Stereospecific Substitution Reactions of Epoxy Sulfides
37
O
SPh
R
H
H
O
R
H
H
Al
Me
Me
Me
SPh
R
O
H
Al
Me
Me
Me
S
Ph
Me 3 Al
O
R
SPh
H
H
Me 3 Al
O
H
R
H
Al
Me
Me
Me
SPh
R
O
H
Al
Me
Me
Me
S
Ph
R
SPh
OH
Me
R
S Ph
OH
Me
1
3
2
4
14
15
16
17
Scheme 6.6
Are the solvent effects in agreement with th t e proposed mechanism? The reaction rate is increased when more polar dichloromethane is employed instead of
m
less polar hexane. These results are reasonab a le, as polar intermediates (like episulfonium ions) are better stab a ilized in polar solvents.
In Summary
Me 3 Al acts as a nucleophile in the regioselective and stereospecific ring-opening
of 1-(phenylthio)-2,3-epoxialkanes 1 and 2. The reaction proceeds through a
neighboring group mechanism with formation of an episulfonium ion intermediate
and leads to the products with retention of the configuration at the C2 position.
Additional Comments
This problem is based on the work by Sasaki M, Tanino K, Miyashita M (2001) J.
Org. Chem. 66:5388-5394 and on the work by Liu Q, Simms MJ, Boden N,
Rayner CM (1994) J. Chem. Soc. Perkin Trans. 1 1363-1365.
Subjects of Revision
Neighboring group participation. Epoxides: ring-opening mechanisms. Regioselectivity. Stereospecificity.