to proceed via cyclic oxetane/azetidine intermediates resulting
from photocycloaddition of the C dC and C4′dO/NH bonds
chosen as models and tris(4-bromophenyl)aminium hexachlo-
roantimonate (also named BAHA or magic blue) as one-
9
5
6
of two bases (in a Paterno-B u¨ chi [2 + 2] cycloaddition). It
has been demonstrated that in many organisms 6-4 photo-
products can be efficiently repaired by regeneration of the
oxetane/azetidine moiety, in an electron transfer mechanism
resembling that employed by CPD photolyases; this is not
unexpected in view of the similarity between CPD and 6-4
photoproducts in terms of structure and binding of cofactors.
Thus, cycloreversion (CR) of azetidine and oxetane radical
ions appears to be involved in the enzymatic repair of DNA.
Moreover, the possible involvement of both the anionic and
the cationic pathways in the regeneration of native pyrim-
idines from azetidine and oxetane lesions has been recently
electron oxidizing agent.
Although the synthesis of azetidines 2 has been the subject
of a previous publication, their isolation and characterization
as pure separated stereoisomers has not been completely
reported. In the present work, c-2 and t-2 were prepared as
depicted in Scheme 1: copper-catalyzed reaction of pheny-
Scheme 1. Synthesis of c-2 and t-2
7
studied by means of theoretical calculations.
In this context, the electron transfer CR of model oxetanes
8
has been previously described; however, there seems to be
no report dealing with the same type of process in azetidines.
Hence, the goal of the present work was to gain insight into
the mechanistic aspects of azetidine CR via electron transfer.
For this purpose 1,2,3-triphenylazetidines c-2 and t-2 were
(
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For the electron transfer experiments, a solution of c-2 or
t-2 in acetonitrile was reacted with an equimolar amount of
BAHA, which was added dropwise in the same solvent. At
the end of the addition the initial blue color, typical of tris(4-
bromophenyl)aminium radical cation, changed to red-brown.
Analysis of the reaction mixture by GC-MS showed the
presence of cis-stilbene (c-3) and/or trans-stilbene (t-3),
toghether with lower amounts of imine 4 (Scheme 2). For
quantitation, known amounts of appropriate standards were
added prior to analysis. The structures of all of the products
were confirmed by comparison with authentic samples.
The obtained results can be explained by initial electron
(
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transfer from c-2 or t-2 to BAHA. Subsequent C
2
-N/C
3
-C
4
bond cleavage (pathway a) of the generated azetidine radical
cation would lead to formation of the two stilbene isomers.
Alternatively, the initial azetidine radical cation could
undergo C
2
3 4
-C /C -N cleavage (pathway b), ultimately
(
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resulting in formation of 4. The proposed mechanism is
illustrated in Scheme 3. A control experiment showed that
no cycloreversion takes place in the absence of BAHA.
To rule out a possible interconversion of the stilbenes via
radical cation under the employed reaction conditions, c-3
2
(
1
(
6
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