser flash photolysis (LFP), which supports an electron-
transfer mechanism. Taking into account that the photo-
sensitizer fluorescence is quenched by oxetanes, it has
been proposed that the reaction occurs from the singlet
excited state.
More recently, the mechanistic aspects of the reductive
intermolecular CR of the model compound 2-(p-cyano-
phenyl)-4-methyl-3-phenyloxetane using 1-methoxynaph-
thalene (1-MN) as the electron-donor photosensitizer
have been studied.16 Clear evidence has been obtained
confirming that CR of the oxetane takes place from the
singlet excited state of the sensitizer. Ring splitting of
the oxetane radical anion occurs with cleavage of the
O-C2 and C3-C4 bonds, leading to products (acetalde-
hyde and p-cyanostilbene) different from the reagents
used in the Paterno-Bu¨chi synthesis of the oxetane.
Moreover, the radical anion of p-cyanostilbene involved
in the PET process has been detected by LFP.
As 1-MN has been shown to be a good electron-donor
photosensitizer for the CR of p-cyanophenyl-substituted
oxetanes, it appeared interesting to study the analogous
intramolecular process in oxetanes 1 and 2, which contain
a chiral methoxynaphthalene covalently linked to the
oxetane ring (Chart 1).
The main goal of this work was to detect a possible
stereodifferentiation in the intramolecular PET process,
either in the quenching of the reactive singlet excited
state or in the kinetics of oxetane disappearance with
formation of CR photoproducts.
For this purpose, compounds 1 and 2 have been
submitted to preparative photolysis and fluorescence
(steady-state as well as time-resolved) studies, in both
acetonitrile and chloroform, to compare their photophysi-
cal and photochemical behavior.
Cycloreversion has been found to occur with cleavage
of the O-C2 and C3-C4 bonds. Interestingly, there is a
significant stereodifferentiation in chloroform, while a
higher photoreactivity is observed in acetonitrile. Ac-
cordingly, intramolecular fluorescence quenching is also
more efficient in acetonitrile, whereas stereodifferentia-
tion is markedly higher in chloroform. Thus, a good
correlation can be established between the results from
steady-state irradiations and fluorescence measurements.
Results and Discussion
Synthesis of Oxetanes 1 and 2. The target com-
pounds 1 and 2 were prepared by esterification of (S)-
(15) Joseph, A.; Falvey, D. E. Photochem. Photobiol. Sci. 2002, 1,
632-635.
(16) Pe´rez-Ruiz, R.; Izquierdo, M. A.; Miranda, M. A. J. Org. Chem.
2003, 68, 10103-10108.
To compare the photoreactivity of 1 and 2 under
identical conditions and to follow the reaction by 1H
NMR, direct irradiation of a 1:1 diastereomeric mixture
J. Org. Chem, Vol. 70, No. 4, 2005 1377

Pe´rez-Ruiz et al.
FIGURE 1. (a) Aliphatic and (b) aromatic part of the phase-
sensitive 2D NOESY spectrum of 1 recorded in CDCl3.
was performed using deuterated chloroform and aceto-
nitrile as solvents. At the beginning of the reaction, the
two doublets in the region δ ) 5.60-5.80 ppm, corre-
sponding to the H2 protons of the oxetane rings of 1 and
2, were of the same intensity. These signals decreased
with increasing irradiation time, giving rise to a new AB
system centered at δ ) 6.65 ppm in chloroform and δ )
6.80 ppm in acetonitrile, corresponding to the olefinic
protons of cis-p-cyanostilbene.
FIGURE 2. (a) Aliphatic and (b) aromatic part of the phase-
sensitive 2D NOESY spectrum of 2 recorded in CDCl3.
carried out looking for intramolecular interactions in the
ground and the excited states. Thus, the ultraviolet
absorption spectra of 1, 2, NPX, and 2-(p-cyanophenyl)-
4-hydroxymethyl-3-phenyloxetane were recorded in ace-
tonitrile and chloroform, and then difference spectra [1
- (NPX + oxetane)] and [2 - (NPX + oxetane] were
obtained. Figure 3 shows the results in acetonitrile.
Further material can be found in the Supporting Infor-
mation, pages S20-S24. Interestingly, a new band was
clearly observed at 336-337 nm in both compounds that
can be attributed to a CT ground-state complex.
