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

156 Consecutive Radical Reactions
Fused cyclic ethers can be derived from appropriately substituted sugars. An
example is given with the stereoselective 5-exo radical cyclization of allylic
2-bromo-2-deoxysugars, in the presence of 1,1,2,2-tetraphenyldisilane as the
radical mediator and AIBN in refluxing ethyl acetate. The corresponding cisfused
bicyclic sugars have been prepared in moderate to good yields (Reaction
7.28) [39].
AcO
AcO
O
AcO Br
O
X
Ph 2 Si(H)Si(H)Ph 2
AIBN, 77 �C
X = H
X = Ph
AcO
AcO
AcO
O
78%
48%
X
O
(7.28)
The chemoselective addition of silyl radical to the double bond of the
b-alkenyloxyenone derivative 24 was instead planned in Reaction (7.29) and
accompanied by a 5-exo-trig radical cyclization leading to the diastereomeric
cyclic ether products [40].
O
O
24
(TMS) 3 SiH
AIBN, 110 �C
O
O
Si(TMS) 3
25% 58%
Si(TMS) 3
+ (7.29)
O
O
Acyl radical cyclization has been also used for the synthesis of oxygencontaining
rings [27], using (TMS) 3SiH as the mediator and phenylseleno
esters as acyl radical precursors. Reaction (7.30) shows again the use of vinylogous
sulfonates, such as 25, which affords the cis-2,5-disubstituted cyclic
derivatives in good yields on exposure at room temperature to (TMS) 3SiH
and Et3B=O2 combination as the initiator, following an exo-mode ring closure
[35]. The selectivity is explained in the standard manner through a chair-like
transition state. Related reactions in which CO2Me replaced the SO2Ph group
behaved similarly [41]. Reaction (7.31) is relevant as the key step for the
enantioselective synthesis of nonisoprenoid sequiterpene ( )-kumausallene
[42]. The radical cyclization at low temperature furnished the cyclic ethers in
a 32:1 mixture in favour of 2,5-cis diastereoisomer, whereas the analogous
reaction at room temperature gave only modest diastereoselectivity (cis:trans
¼ 8:1).