4.6 Photochemical Processes
In the course of the total synthesis of brevetoxin B, Nicolaou et al. have designed a
new and elegant access to oxepenes from dithiono derivatives. Thus, irradiation of
159 can generate a bi-diradical intermediate which can furnish after recombination
of the bicyclic 1,2-dithietane 160. Under UV irradiation, this unstable structure can
lose sulphur leading to the formation of a C¼C bond (Scheme 60). The initially
attempted transannular process was unsuccessful due to a problematic dithionation
of bis-lactone precursors. Therefore, the reaction was efficiently developed on a
large number of acyclic structures related to the complex target molecule
(Scheme 61) [32].
O
O
CO 2 Me
CHO
O
MeO 2 C
Me
O
O
O
O
O
O
MeO 2 C
HO
H
H
H
H
H
156
SmI 2
MeOH
THF, rt
155
90%
O O
O
H
H
OMe
O
H
SmI 2
O
HO
MeO 2 C
Scheme 59 Simultaneous formation of the F and H rings of gambierol promoted by samarium
diiodide
O
O
O
OMe
O
O
O
S
OMe
S
O
O
S
OMe
S
O
O
S
S
OMe
O
O
OMe
O
O
O
Lawesson's
reagent (3 equiv)
TMTU (3 equiv)
Xylene, 160 °C, 2 h
159
hn
Hanovia
lamp
Ph-Me
70 °C, 2 h
63%
161
HCl (2M)
rt
162
158
160
47%
- S 2
80%
Scheme 60 Access to oxopanone 162 through the formation and cleavage of a 1,2-dithietane
generated under UV irradiation
310
O. Piva