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

H
H
REACTIONS OF DIPOLAR 1,3-CYCLOADDITION ([3 + 2] CYCLOADDITION) 349
O
R
anti-exo
N
O
O
H
H
OBu t
OBu t
616 R = H
617 R = CH 2OAc
Bu t O
Bu t O
Bu t O
Bu t O
O
O
O
N
O
N
O
O
(a)
O
R
O
O
D7a:
R = H
D7c : R = CH2OAc O CH 2OAc
(b)
O
Bu t O
N
O
+
−
614
Scheme 2.270
CH2OAc
CH 2OAc
O CH 2OAc
OBu t
O
CH2OAc
D7d
AcOH2C
AcOH 2C
O
O
O
N
O
O
N
O
CH2OAc
H
O
N
syn-exo
H
O
OBu t
H
618
OBu t
O
OBu t
O
H
H
OBu t
OBu t
OBu t
Fig. 2.41 Schematic pathway of 1,3-dipolar cycloaddition of nitrone 614 to lactones D7c
(a) and D7d(b).
The 1,3-dipolar cycloadditions of nitrones (551), (595), (614), (615) and
their enantiomers (595 ent), (614 ent), (615 ent) (Fig. 2.40) to α,β-unsaturated
γ-lactones, such as achiral D7 g and D-glycero D7 h, provide an interesting
example of double asymmetric inductions. The reactions are kinetically controlled.
However, on heating and at longer reaction times, the reversibility of the
cycloaddition (595 + D7 h) was observed, and the presence of a more stable thermodynamic
product (620) was detected. Moreover, in the case of lactone D7 h, a