L Le ev ve el l 1 1 C Ca as se e 1 13 3
O Ox xy yg ge en n V Ve er rs su us s S Su ul lf fu ur r S St ta ab bi il li iz za at ti io on n
o of f C Ca ar rb be en ni iu um m I Io on ns s
K Ke ey y p po oi in nt t: : K Ki Ki Ki K Ki Ki K Kin i in in in i i e et ti ti ti t ti ti t tic ic ic ic i i I Is Is I Is Is Is I I o ot to to to to to t top op op pe e E Ef Ef E Ef E E f
f ff ff ff ff f f e
fe f fe f fe fe f fec ct ts ts t ts ts ts ts t t
X
OR
X
XH
OR
X = O, S
X
OH
H 2 O
H 2 O
O Ox xy yg ge en n V Ve er rs su us s S Su ul lf fu ur r S St ta ab bi il li iz za at ti io on n o of f C Ca ar rb be en ni iu um m I Io on ns s
The general mechanistic features of the acid-catalyzed hydrolysis of glycopyranosides have been recognized for about 30 years. It is generally accepted that
the hydrolysis occurs via specific acid catalysis. The rate-determining step involves exocyclic C-O bond cleavage and an oxacarbenium ion is formed (Scheme
n
13.1).
O
OR
H
O
OR
H
-ROH
O
H 2 O
O
OH
slow
fast
Scheme 13.1
In contrast, little is known about the mechanism of hydrolysis of 5-thioglycopyranosides, apart from the fact that they hydrolyze several times faster than the
corresponding glycopyranosides. As the relative capabilities of sulfur and oxygen
atoms to stabilize an adjacent carbenium ion center has been a controversial subm
ject, an alternative hydrolysis mechanism for both 5-thio- and glycopyranosides
has been proposed. This alternative considers that the reaction involves specific
acid-catalyzed protonation on the ring heteroatom followed by rate limiting C-X
bond cleavage (Scheme 13.2).
O Ox xy yg ge en n V Ve er rs su us s S Su ul lf fu ur r S St ta ab bi il li iz za at ti io on n
o of f C Ca ar rb be en ni iu um m I Io on ns s
K Ke ey y p po oi in nt t: : K Ki Ki Ki K Ki Ki K Kin i in in in i i e et ti ti ti t ti ti t tic ic ic ic i i I Is Is I Is Is Is I I o ot to to to to to t top op op pe e E Ef Ef E Ef E E f
f ff ff ff ff f f e
fe f fe f fe fe f fec ct ts ts t ts ts ts ts t t
X
OR
X
XH
OR
X = O, S
X
OH
H 2 O
H 2 O
O Ox xy yg ge en n V Ve er rs su us s S Su ul lf fu ur r S St ta ab bi il li iz za at ti io on n o of f C Ca ar rb be en ni iu um m I Io on ns s
The general mechanistic features of the acid-catalyzed hydrolysis of glycopyranosides have been recognized for about 30 years. It is generally accepted that
the hydrolysis occurs via specific acid catalysis. The rate-determining step involves exocyclic C-O bond cleavage and an oxacarbenium ion is formed (Scheme
n
13.1).
O
OR
H
O
OR
H
-ROH
O
H 2 O
O
OH
slow
fast
Scheme 13.1
In contrast, little is known about the mechanism of hydrolysis of 5-thioglycopyranosides, apart from the fact that they hydrolyze several times faster than the
corresponding glycopyranosides. As the relative capabilities of sulfur and oxygen
atoms to stabilize an adjacent carbenium ion center has been a controversial subm
ject, an alternative hydrolysis mechanism for both 5-thio- and glycopyranosides
has been proposed. This alternative considers that the reaction involves specific
acid-catalyzed protonation on the ring heteroatom followed by rate limiting C-X
bond cleavage (Scheme 13.2).