Base-Induced Photocyclization of 1,2-Diaryl-1-tosylethenes. A Mechanistically Novel Approach to Phenanthrenes and Phenanthrenoids

Article abstract of DOI:10.1021/ol0357954

(Equation presented) The irradiation with UV light of a number of 1,2-diaryl-1-tosylstilbenes, in the presence of base, leads to the corresponding phenanthrenes and heterocyclic analogues. These results are consistent with a mechanism involving the base-induced elimination of p-toluenesulfinic acid from an intermediate 9-tosyl-4a,4b-dihydrophenanthrene, formed by photochemical cyclization of the starting 1,2-diaryl-1-tosylstilbenes.

Full text of DOI:10.1021/ol0357954

ORGANIC
LETTERS
2003
Vol. 5, No. 26
4939-4941
Base-Induced Photocyclization of
1,2-Diaryl-1-tosylethenes. A
Mechanistically Novel Approach to
Phenanthrenes and Phenanthrenoids
Jesu´s F. Almeida, Luis Castedo, Dolores Ferna´ndez, Ana G. Neo,
V´ıctor Romero, and Gabriel Tojo*
Departamento de Qu´ımica Orga´nica y Unidad Asociada al CSIC,
UniVersidad de Santiago de Compostela, 15782-Santiago de Compostela, Spain
qogatojo@usc.es
Received September 17, 2003 (Revised Manuscript Received November 24, 2003)
ABSTRACT
The irradiation with UV light of a number of 1,2-diaryl-1-tosylstilbenes, in the presence of base, leads to the corresponding phenanthrenes and
heterocyclic analogues. These results are consistent with a mechanism involving the base-induced elimination of p-toluenesulfinic acid from
an intermediate 9-tosyl-4a,4b-dihydrophenanthrene, formed by photochemical cyclization of the starting 1,2-diaryl-1-tosylstilbenes.
On UV irradiation, stilbenes (1a, 2a) undergo an initial
equilibration to a photostationary mixture of E and Z
isomers,1 which is followed by a photochemically allowed
ring closure of the Z-isomer (2a) to an unstable dihydro-
phenanthrene (3a) (Scheme 1). When an oxidant is present,
the dihydrophenanthrene is transformed into a phenanthrene
(4).2 This is the most commonly used procedure for the
preparation of phenanthrenes and related heterocyclic com-
pounds.3 An interesting variant of this reaction consists of
the irradiation, in the presence of a base, of a stilbene
containing a good leaving groupsnormally Cl or Brsat the
ortho position of one of the phenyl rings (1b, 2b). In this
case, the unsubstituted phenanthrene 4 is produced by base-
induced elimination of HL from the intermediate susbstituted
dihydrophenathrene (3b).4
We speculated that irradiation of a stilbene containing a
good leaving group (L) on the central double bond (5) would
lead to the substituted dihydrophenanthrene 6, which, in the
56, 2240. Beccalli, E. M.; Marchesini, A.; Pilati, T. Tetrahedron 1993, 49,
4741. Puls, C.; Stolle, A.; de Meijere, A. Chem. Ber. 1993, 126, 1635.
Plater, M. Tetrahedron Lett. 1994, 35, 6147. Brooks, P.; Donati, D.; Pelter,
A.; Poticelli, F. Synthesis 1999, 8, 1303. Ishikura, M.; Hino, A.; Yaginuma,
T.; Agata, I.; Katagiri, N. Tetrahedron 2000, 56, 193. Piers, E.; Britton,
R.; Andersen, R. J. J. Org.Chem. 2000, 63, 530. Castedo, L.; Delamano,
J.; Enjo, J.; Ferna´ndez, J.; Gra´valos, D. G.; Leis, R.; Lo´pez, C.; Marcos, C.
F.; R´ıos, A.; Tojo, G. J. Am. Chem. Soc. 2001, 123, 5102. Enjo, J.; Castedo,
L.; Tojo, G. Org. Lett. 2001, 3, 1343.
(4) Wood, C. S.; Mallory, F. B. J. Org. Chem. 1964, 3373. Kupchan, S.
M.; Wormser, H. C. J. Org. Chem. 1965, 30, 3792. Kanaoka, Y.; Itoh, K.
J. Chem. Soc., Chem.Commun. 1973, 647. Gellmann, K. H.; Ku¨hle, W.;
Weller, H.; Wolff, T. J. Am. Chem. Soc. 1981, 103, 6889. Mallory, F. B.;
Rudolph, M. J.; Oh, S. M. J. Org. Chem. 1989, 54, 4619. Mallory, F. B.;
Rudolph, M. J.; Oh, S. M. J. Org. Chem. 1989, 54, 4619. Jayalaban, L.;
Shannugan, P. Synthesis 1990, 789. Benedetti-Doctorovich, V.; Huang, F.
Y.; Lambropoulos, J.; Burgess, E. M.; Zalkow, L. H. Synth. Commun. 1995,
25, 3701.
(1) Lewis, G. N.; Magel, T. T.; Lipkin, D. J. Am. Chem. Soc. 1940, 62,
2973.
(2) Parker, C. O.; Spoerri, P. E. Nature 1950, 166, 603. Buckles, R. E.
J. Am. Chem.Soc. 1955, 77, 1040. Mallory, F. B.; Wood, C. S.; Gordon, J.
T.; Linquist, L. C.; Savitz, M. L. J. Am. Chem. Soc. 1962, 84, 4361.
(3) Mallory, F. B.; Mallory, C. W. Organic Reactions; Wiley: New York,
1984; Vol. 30, p 1. Ben, I.; Castedo, L.; Saa´, J. M.; Seijas, J. A.; Suau, R.;
Tojo, G. J. Org. Chem. 1985, 50, 2236. Rawal, V. H.; Jones, R. J.; Cava,
M. P. Tetrahedron Lett. 1985, 26, 2423. Rawal, V. H.; Jones, R. J.; Cava,
M. P. J. Org.Chem. 1987, 52, 19. Mirsadeghi, S.; Prasad, G. K. B.;
Whittaker, N.; Thakker, D. R. J. Org. Chem. 1989, 54, 3091. Kaliakoudas,
D.; Eugster, C. H.; Ruedi, P. HelV. Chim. Acta 1990, 73, 48. Yang, B.;
Liu, L.; Katz, T. J.; Liberko, C. A.; Miller, L. L J. Am. Chem. Soc. 1991,
113, 8993. Karminski-Zamola, G.; Pavlicic, D.; Bajic, M.; Blazevic, N.
Heterocycles 1991, 12, 2323. Dorst, K. J.; Cava, M. P. J. Org. Chem. 1991,
10.1021/ol0357954 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/03/2003

