Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

354 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
X
H
O
N
+
−
H
H
CO2Bu t
endo-635
endo O
exo
Scheme 2.277
H
H
N
+
O
X
−
H
O
CO2Bu t
exo-635
the formation of (635) (Scheme 2.277) was generally detected. Ratio (endo-635)/
(exo-635) changed from 50:50 up to 100:0; (a) 64:36, (b) 66:34, (c) 87:13, (d)
100:0, (e) 67:33 and 55:45, and (f) ratio changed from 50:50 up to 75:25 (depending
on the reaction conditions). Obviously, the main reason of the preferential
endo-approach is the steric interactions induced by the bulky CO2 t Bu group in
the exo approach (Scheme 2.277) (794).
Recently, a series of cycloadducts possessing unusual ßipping modes have
been isolated from the 1,3-dipolar cycloaddition of 3,4,5,6-tetrahydropyridine
N -oxide to piperidides of cinnamic acid and para-substituted cinnamic acids
(791).
Dipolarophiles D9 . Various α-methylenelactams D9 n (n = 0,1,2) with fourto
six-membered rings (Table 2.25), in reactions with nitrones (637), (Table
2.26) afforded good yields of spiro adducts (638) and (639) via regiospeciÞc
1,3-cycloadditions (Scheme 2.278). Owing to the creation of at least two new
asymmetric centers, mixtures of diastereoisomers were obtained (792).
Four asymmetric carbon atoms (structures 641 and 642) were created in the
reaction of nitrone (637a) with bislactam (640) (Scheme 2.279).
Dipolarophiles D10 . Many different catalysts have been used in asymmetric
1,3-dipolar cycloadditions of nitrones (643) tooleÞns D10 (Scheme 2.280) (793).
Table 2.25 Structures of α-methylenelactams D9 n
R 2
( ) n
N
R 1
D9 0
D9 0
D9 0
D9 1
D9 1
D9 2
D9 2
CH 2
O
n R 1 R 2
0
0
0
1
1
2
2
p-MeO-C6H4 p-Me-C6H4p-O2N-C6H4-
-CH3 -CH3
-CH3 Ph-CH2-
-H
-H
-H
-H
Ph
-H
-H