Previously, the fluorescence emission as well as the
triplet-triplet (T-T) absorption of 1-methoxynaphtha-
lene (1-MN) has been studied in the presence of increas-
ing amounts of 2-(p-cyanophenyl)-4-methyl-3-phenyloxe-
tane, to establish the nature of the excited state involved
in the intermolecular PET process.16 Although both
singlet and triplet quenching were observed, product
studies in the presence of triplet quenchers, together with
the calculated free energy changes, provided a clear
1
The relevant part of the H NMR spectra is shown in
the Supporting Information, page S25. It is remarkable
that a significant stereodifferentiation was found in
chloroform, where compound 2 (monitored through its
more deshielded doublet) reacted faster than its diaste-
reomer 1. On the other hand, higher conversions were
achieved in acetonitrile after shorter irradiation times;
however, a lower stereodifferentiation was observed in
this solvent. This is in agreement with the fact that
reactivity differences are usually better observable when
the reaction is not too fast.
Intramolecular Interactions in the Ground and
Excited States. To gain some insight into the mecha-
nistic aspects of the reaction, experimental studies were
1378 J. Org. Chem., Vol. 70, No. 4, 2005

Photochemical Cycloreversion of MN-Oxetane Dyads
SCHEME 1
depend on the singlet energies (89 kcal/mol for 1-MN vs
86 kcal/mol for 2-MN) and on the oxidation potentials
(1.4 V vs SCE for 1-MN vs 1.5 V vs SCE for 2-MN). On
this basis, taking into account the presence of a chiral
2-methoxynaphthalene as donor in compounds 1 and 2,
singlet excited-state involvement in the intramolecular
reaction was also expected.
The required information on intramolecular excited-
state quenching was obtained by fluorescence (steady-
state and time-resolved) and LFP of 1 and 2, in both
acetonitrile and chloroform. The results were compared
with those obtained for NPX, the corresponding isolated
chromophore.
In acetonitrile, the emission maxima of 1 and 2
appeared at ∼350 nm, as in the case of NPX. However,
the fluorescence intensity became strongly reduced in the
two diastereomers (Figure 4a), indicating significant
quenching attributable to electron transfer from the
excited naphthalene moiety to the p-cyanophenyl group.
The quenching rate constants kq(S1) were calculated from
eq 1 using the experimental fluorescence lifetimes (τs) of
1/τs - 1/τs,0 ) kq(S1)
(1)
1 and 2 (2.6 and 2.2 ns, respectively). As the lifetime of
NPX is known (τs,0 ) 10.8 ns),17 the values of kq(S1) for
singlet quenching can be estimated as 2.9 × 108 s-1 (for
1) and 3.6 × 108 s-1 (for 2).
The fluorescence quantum yields (Φf) of the diastereo-
meric oxetanes were obtained from the integrated emis-
sion spectra, using NPX as the reference (Φf ) 0.47).17
They were found to be Φf ) 0.075 (1) and 0.055 (2). The
decrease of the Φf values as compared with those of NPX
and the differences found between the two isomers are
in good qualitative agreement with the trends observed
in the lifetime measurements. This confirms that (apart
from ET) the radiative and radiationless transitions in
NPX and compounds 1 and 2 are similar.
As regards the LFP experiments, the expected T-T
absorption of NPX at 440 nm was detected for 1 and 2.
The reduced intensity of this signal observed for the two
diastereomers right after the laser pulse (Figure 4b)
FIGURE 3. Difference UV-spectra in acetonitrile: (a) 1 -
[(NPX + 2-(p-cyanophenyl)-4-hydroxymethyl-3-phenyloxe-
tane)] and (b) 2 - [(NPX + 2-(p-cyanophenyl)-4-hydroxy-
methyl-3-phenyloxetane)], in the long wavelength region.
support for the reaction taking place from the singlet
excited state of the sensitizer 1-MN. The redox properties
of 2-methoxynaphthalene (2-MN) in the excited states are
expected to be very similar to those of 1-MN. They only
(17) (a) Martinez, L. J.; Scaiano, J. C. Photochem. Photobiol. 1998,
68, 646-651. (b) Moore, D. E.; Chappuis, P. P. Photochem. Photobiol.
1988, 47, 173-180.
(18) Weller, A. Z. Phys. Chem. (Muenchen) 1982, 133, 93-98.
J. Org. Chem, Vol. 70, No. 4, 2005 1379

Pe´rez-Ruiz et al.
