Chemistry Reference
In-Depth Information
AcHN
COOEt
AcHN
COOMe
COBHN
COOEt
99% ee
99% ee
98% ee
AcHN
COOEt
AcHN
COOEt
N
COOEt
92% ee
95% ee
Figure 7.18.
Asymmetric hydrogenation of cyclic β - dehydroamino acid derivatives.
2
- dehydroamino
acids. Robinson, Jackson, and others reported the synthesis and hydrogenation of a
series of substrates of this type with moderate selectivities [138]. A few examples of
phthaloyl -
An interesting variation of the substrates described above is the
β
2
-dehydroamino acid derivatives were hydrogenated with up to 99% ee using
an Rh-TangPhos catalyst [139]. A series of
β
2
-amino acids were synthesized via asym-
β
metric hydrogenation of
-(phthalimidomethyl) acrylates [140]. Up to 99% ee was
achieved using Rh - BoPhoz - type ligand complexes (Eq. 7.3 ).
α
CO
2
R
CO
2
R
N
N
Rh-(
R,R
)-Me-DuPhos
R'
O
R'
O
>99% ee
37% conv.
(7.4)
CO
2
Et
Rh-(
R,R
)-Me-DuPhos
CO
2
Et
N
N
5 atm, 25°C
O
OMe
O
OMe
95.5% ee
100% conv.
(7.5)
Several other unusual substrates were also worth of note. Asymmetric hydrogenation
of a cyclic substrate was studied in the synthesis of homoproline derivative [141]. Among
the catalyst screened, Rh-DuPhos was found to be the most selective catalyst (> 99% ee)
although with a low 37% conversion (Eq. 7.4). A structurally similar substrate was docu-
mented in a patent by Solvay [142]. Good enantioselectivities and conversions were
observed under mild conditions (Eq. 7.5).
7.3.1.3. Enamides
Chiral amines are another important class of compounds that are
frequently employed as chiral auxiliaries, key building blocks, resolving agents, and