Scheme 1. Photochemical Preparation of Phenanthrenes an
Table 1. Base-Induced Photocyclization of Tosylstilbenes to
Phenanthrenesa
Phenanthrenoids
presence of base, might likewise lose HL to afford the
unsubstituted phenanthrene (Scheme 2).5
Scheme 2. Irradiation of Stilbenes Containing a Leaving
Group in the Central Double Bond
To check this hypothesis, we prepared 1,2-p-tolyl-1-
tosylethene 7, a 5:1 mixture of E and Z isomers, by
condensation of the LDA-generated anion of p-methylbenzyl
p-tolyl sulfone with p-tolualdehyde, followed by dehydration
of the resulting â-hydroxysulfone with mesyl chloride and
triethylamine.6 The stereochemistry of these isomers was not
determined, being irrelevant in view of the photochemical
equilibration in the next step.
Irradiation of 7 in the presence of base leads to the desired
phenanthrene (8) in yields which ranged from 30% to 95%
depending on the base (DBU, Et3N, or CaCO3) and the
solvent (THF, benzene, CH2Cl2, or MeOH).7
An optimum yield of 95% is obtained by irradiation of a
0.2 M solution of sulfone 7 in THF containing 5 equiv of
DBU. The cyclization conditions were applied to other
tosylstilbenes and stilbenoids leading to excellent yields of
the corresponding phenanthrenes and phenanthrenoids
(Table 1).
a A 0.2 M solution of starting stilbene or stilbenoid in THF containing
5 equiv of DBU was irradiated with a 450 W medium-pressure Hannovia
mercury lamp until TLC showed consumption of most of the starting
material.
In keeping with the proposed mechanism, irradiation of
sulfone 7 in the absence of base led to a photostationary 1:1
mixture of E and Z isomers which fails to yield phenathrene
8 even after prolonged times. In the presence of I2 as oxidant,
the expected tosyl-substituted phenathrene 21 (Figure 1) is
obtained in 80% yield.
A clue as to the mechanism of the based-induced conver-
sion of the intermediate tosyl-substituted dihydrophenan-
threnes into phenanthrenes is gained by the fact that
(5) For the photochemistry of stilbenes with a bromine on the central
double bond, see: Kimatura, T.; Kobayashi, S.; Taniguchi, H. J. Org. Chem.
1990, 55, 1801. Kitamura, T.; Kabashima, T.; Taniguchi, H. J. Org. Chem.
1991, 56, 3739.
(6) Antelo, B.; Castedo, L.; Delamano, J.; Go´mez, A.; Lo´pez, C.; Tojo,
G. J. Org. Chem. 1996, 61, 1188. Kondo, K.; Tunemoto, D. Tetrahedron
Lett. 1975, 1007. Cardillo, G.; Savoia, D.; Umani-Ronchi, A. Synthesis 1975,
453.
(7) The following yields were obtained: Et3N (40 equiv), benzene, 24 h
(53%); Et3N (40 equiv.), MeOH, 10 h (30%); Et3N (40 equiv), CH2Cl2, 39
h (75%); Et3N (40 equiv), THF, 11 h (75%); DBU (4 equiv), THF, 13 h
(95%); DBU (4 equiv), benzene, 18 h (90%); CaCO3 (10 equiv), THF, 15
h (62%); CaCO3 (3 equiv), MeOH, 25 h (58%).
4940
Org. Lett., Vol. 5, No. 26, 2003

