Chemistry Reference
In-Depth Information
R
1
OEt
OEt
EtO
OEt
518
R
2
O
TsOH
Me
CH
2
Cl
2
rt
N
Ph
N
Ph
N
Ph
Ar
O
O
O
516
517
519
Me
Me
Ar
R
1
R
2
520
(%)
521
(%)
OEt
-H
+
OEt
Ar
O
OMe
Me
27
43
N
N
Me
OMe
43
36
Ph
O
Ph
520
521
SCHEME 10.85.
observed products. Unfortunately, the appeal of this transformation is tempered by
the lack of selectivity observed during this final step, which may reflect a nonoptimal
stereoelectronic arrangement for involvement of the imine nitrogen in ring closure to
the five-membered ring.
Falvey and Cramer have demonstrated that the parent diphenyl nitrenium ion
523
, generated through the photolysis of
N-
amino-2,4,6-trimethylpyridinium tetra-
fluoroborate
, also undergoes reaction with electron-rich alkenes, including
ketene acetals and silyl enol ethers (Scheme 10.86).
201
In the presence of the latter
p
522
522
524
525,
526a
, the
products resulting from
para
-addition,
N
-addition, and annulation. The less reactive
enol ethers display more selectivity for attack at the ring positions of
-nucleophiles, photolysis of
generates a mixture of
,
and
and do not
undergo
N
-addition. While Falvey has also observed annulation between cyclo-
pentadiene and
523
523
, the yield of the formal [2
þ
3] cycloadduct in this case proved to
be very low (3%).
202
Since its discovery by Sheppard
et al.
at Dupont,
203
who prepared it by the
addition of HCN to cyanogen, diiminosuccinonitrile (
, DISN) has proven to be a
useful reagent for the synthesis of heterocycles through condensation and displace-
ment reactions (Scheme 10.87).
204
For example, treatment of electron-rich olefins,
such as 1,2-dimethoxyethylene (
527
), with DISN in acetonitrile generates tetrahy-
drodicyanopyrazines, which in the case of
528
529
undergo elimination of methanol to
provide 2,3-dicyanopyrazine (
).
Although it is tempting to assume that the transformation shown in Scheme 10.87
proceeds via a concerted, inverse-demand [4
530
2] cycloaddition (Scheme 10.88,
pathway a), work by Fukunaga suggests that this reaction may actually take place in
such a way that in its approach to the diene, the 1,4-diazabutadiene unit in
þ
527
is
progressively polarized to zwitterionic nitrenium ion
532
, which actually undergoes
