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
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
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10.1021/ol0357954 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/03/2003