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

Intramolecular Formation of Carbon–Carbon Bonds (Cyclizations) 155
O
S
O Ar
Br
19
(TMS) 3 SiH
AIBN, 6h
80� C
19-(E)
19-(Z)
O
O
S
Ar +
20 (1R,3S) 21 (1R,3R)
O
93% (20:21 = 82:18)
90% (20:21� 2:98)
O
S
Ar
(7.25)
cis-2,5-Disubstituted tetrahydrofurans and cis-2,6-disubstituted tetrahydropyrans
are conveniently formed by radical cyclization of b-alkoxyacrylates. A
strategy using b-alkoxymethacrylate 22 led to the stereoselective preparation of
the benzyl ether of (þ)-methyl nonactate, resulting from exclusive formation of
the threo product (Reaction 7.26) [37]. It is worth mentioning that in this
strategy the ‘cis-2,5’ selectivity in the forming tetrahydrofuranyl ring is coupled
with the ‘threo’ selectivity at the exocyclic a sites (23), and it should offer easy
access to a number of natural products.
OBn
O
22
I
CO 2Me
R
(TMS) 3SiH
Et 3B, −20 �C
H
O H
Me CO2Me OBn
H H
O
90%, de>96%
CO 2 Me
(7.26)
23
In the next example, the strategy for the formation of fused cyclic ethers
utilized the formation of an intermediate a-heterosubstituted carbon radical,
generated by the reaction of (TMS) 3Si: radical with N-(ethoxycarbonyl)-1,
3-thiazolidine derivative [38]. This intermediate gives intramolecular C C
w
bond formation in the presence of proximate 1,2-disubstituted double bonds
(Reaction 7.27). However, when terminal double bonds are used, the hydrosilylation
of the double bond by (TMS) 3SiH can compete with the reduction and
prevent forming the desired C C bond formation.
w
(TMS) 3SiS
S
O
N
CO2Et (TMS) 3SiH
AIBN, 90 �C
H
N
O
H
CO 2 Et
79%, endo:exo = 2.4:1
(7.27)