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

REACTIONS OF DIPOLAR 1,3-CYCLOADDITION ([3 + 2] CYCLOADDITION) 355
Table 2.26 Structures of nitrones 637
R 3
C N
R4 + O
H
a
b
c
d
e
R 2
−
R 3 R 4
PhCO-
Ph-
Ph-
Ph-
Ph-
( ) n
N
R 1
CH 2
O
Ph-
Ph-
PhCH2-
Bu t
Me-
+
R 3
H
D9 n 637
O
+
N
R 4
−
Scheme 2.278
R 1
R 1
N
N
( )n
( )n
O
O
638
O N
R 3
O N
For example, cycloaddition of nitrone (643, R 1 = Ph, R 2 = Me) to D10, catalyzed
by chiral phosphine-palladium complexes (Fig. 2.42), gave isoxazolidines (644)
in high yield with high enantioselectivity (794).
Cycloaddition reactions of alkenes D10 with nitrones were also catalyzed by
Yb(OTf)3, by Sc(OTf)3 (795), by chiral 2,6-bis(4 R-trialkylsiloxymethyloxazolinyl)pyridine/Ni(II)
(pybox) (Fig. 2.43) (796a), and by chiral bis(2oxazolinyl)xanthene
(xabox) (Fig. 2.44) (796b).
The utility of a new ßuorine supported chiral auxiliary was established in
a series of catalyzed and uncatalyzed 1,3-dipolar cycloaddition reactions with
diphenylnitrone (637b) (Scheme 2.281) (797). The yields and selectivities of the
cycloadducts (645a–d) compare favorably with those obtained with conventional
Evans-type auxiliaries (798). PuriÞcation of the products was greatly improved
by using ßuorous solid phase extraction (FSPE).
The cyclo-adducts were removed from the auxiliaries by reductive cleavage.
The auxiliary group was readily re-functionalized and reused in subsequent
+
639
R 3
R 4
R 4