3.3 Reductive Cyclisation of Hydroxyl Ketones
Access to cyclic ethers can be easily achieved by reductive cyclisation of
hydroxyketones. Combination of Et 3 SiH with a strong Lewis acid has been widely
used for this purpose [32]. Tripathi and Kumar have used these conditions to
synthesise (+)-isolaurepan 49 (Scheme 20). Starting from the enantioenriched
starting material 48, they noticed the exclusive formation of the 2,7-disubstituted
oxepane 49 in 84 % yield [33–35].
OH
OBn
O
OMPM
BnO
O
OMPM
H
OH
H
(Bu 3 Sn) 2 O
Ph-Me, D
1)
2) Zn(OTf) 2
40
41
75%
O
Cl
H
Br
H
(+)-Rogioloxepane
(MPM = 4-methoxybenzyl)
Scheme 16 Access to the core structure of rogioloxepane by intramolecular ring opening of
epoxide 40
H
O
O
O
t-BuO
H
O
O
O
O
H
OAc
O
H
1) BF 3 .OEt 2 , CH 2 Cl 2
-40 °C,10-30 min
then H 2 O
2) Ac 2 O, Et 3 N, CH 2 Cl 2
42
43
60%
Scheme 17 Double-epoxide ring opening in a cascade process promoted by an L.A.
OH
O
OBn
O
O
Bn
O
Bu 3 Sn
L.A.
O
HO
OBn
39
38
(Bu 3 Sn) 2 O
Ph-Me
1)
2) Yb(OTf) 2
96%
Scheme 15 Lewis acid catalysed intramolecular ring opening of an epoxyalcohol
292
O. Piva