Scheme 3. Hypothetic Mechanism for the Photocyclization of
Stilbenoids in the Presence of a Base
Figure 1.
group. This can be explained by the selective deprotonation
of intermediate (22a, 22b) to the sulfone-stabilized anion
25. Protonation of this anion leads to intermediate 26, which
yields phenanthrenoids 18 and 20 by loosing p-toluene-
sulfinic acid. Phenanthrenoids 24 are not obtained, as the
formation of anion 23 would be thermodynamically unfavor-
able (Scheme 3).8
This work shows a mechanistically novel photocyclization
of stilbenes to phenanthrenes, which may prove useful for
the preparation of biologically important phenanthrenes and
phenanthrenoids. The starting tosylstilbenoids are easily
obtained by condensation of arylmethyl sulfones with
aromatic aldehydes6 and the key photocyclization gives good
yields in a range of different benzenic and heterocyclic
compounds.
Acknowledgment. We thank the Spanish Ministry of
Science and Technology (SAF2001-3120) and the Xunta de
Galicia (PGIDIT02RAG20901PR and PGIDIT02PXIC-
20902PN) for financial support.
compounds 17 and 19 both lose their tosyl groups rather
Supporting Information Available: Experimental details
and analytical data for all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
than the tosyloxy substituent, which is a much better leaving
(8) For precedents of base-induced tautomerization of 4a,4b-dihydro-
phenanthrenes, see: Somers, J. B. M.; Couture, A.; Lablache-Combier, A.;
Laarhoven, W. H. J. Am. Chem. Soc. 1985, 107, 1387.
OL0357954
Org. Lett., Vol. 5, No. 26, 2003
4941