using Sn(OTf) 2 and resulted in the generation of tetrahydrofuran 101 in 89 % yield
as an 11:1 mixture of 2,5-cis- and 2,5-trans-isomers.
3.2 Prins Cyclizations
Although Prins cyclization normally gives tetrahydropyrans, Cho et al. found
that the reaction can give tetrahydrofurans by utilizing propargylsilanes as substrates [28]. For example, the condensation of propargyl alcohol 102 with benzaldehyde in the presence of TMSOTf gave allenyl furan 103 in 91 % yield
(Scheme 28). The reaction was amenable to the use of aldehydes, ketones, and
acetals as coupling partners.
Building on their propargylsilane results, Cho and coworkers found internal
alkynes to also give 2,5-cis-tetrahydrofurans but having a vinyl triflate at the
three-position when TMSOTf was used to promote the reaction (Scheme 29)
[29]. By subjecting optically active propargyl alcohol 104 to benzaldehyde and
TMSOTf, they generated tetrahydrofuran 105 in 84 % yield as a single
CO 2 CH 3
CO 2 Bn
O
CH 2 Cl 2 , rt
(89%)
101 (dr = 11:1)
100
99
N
Ts
CHO
Sn(OTf)2
CO 2 CH 3
BnO 2 C
N
Ts
N
O
O
O
H
N
OH
OH
OH
(+)-Isatisine A
+
Scheme 27 [3+2]-Cycloaddition to the tetrahydrofuran moiety in (+)-isatisine A by Kerr
et al. [27]
Ph
OH
TMS PhCHO, TMSOTf (1.1 equiv)
Et 2 O, -78 °C
91%
O
Ph
Ph
102
(dr = 40:1)
103
Scheme 28 Propargylsilane Prins cyclization to substituted tetrahydrofurans by Cho et al. [28]
OH
PhCHO, TMSOTf (3 equiv)
CH 2 Cl 2 , -78 °C to rt
84%
O
Ph
105 (dr > 99:1)
BzO
H
H
BzO
TfO
104
Scheme 29 Alkyne-Prins cyclization to tetrahydrofurans by Cho et al. [29]
Synthesis of Substituted Tetrahydrofurans
13