A R T I C L E S
Amorati et al.
in 2-(methylthio)phenol the S-CH3 bond points perpendicular
to the ring plane, as shown in Scheme 1 for compound 1.15
Therefore, in systems where the ortho alkylthio substituent
cannot assume this geometry, the strength of the hydrogen
bonding interaction may differ significantly and the contribution
to the phenolic BDE(O-H) of an ortho -SR group may be
quite far from the value of -0.8 kcal/mol found in 1.
This peculiarity may be important for understanding the role
of the -SR linkage between the Tyrosine 272 and Cysteine
228 in GOase as, in the enzyme crystallized without the
substrate, the S-R bond assumes a conformation nearly coplanar
(∼7°) with the aromatic ring (Figure 1).9 Actually, the reactivity
and the BDE of the phenolic OH of the modified tyrosine of
GOase has been estimated using 2-(methylthio)phenols,13 which
seem to be poor models since they do not allow us to take into
account the coplanarity with the aromatic ring of the -SR
substituent.
Figure 1. X-ray crystal structure of the active site of GOase9 where the
oxidizable substrate is replaced by a water molecule.
In order to study the effect of the conformation of ortho-SR
groups on the phenolic BDE(O-H) and on the reactivity of
the hydroxyl group toward free radicals, we synthesized
compounds 3 and 4, in which the critical substituent was
expected to be in a nearly planar geometry (see later).
Compounds 5 and 6, both accessible through the same synthetic
pathway, were prepared to have a reference compound to probe
the ortho Vs para electronic effects of the -SR group (5) and
to compare the effects of S and O atoms on the BDE(O-H) of
ortho substituted phenols (6).
Despite the GOase catalytic cycle being thoroughly investi-
gated, the role of the Tyr-Cys linkage remains a controversial
point. Although the thioether bond seems essential for the
function of GOase,12 studies on model compounds13 and
theoretical simulations of the catalytic cycle10a show no major
effects of an ortho thioether substituent on the redox behavior
of tyrosine. Therefore, it has been suggested that the role of
the Tyr-Cys cross-link is basically structural; that is, it contrib-
utes to maintaining the three-dimensional structure of the
enzyme.10a
Clearly, the knowledge of the contribution of the ortho thio-
substituents to the phenolic BDE(O-H) is an important
prerequisite for understanding the radical reactions involving
the tyrosine OH group.
Results and Discussion
Synthesis. Heterocycles 3-6 (Scheme 2) were prepared by
the inverse electron demand hetero Diels-Alder reaction of an
electron-poor dienic o-thioquinone with 4-methoxystyrene used
as an electron-rich dienophile. Following our original synthetic
strategy,16 transient o-thioquinones have been obtained from the
corresponding o-hydroxy-N-thiophthalimides which, in turn, are
the product of the reaction of phthalimidesulfenyl chloride
(PhtNSCl, Pht ) Phthaloyl) with the required phenol. Deriva-
tives 3-6 were prepared using as starting materials com-
mercially available 4,6-di-tert-butylresorcinol (3a), 3,5-di-tert-
butylcatechol (6a), and 4,6-di-n-propyl- and 2,4-di-n-
propylresorcinol (4a and 5a, respectively) which were prepared
from O,O′-diallyl resorcinol Via a double Claisen rearrangement
followed by separation and hydrogenation as previously reported
(see Supporting Information).17 Remarkably, for phenols 3a, 4a,
and 6a the sulfenylation and the cycloaddition reactions can be
carried out without protection of the extra phenolic OH not
involved in the formation of o-thioquinone. For 5a the protection
In a recent work, the BDE(O-H) of ortho and para alkylthio
substituted phenols have been measured.14 The additive con-
tributions of ortho and para -SCH3 groups to the phenolic
BDE(O-H) were obtained as -0.8 and -3.5 kcal/mol, respec-
tively, by studying compounds 1 and 2 (see Scheme 1). The
large difference between these two values has been explained
as being due to the formation of an Intramolecular Hydrogen
Bond (IHB) that preferentially stabilizes the parent phenol of 1
with respect to the phenoxyl radical 1′ (Scheme 1).14 These
results are similar to those previously found in the case of ortho
methoxyphenols, where the relatively strong IHB raises the
BDE(O-H) to about 3.7 kcal/mol.5,6
However, it should be pointed out that a striking difference
between ortho -OR and -SR groups in the formation of IHB
with the phenolic OH resides in the directionality of such an
interaction: 2-methoxyphenol adopts a planar geometry,5 while
(10) (a) Himo, F.; Eriksson, L. A.; Maseras, F.; Siegbahn, P. E. M. J. Am. Chem.
