The transformation of thioacetals into mono- or bis-sulfoxides presents intriguing stereochemical possibilities. In a symmetric thioacetal of an aldehyde other than
formaldehyde, the sulfur atoms are enantiotopic and each of them contains two
diastereotopic nonbonded pairs of electrons (Scheme 2.159). Unfortunately, most
of the products from asymmetric oxidation of thioacetals are of low to moderate
optical purity [1339, 1340]. Two exceptions, however, are worth mentioning.
Oxidation of 2-tert-butyl-1,3-dithiane by Helminthosporium sp. gave the (1S,2R)monosulfoxide in 72% optical purity [1341] and formaldehyde thioacetals were
oxidized by Corynebacterium equi to yield (R)-sulfoxide-sulfone products [1342]
with excellent e.e.
In order to avoid poor recoveries of the water-soluble sulfoxide from considerable amounts of biomass when using whole cells, isolated (flavin-dependent)
monooxygenases have been successfully employed for thioether-oxidations
together with NAD(P)H-recycling (for biocatalytic sulfur oxidation using peroxidases, see Sect. 2.3.4). In particular, cyclohexanone monooxygenase (CHMO)
[1343–1345], phenylacetone monooxygenase (PAMO) [1346] and
hydroxyacetone monooxygenase (HAPMO) [1347, 1348] were shown to be
most useful. Overall, recoveries and stereoselectivities were high, and some
enzymes exhibited stereo-complementary properties on selected substrates
(Scheme 2.160).
R
2
S
S
R 1
t-Bu
S
S
O
t-Bu
S
S
R
3
H
R 2
S
R
1
S
O
O
O
R 2
S
S
R
1
Corynebacterium equi
Helminthosporium sp.
O 2
O 2
R
S
R
R 1
R 2
e.e. [%]
n-C 4 H 9
n-C 4 H 9
>95
CH 3
Ph
>95
Scheme 2.159 Microbial oxidation of dithioacetals
188
2 Biocatalytic Applications
formaldehyde, the sulfur atoms are enantiotopic and each of them contains two
diastereotopic nonbonded pairs of electrons (Scheme 2.159). Unfortunately, most
of the products from asymmetric oxidation of thioacetals are of low to moderate
optical purity [1339, 1340]. Two exceptions, however, are worth mentioning.
Oxidation of 2-tert-butyl-1,3-dithiane by Helminthosporium sp. gave the (1S,2R)monosulfoxide in 72% optical purity [1341] and formaldehyde thioacetals were
oxidized by Corynebacterium equi to yield (R)-sulfoxide-sulfone products [1342]
with excellent e.e.
In order to avoid poor recoveries of the water-soluble sulfoxide from considerable amounts of biomass when using whole cells, isolated (flavin-dependent)
monooxygenases have been successfully employed for thioether-oxidations
together with NAD(P)H-recycling (for biocatalytic sulfur oxidation using peroxidases, see Sect. 2.3.4). In particular, cyclohexanone monooxygenase (CHMO)
[1343–1345], phenylacetone monooxygenase (PAMO) [1346] and
hydroxyacetone monooxygenase (HAPMO) [1347, 1348] were shown to be
most useful. Overall, recoveries and stereoselectivities were high, and some
enzymes exhibited stereo-complementary properties on selected substrates
(Scheme 2.160).
R
2
S
S
R 1
t-Bu
S
S
O
t-Bu
S
S
R
3
H
R 2
S
R
1
S
O
O
O
R 2
S
S
R
1
Corynebacterium equi
Helminthosporium sp.
O 2
O 2
R
S
R
R 1
R 2
e.e. [%]
n-C 4 H 9
n-C 4 H 9
>95
CH 3
Ph
>95
Scheme 2.159 Microbial oxidation of dithioacetals
188
2 Biocatalytic Applications