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

O
HO
REACTIONS OF DIPOLAR 1,3-CYCLOADDITION ([3 + 2] CYCLOADDITION) 333
O
Bn
Bn
H 2NOC
N O
540 b
N O
N S
O
541
CN
N
S
R = Et (74%)
R = Me (80%)
100%
N
Bn
NH3
MeOH
CO 2R
OH
+
H3N CO2Et
HS
−
Cl
MeOH
(76%) N O
Bn
1. EtOH
p-TosOH
2. SiO2
NaIO4
3. NaBH4
MeOH
Scheme 2.254
O
O
O
O
Bn
N O
N
MnO2
S
CO2R
R = Et (70%)
R = Me (6%)
N
S
CO2R
R = Et (90%)
R = Me (88%)
cycloaddition reactions. Using (544a) the isoxazolidines were obtained in high
yield and with high regio-, endo-, and enantioselectivities (Table 2.23) (755–758).
A variety of imidazolidinium and pyrrolidinium salts (Fig. 2.38) have been
found to catalyze the reaction between nitrones and cyclic α,β-unsaturated aldehydes,
affording bicyclic adducts with high diastereoselectivity and enantioselectivity
(Scheme 2.256) (759).
1,3-Cycloaddition of α-aryl-N -phenylnitrones to the C16-C17 π-bond in
16-dehydropregnenolone-3β-acetate (545) involves only the minor rotamer
(E-form) of the nitrones. It proceeds regio-, stereo- and π-facial-selectively to
give steroido[16,17-d]isoxazolidines (546) in high yield (Scheme 2.257), (Table
2.24) (760). Similarly the cycloaddition of α,N -diphenylnitrones proceeds with
Þve-membered heterocyclic enones (761).
Regio- and stereospeciÞc 1,3-cycloaddition of di-tert-butylated acyl nitrone
(548), generated in situ from (547), with Z -2-cyclodecanone and subsequent
aromatization is the key step in the synthesis of biomimetic pyridomacrolidin