Chemistry Reference
In-Depth Information
Ph
O
Ph
O
NO
h
ν
O
N
N
(OC)
5
Cr
N
H
O
8.132
8.133
Scheme 8.36
R'
Ph
Ph
O
R'
O
RO
2
C
NH
2
O
N
N
h
ν
RO
2
C
H
(OC)
5
Cr
R
R
8.134
8.135
Scheme 8.37
Ph
O
Ph
O
1.
n
-BuLi
2. PhCHO
N
N
CO,
h
ν
(OC)
5
Cr
(OC)
5
Cr
Me
HO
8.136
8.137
Ph
Ph
O
Ph
O
(OC)
5
Cr
N
O
N
O
•
O
HO
8
1
8
8
1
Ph
Ph
Scheme 8.38
-lactams
8.131
are obtained.
33
If a carbene
8.132
with a chiral auxiliary is used, high diastereoselectivity may be obtained
(Scheme 8.36).
34
Electron-rich alkenes react to give cyclobutanones
8.130
, showing the same patterns of
reactivity and stereoselectivity as are observed with ketenes generated in classical ways.
35,36
While alkoxy
carbenes work well, the more electron rich
N
-alkyl carbenes do not give cyclobutanones. The less electron
rich
N
-aryl and
N
-pyrrolocarbenes, on the other hand, do react.
37
In both cases, the nitrogen lone pairs
are delocalized over aromatic systems. The reaction with aldehydes to give
Alcohols give esters
8.128
, and amines give amides
8.129
. In the presence of imines,
-lactones works best in an
intramolecular fashion.
38
Photolysis of amino carbenes in the presence of alcohols results in the formation of amino acid derivatives
(Scheme 8.37).
39
Once again, the use of a carbene
8.134
with a chiral auxiliary gives useful stereoselectivity.
Esters of amino acids and even short peptide chains can be used as the nucleophile in a highly unusual way
to build up peptides.
40
Double diastereoselectivity may be observed as a result of the chiral centres in both
the carbene and the nucleophile.
The carbene photochemistry may be combined with the enolate-like chemistry (Scheme 8.38).
41
Both
reactions are capable of showing high diastereoselectivity.