•-
The reduction potential (EA/A ) was taken to be -1.52
V versus SCE, as in the model oxetane previously used.16
•+
The oxidation potential of NPX (ED /D ) 1.45 V vs SCE)
and its singlet and triplet energies [E*(S1) ) 85 kcal/mol,
E*(T1) ) 62 kcal/mol] have already been reported.19
Finally, the value of the dielectric constant in acetonitrile
is 35.9.20 By fitting the above data in eq 3, the following
free energy changes resulted: ∆GET(S1) ) -17.8 kcal/mol
and ∆GET(T1) ) +5.1 kcal/mol. As expected, the intramo-
lecular CR from the singlet excited state would be an
exergonic process, whereas triplet involvement does not
appear to be thermodynamically possible.
In chloroform, a different tendency was observed when
fluorescence (steady-state as well as time-resolved) and
LFP studies were performed. Although the emission
maxima of 1, 2, and NPX appeared at ∼352 nm, fluores-
cence quenching was clearly detected in the case of 2,
while the emission intensity of 1 was very similar to that
of NPX (Figure 5a). As a matter of fact, the fluorescence
lifetimes of NPX and 1 were in the same range (∼3.5 ns),
whereas, in the case of 2, τs was found to be 2.5 ns. From
these data the quenching rate constant was only calcu-
lated for 2, and its value was estimated as kq(S1) ) 1.1 ×
108 s-1
.
On the other hand, the transient T-T absorption of
NPX and the two oxetanes was also observed in chloro-
form at 440 nm by means of LFP. Again, the reduced
intensity of the signal observed for compound 2 right
after the laser pulse (Figure 5b) is simply a consequence
of fluorescence quenching. The triplet lifetimes were
determined and found to be 1.1 µs (1), 0.7 µs (2), and 0.7
µs (NPX). Hence, intramolecular triplet quenching does
not appear to occur in oxetanes 1 and 2.
The free energy changes associated with intramolecu-
lar PET in chloroform were estimated with eq 3, taking
into account that the dielectric constant in this solvent
is 4.8.20 The obtained values were ∆GET(S1) ) -7 kcal/
mol and ∆GET(T1) ) +16 kcal/mol. Thus, the process
would be thermodynamically possible from the singlet
excited state, but it would be disfavored from the
triplet.
A possible reaction mechanism that is in agreement
with the above data is shown in Scheme 2. It includes
both the radical ion pair 3a•-/4•+ and the biradical
resulting from back electron transfer. The former path-
way would be analogous to the known intermolecular
process;16 however, the radical anion of p-cyanostilbene
(λmax ) 500 nm)16 was not detected as a transient
intermediate. Although formation of this species cannot
be completely ruled out (it could be masked by the intense
naphthalene T-T absorption between 350 and 550 nm;
see Supporting Information, pages S12-S17), it appears
more likely that fast intramolecular BET occurs in the
biradical zwitterionic intermediate, to give a biradical as
immediate precursor of the final products.
FIGURE 4. (a) Emission spectra of NPX (s), 1 (- - -), and 2
(‚‚‚) in acetonitrile, recorded after excitation at 320 nm under
nitrogen. (b) Decay traces of the T-T absorption of NPX (×),
1 (O), and 2 (9) measured at 440 nm in acetonitrile after LFP
at 308 nm. All concentrations were fixed, adjusting the
absorbance of the solution at an arbitrary value between 0.3
and 0.32 at the excitation wavelength, under nitrogen.
simply reflects the above-mentioned singlet excited-state
quenching. Besides, there was a significant shortening
of the triplet lifetimes τt of 1 and 2 (560 and 550 ns,
respectively) as compared with those of NPX (τt,0 ∼ 1 µs
under the same conditions). Hence, the triplet quenching
rate constants kq(T1) were estimated using eq 2 and found
to be 7.8 × 105 s-1 (for 1) and 8.1 × 105 s-1 (for 2).
1/τt - 1/τt,0 ) kq(T1)
(2)
Conclusions
Free energy changes associated with electron transfer
from both the singlet and triplet excited states were
estimated using the Weller18 equation (eq 3):
Overall, the above results support the view that the
PET CR of 1 and 2 occurs from the singlet excited state
(19) (a) Bosca´, F.; Martinez-Ma´n˜ez, R.; Miranda, M. A.; Primo, J.;
Soto, J.; Van˜o´, L. J. Pharm. Sci. 1992, 81, 479-482. (b) Bosca´, F.;
Mar´ın, M. L.; Miranda, M. A. Photochem. Photobiol. 2001, 74, 637-
655.
