Chemistry Reference
In-Depth Information
the corresponding cyclopentenone products
99a-d
at 10-98% yields. A detailed study of
the PKR of various 1,7-enynes,
98
, showed that an increase in the electron-deficiency of
98
reduced the yield to 10% (Scheme 4.26).
O
O
O
H
H
H
(i) Co
2
(CO)
8
(ii) NMO
O
O
O
O
BocN
N
N
Ph
R
Ph
Ph
(3 stpeps)
R
Ph
Ph
Ph
99a-d
97
98a-d
a
:(R=CH
2
= CH-CH
2
OAc)
b
:(R=CH
2
=C(CH
3
)-CH
2
Br)
c
:(R=CH
2
=C(CH
2
OTBS)-CH
2
Br)
d
:(R=CH
2
=C(CH
2
CH
2
CH
2
OTBS)-CH
2
Br
a
:yield65%
b
:yield<50%
c
:yield<50%
d
:yield<50%
Scheme 4.26
Intramolecular Pauson-Khand reaction stereocontrolled by easily available chi-
ral glycinates.
Also in 2008, Honda
et al.
established an alternative stereoselective chiral synthesis of
(
-skytanthine
102
that employed an intramolecular PKR of the corresponding enyne
100
.
29
In this study, it was determined that the configuration at the 4a position of the
cyclized product was stereoselectively constructed by the reflection of the stereochemistry
of the methyl group in the starting enyne. Moreover, it was found that the nitro group was
reduced to an amino group by PKR even when hydrogen-free solvents were used. Although
the mechanism for the reduction of the nitro group was not described, the highest yield
(71%) of the desired product,
101
, was obtained when trimethylamine
N
-oxide was used
as a promoter (Scheme 4.27).
+
)-
Me
Me
H
H
Co
2
(CO)
8
TMANO
.
2H
2
O
THF/H
2
O(3:1)
O
N
NMe
NNs
O
2
H
Me
Me
NH
2
100
101
102
Scheme 4.27
Stereoselective chiral synthesis of (
+
)-
α
-skytanthine
102
by employing an in-
tramolecular Pauson-Khand reaction.
In 2008, Tanyeli
et al.
reported a simple route to the synthesis of camphor-anchored achi-
ral spiro cyclopentenone-pyran systems
106
and
107
with excellent stereoselectivities based
on the intramolecular PKR of camphor-tethered enynes
104, 105
.
30
The camphor-tethered
enynes, which were derived from homoallyl, homomethallyl, and homopropargyl alcohols
103a-c
, were easily prepared in two steps from commercially available (1
R
)-(
)camphor
via the addition of alkylmagnesium bromide followed by consecutive O-propargylation,
O-methanylation, and O-allylation. The addition of alkylmagnesium bromides resulted in
excellent endo-face diastereoselectivity, as expected. The enantiomerically pure enynes,
104-105
, afforded their respective spirocyclic cyclopenta[c]pyran products,
106-107
,in
+
