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

Intramolecular Formation of Carbon–Carbon Bonds (Cyclizations) 157
Me
O
C(O)SePh
n
SO
O
2Ph
(TMS) 3SiH
Et3B, O2 , r.t. Me
n
O
25 n = 1
n = 2
n = 3
BnO C(O)SePh
O
CO 2 Me
(TMS) 3SiH
Et 3B, O 2
–78 �C
BnO
SO 2 Ph
87%, cis:trans = 17:1
90%, cis.trans = 6:1
63%, cis:trans = 17:1
O
O
H H
CO 2 Me
92 %, cis:trans = 32:1
(7.30)
(7.31)
In order to improve the poor diastereoselectivity of 6-exo-trig cyclizations, the
configuration of a double bond was carefully considered (Reaction 7.32). Indeed,
the Z-vinylogous sulfonate 26 under standard experimental conditions furnished
the expected cyclic ethers with an excellent diastereoselectivity in favour of cis,
thus confirming the relevance of the double bond geometry to the rotamer
population. Indeed, the higher selectivity observed for 26-(Z) is thought to
arise from stronger 1,3-diaxial interactions in the chair-like transition state for
the formation of the minor isomer [35]. This approach has been applied to
cyclization of 27, which afforded the cis-2,6-disubstituted tetrahydropyran-
3-one in 81 % yield as a single diastereoisomer, with close structural requirements
to C16–C26 fragment of the antitumour agent Mucocin (Reaction 7.33) [43].
Improved selectivity in 6-exo-trig cyclization was also found in substrates with a
preexisting ring such as 28, which gave high diastereoselectivity arisen from an
increased rigidity imposed to the transition state (Reaction 7.34) [44].
O
PhO 2 S
PhSe
Ph
O
27
O
OBn
O
SePh
SO2Ph 26-(E)
26-(Z)
OPMB
(TMS) 3SiH O
Et3B, O2 , r.t. Ph O
SO2Ph (7.32)
(TMS) 3 SiH
Et 3 B, O 2 , r.t.
PMB = p-MeOC 6H 4CH 2
85%, cis:trans = 7:1
84%, cis:trans = 35:1
PhO 2 S
O
O
81%
OBn
OPMB
(7.33)