Soc. 2000, 122, 8031-8036. (b) Rothlisberger, U.; Carloni, P.; Doclo, K.;
Parrinello M. J. Biol. Inorg. Chem. 2000, 5, 236-250. (c) Rokhsana, D.;
Dooley, D. M.; Szilagyi, R. K. J. Am. Chem. Soc. 2006, 128, 15550-
15551.
(11) (a) Wang, Y.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Stack, T. P. D.
Science 1998, 279, 537. (b) Itoh, S.; Taki, M.; Takayama, S.; Nagatomo,
S.; Kitagawa, T.; Sakurada, N.; Arakawa, R.; Fukuzumi, S. Angew. Chem.,
Int. Ed. 1999, 38, 2774-2776. (c) Guidoni, L.; Spiegel, K.; Zumstein, M.;
Rothlisberger, U. Angew. Chem., Int. Ed. 2004, 43, 3286-3289. (d) Pratt,
R. C.; Stack, T. D. P. J. Am. Chem. Soc. 2003, 125, 8716-8717. (e)
Thomas, F.; Gellon, G.; Gautier-Luneau, I.; Saint-Aman, E.; Pierre, J.-L.
Angew. Chem., Int. Ed. 2002, 41, 3047-3050.
(12) Baron, A. J.; Stevens, C.; Wilmot, C. M.; Seneviratne, K. D.; Blakeley,
V.; Dooley, D. M.; Phillips, S. E. V.; Knowles, P. F.; McPherson, M. J. J.
Biol. Chem. 1994, 269, 25095-25105.
(13) Itoh, S.; Taki, M.; Fukuzumi, S. Coord. Chem. ReV. 2000, 198, 3-20.
(14) Amorati, R.; Fumo, M. G.; Menichetti, S.; Mugnaini, V.; Pedulli, G. F. J.
Org. Chem. 2006, 71, 6325-6332.
(15) (a) Schaefer, T.; Wildman, T. A.; Salman, R. S. J. Am. Chem. Soc. 1980,
102, 107-110. (b) Schaefer, T.; Salman, R. S.; Wildman, T. A.; Clark, P.
D. Can. J. Chem. 1982, 60, 342-348. (c) Schaefer, T.; McKinnon, D. M.;
Sebastian, R.; Peeling, J.; Penner, G. H.; Veregin, R. P. Can. J. Chem.
1987, 65, 908-914. (d) Himo, F.; Eriksson, L. A.; Blomberg, M. R. A.;
Siegbahn, P. E. M. Int. J. Quantum Chem. 2000, 76, 714-723. (e) Schaefer,
T.; Penner, G. H. Can. J. Chem. 1988, 66, 1229-1238.
(16) (a) Capozzi, G.; Falciani, C.; Menichetti, S.; Nativi, C. J. Org. Chem. 1997,
62, 2611-2615. (b) Capozzi, G.; Lo Nostro, P.; Menichetti, S.; Nativi, C.;
Sarri, P. Chem. Commun. 2001, 551-552. (c) Menichetti, S.; Aversa, M.
C.; Cimino, F.; Contini, A.; Tomaino, A.; Viglianisi, C. Org. Biomol. Chem.
2005, 3, 3066-3072. (d) Amorati, R.; Fumo, M. G.; Pedulli, G. F.,
Menichetti, S.; Pagliuca, C.; Viglianisi, C. HelV. Chim. Acta 2006, 89,
2462-2472; e) Amorati, R.; Cavalli, A.; Fumo M. G:, Masetti, M.;
Menichetti, S.; Pagliuca, C.; Pedulli, G. F.; Viglianisi, C. Chem.sEur. J.
2007, 13, 8223-8230.
(17) Liu, K.; Xu, L.; Berger, J. P.; MacNaul, K. L.; Zhou, G.; Doebber, T. W.;
Forrest, M. J.; Moller, D. E.; Jones, A. B. J. Med. Chem. 2005, 48, 2262-
2265.
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238 J. AM. CHEM. SOC. VOL. 130, NO. 1, 2008