Chemistry Reference
In-Depth Information
PIFA
MeOH
8'
4 steps
2
O
N
O
N
O
NH
OMe
CH
2
Cl
2
-78 to -20°C
MeO
O
MeO
9'
MeO
455
456
(90%)
457
(38%)
O
O
1. O
3
/O
2
; Me
2
S
1. H
2
, Pd/C
2. Mo(CO)
6
O
N
O
N
2. CH
3
CHI
2
CrCl
2
MeO
H
458
(40%)
(±)-Adalinine (
412
) (98%)
SCHEME 10.73.
stereogenic center of the target molecule. Ozonolytic cleavage of the dienone moiety
of
, which, through a
sequence of functional group manipulations and deprotection steps, was converted to
(
452
and
in situ
reduction then yielded
b
-hydroxy ester
454
413
).
Wardrop and Landrie, through their total synthesis of the piperidine alkaloid
)-dysibetaine (
(
), have demonstrated that the azaspirane dienone ring can also be
viewed as a latent 1,6-dicarbonyl group.
172
This natural product was isolated from
the abdomen of the European two-spotted ladybird beetle,
Adalia bipunctata
and
found to be toxic to amphibians, reptiles, birds, and other insects.
173
Synthesis of
)-adalinine (
412
412
commenced with the cyclization of
, which, through a four-
step sequence featuring reduction and regioselective dehydration, was converted to
457
455
to form dienone
456
. Ozonolytic cleavage of this cyclohexene and chemoselective Takai ethyl-
idenation of the resulting 1,6-ketoaldehyde then generated
, which was readily
converted to the natural product through hydrogenation and N
458
O bond cleavage
(Scheme 10.73).
10.8 INTERMOLECULAR ALKENE ADDITION OF STABILIZED
NITRENIUM IONS
10.8.1 Alkene Addition of N-Stabilized Nitrenium Ions
In spite of the fact that the reactions between alkenes and electrophiles, including
halogens, pseudohalogens, carbenes, and nitrenes, are among the most fundamen-
tally important and widely studied processes in organic chemistry, the involvement
of singlet nitrenium ions in this context has received only a fraction of the attention
garnered by these other electrophiles. In common with carbenes and nitrenes, their
isoelectronic congeners, singlet nitrenium ions
459
undergo concerted cycloaddition
(Scheme 10.74).
48
The potential
importance of this transformation is apparent when one considers the recent
with alkenes to generate aziridinium ions
460