(20) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993; p 284.
∆GET (kcal/mol) )
23.06[ED /D - EA/A•- + (2.6/ꢀ) - 0.13] -
•+
E*(S1 or T1) (3)
1380 J. Org. Chem., Vol. 70, No. 4, 2005

Photochemical Cycloreversion of MN-Oxetane Dyads
SCHEME 2
Experimental Section
Cycloreversion Reaction. A solution of the isomer mix-
ture (10-2 M) in CD3CN and CDCl3 (0.75 mL) was placed in
NMR tubes and bubbled with nitrogen. Then, the solution was
irradiated for 60 min in a multilamp photoreactor, using 8 W
lamps (4×) with emission maxima at λ ) 300 nm. The reaction
was followed by 1H NMR, which was recorded before and after
the irradiation. A control experiment showed that the photo-
cycloreversion does not take place in the dark.
Fluorescence Spectroscopy. The steady-state fluores-
cence spectra were obtained with a conventional fluorimeter,
equipped with a 450 W xenon lamp. The time-resolved
fluorescence determinations were performed using a hydrogen-
nitrogen flash lamp (2 ns pulse width). The samples were
placed into quartz cells of 1 cm path length. The concentrations
of substrates 1 and 2, as well as that of NPX, were fixed by
adjusting the absorbance of the solution at an arbitrary value
between 0.3 and 0.4, and deoxygenation was performed by
bubbling nitrogen.
Time-Resolved Absorption Spectroscopy. The laser
flash photolysis system has been described elsewhere.16 Briefly,
it was based on a pulsed XeCl Excimer laser, using 308 nm as
the excitation wavelength. The single pulses were ∼17 ns
duration, and the energy was ∼100 mJ/pulse. A xenon lamp
was employed as the detecting light source. The laser flash
photolysis apparatus consisted of the pulsed laser, the Xe lamp,
a monochromator, a photomultiplier (PMT) system, and an
oscilloscope. The output signal from the oscilloscope was
transferred to a personal computer for study.
FIGURE 5. (a) Emission spectra of NPX (s), 1 (- - -), and 2
(‚‚‚) in chloroform, recorded after excitation at 320 nm under
nitrogen. (b) Decay traces of the T-T absorption of NPX (×),
1 (O), and 2 (9) measured at 440 nm in chloroform, after LFP
at 308 nm. All concentrations were fixed, adjusting the
absorbance of the solution at an arbitrary value between 0.3
and 0.32 at the excitation wavelength, under nitrogen.
of the naphthalene-like chromophore. The efficiency of
the photoreaction was higher in acetonitrile, which is
consistent with the higher values of the fluorescence
quenching rate constants found in this solvent, where the
free energy changes associated with the process are more
negative. Remarkably, the stereodifferentiation observed
in the preparative photolysis (with oxetane 2 reacting
significantly faster) agrees well with the relative kq(S1)
values obtained from fluorescence measurements.
It is interesting to note that stereodifferentiation is
higher in chloroform, which can be explained in terms of
the well-known reactivity/selectivity principle. Finally,
it is also worth mentioning that the higher reactivity of
oxetane 2 must be related to the contribution of folded
conformations, with a significant interaction between the
electron-donating methoxynaphthalene chromophore and
the electron-accepting p-cyanophenyl group; this is in
good agreement with the NOESY data.
Acknowledgment. Financial support by the MCYT
(Grant No. BQU2001-2725) and by Generalitat Valen-
ciana (Grups 03/082) is gratefully acknowledged.
Supporting Information Available: Additional experi-
mental details; 1H, 13C NMR of compounds 1, 2, and 4 and
1
2-(p-cyanophenyl)-4-hydroxymethyl-3-phenyloxetane; H, 13C
correlation spectra of compounds 1 and 2; transient absorption
spectra of NPX, 1, and 2; MOPAC (AM1) optimized geometries
for 1 and 2; absorption spectra of NPX, 2-(p-cyanophenyl)-4-
hydroxymethyl-3-phenyloxetane, 1, and 2, as well as difference
1
spectra; H NMR monitoring of the 1:2 mixtures (28 pages).
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO048708+
J. Org. Chem, Vol. 70, No. 4, 2005 1381