"Front Matter". In: Organosilanes in Radical Chemistry - Index of

154 Consecutive Radical Reactions
MeO 2 C CO 2 Me
Me SiR 3
MeO OMe
14
AIBN, 80 �C
R 3 Si = i-Pr 3 Si, t-BuMe 2 Si, PhMe 2 Si
O (TMS) 3SiH Si(TMS) 3
O
AIBN, 90 �C
SiR 3
MeO 2 C CO 2 Me
+
47-84%
15 16 17
67%, 16:17 = 70:30
O Si(TMS) 3
7.3.2 CONSTRUCTION OF CYCLIC ETHERS AND LACTONES
(7.22)
(7.23)
Stereocontrolled synthesis of cyclic ether moieties gives access to a series of
biologically important targets. Methods that utilize radicals as the reactive
intermediates have gained considerable prominence in recent years, primarily
due to the excellent stereocontrol that could be obtained. A thorough study of the
formation of five- to seven-membered cyclic ethers has been performed by the
cyclization of vinylogous sulfonates. An example is given in Reaction (7.24) [35].
Treatment of 18 with (TMS) 3SiH and Et3B at room temperature, in the presence
of air, gave the cyclized product with a high cis-diastereoselectivity, due to the
exo mode and with decreased yields depending on the ring size increases.
Several approaches dealt with the preparation of tetrahydrofuran derivatives,
starting from optically active compounds. The sulfoxide 19 was debrominated
by (TMS) 3SiH and AIBN in refluxing benzene and behaved in a totally regioselective
5-exo mode affording the ring closure. The diastereoselective outcome
of the radical cyclization depended strongly on the configuration of the double
bond, which reacted in the s-trans conformation during the cyclization, i.e., the
attacking alkyl radical is in anti to the bulky p-tolyl group (Reaction 7.25) [36].
Me
Br
n
O
SO2Ph (TMS) 3SiH
Et3B, O2, r.t. Me
n
O
18 n = 1
n = 2
n = 3
SO 2 Ph
99%, cis:trans �19:1
90%, cis.trans �19:1
34%, cis:trans �19:1
(7.24)