Chemistry Reference
In-Depth Information
Table 5.12
R
H
O
NMe
2
O
O
R
O
NMe
2
R
O
NMe
2
S
(OC)
3
Co
Co(CO)
3
S
S
+
NMO (6 equiv),
MeCN, 0 °C or rt, 2-26 h
(
R
)-
84
85a-h
8
a-h
R
Yield (%)
Products
85
:
86
Ratio
n
-Bu
85a
,
86a
74
13.3:1
t
-Bu
55
> 49:1
85b
,
86b
PhCH
2
85c
,
86c
58
13.3:1
p
-Tol
85d
,
86d
49
13.3:1
SiMe
3
85e
,
86e
59
> 49:1
CH
2
OSi
i
Pr
3
85f
,
86f
62
13.3:1
CH
2
CH
2
OSi
i
Pr
3
85g
,
86g
66
> 49:1:1
(CH
2
)
3
Br
85h
,
86h
68
> 49:1:1
omers
85
were the thermodynamically more stable ones. In order to highlight the ap-
plicability of the method in natural product synthesis, the authors reported the efficient
conversion, in three steps, of the cyclopentenone
85f
(separated from its epimer
86f
by
trituration of the mixture with cold hexane) into the natural antibiotic (-)-pentenomycin I
(Scheme 5.58).
58a
The dihydroxylation of
85f
with the stoichiometric pair osmium tetroxide/
N,N,N
,N
-
tetramethylethylenediamine was highly diastereoselective, and took place exclusively on
the less hindered face of the double bond. The resulting stable osmate ester
87
underwent
a clean sulfoxide pyrolysis in refluxing toluene to afford the enone-osmate diester
88
.
Acid hydrolysis of the ester and of the silyl protecting group provided the natural (-)-
pentenomycin I in very high enantiomeric purity. Subsequently,
85f
was also converted
into the (-)-aminocyclopentitol moiety of bacteriohopanetetrol ether, a triterpenoid found
in themembrane of several bacteria, bymeans of an eight-step stereocontrolled sequence.
58b
As it happened in the intramolecular PKR of 1-sulfinyl enynes,
32
the chiral controller cannot
be recovered after the elimination step.
The intermediacy of an internally chelated alkyne-vinyl sulfoxide complex in the reaction
mechanism (Figure 5.2) was supported by DFT calculations.
58b
