Chemistry Reference
In-Depth Information
product. Furthermore, cyclization of amide substrate proceeds in good yield without
olefi n migration.
The enantioselective spiro-ring construction is an important issue because many
natural compounds have chiral spiro centers [78]. Some examples of catalytic spiro
cyclizations have been reported by asymmetric intramolecular Mizoroki-Heck reactions
[79]. In spite of a similar but higher potential, transition metal-catalyzed ene-type car-
bocyclization has never been applied to asymmetric spiro cyclizations. Spiro cyclizations
can be catalyzed by the cationic (
S
,
S
) - skewphos - Rh(I) complex (Scheme 8D.49 ). Cycli-
zation of the six-membered ring is executed to give moderate yield and high enantiose-
lectivity, accompanied with olefi n-migration product in high enantioselectivity. The
reactivity increased dramatically at 80°C, and the cyclization completes within only
40 min to afford olefi n-migration product in high enantioselectivity (44%, 91% ee). The
spiro cyclization is also attained with pyran amide compound. Spiro cyclization proceeds
to provide the desired spiro amide-pyran as an almost sole product with a high level of
enantioselectivity (Scheme 8D.50 ).
CO
2
Me
[Rh{(
S,S
)-skewphos}]
2
(SbF
6
)
2
(5 mol %)
n
CH
2
Cl
2
O
n
CO
2
Me
CO
2
Me
n
+
O
O
97% ee (16%)
91% ee (44%)
88% ee (51%)
n
= 2
n
= 2
n
= 4
r.t., 46 h
80
88% ee (53%)
67% ee (4%)
88% ee (12%)
C, 40 min
°
40
°
C, 17 h
Scheme 8D.49.
O
O
CO
2
Me
[Rh{(
S,S
)-skewphos}]
2
(SbF
6
)
2
(5 mol
%)
CO
2
Me
CH
2
Cl
2
, r.t., 36 h
Ts
Ts
94% ee (72%)
Scheme 8D.50.
The
tropos
or
atropos
nature of BIPHEP-Rh complexes at room temperature
critically depends on the amines complexed therewith. The aliphatic DPEN (1,2-
diphenylethylenediamine) complex is
atropos
, while the aromatic DABN complex is
tropos
. However, at 5°C or below, even amine-free BIPHEP-Rh complexes are
atropos
and hence can be used as chiral catalysts. Therefore, ene-type cyclization in the presence
of enantiopure BIPHEPs-Rh/DABN complexes
31
and
32
achieves the high level of
