2325
1,
R
H;
-
R1
R2
Ac;
-
^QCOC—C— C6H5 OH NHCOQHj
3,R 5,
=
R = R‘
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6, R =
12
The formation of 2 photochemically from 1 could of a concerted [„2a + T2a] reaction;11 however, such a mechanism cycloaddition does not account for the formation of 3.15 An alternative pathway, which can account for the production of both 2 and 3, is an electrocyclic ring-opening reaction that breaks the CeC7 bond of 1 to give the open-chain cation 12. Such a process is directly comparable to that observed upon the photoisomerization of the isoelectronic cyclohexa-1,3-dienes to the bicyclo[3.1.0]hexenes.1116 While the direct isomerization of 12 to give 2 is possible, in view of the constant ratio of 2 to 3 observed in these reactions, it is attractive to consider that a cation resembling 11 might be formed either photochemically, or thermally, from 12 and that this gives the observed products, 2 and 3.
Ri = H;R2
,H =
COCHJ;
R‘=Ac;R2
=
Ac; R2
=
7,
R=
R1
8,
R
H; R1
=
