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

494 NITRONATES
Table 3.7 Physical characteristics of SENAs
Entry Nitronate
1
2
3
4
5
6
7
8
9
10
11
N
OSiMe3
N
N
N
O
OSiMe3
O
OSiMe3
O
OSiMe2Pr i
N
O
OSiMe2Bu t
O
b.p. b.p.
◦
C/torr RefEntry Nitronate
◦
C/Torr Ref.
65–67/17 205 12
67/17 205 13
72–77/18 205 14
43/0.5 217 15
120–140/10 178 16
OSiMe3 65/0.01 178 17
N
O
OSiMe3 70–72/0.35 250 18
N
O
OSiMe2Bu
N
t140/0.005
178 19
O
OSiMe2Bu
N
t90–100/
O
0.01
178 20
OSiMe3 49–51/0.5 171 21
Me3SiO N
N
O
OSiMe3 63–65/0.3 251
Ph N
O
a enantiomerically pure [α d] = + 119 ◦
Ph
MeO 2C
MeO 2C
MeO2C
N
OSiMe3
N
O
OSiMe3 N
O
OSiMe2Bu t
O
OSiMe 3
N
O
85–87/0.5 205
62/0.5 252
80–90/20 178
92–94/1 186
OSiMe2Bu N
t140–150/
O
0.01 82/0.03
178, 253
OSiMe3 110/10 217
MeO2C N
O
OSiMe2Bu
O
N
O
t 67–68/0.03 254
O
OSiMe2Bu
Me2N
N
O
t 116/0.05 202
O
OSiMe2Bu Ph N
t 150/0.01 (m.p. 178
O
68–69)
OSiMe2Bu N
t
O
110/0.03 a 253
4,5–Disubstituted 4,5-dihydroisoxazole N-oxides and 5,6-dihydro-[4H ]oxazines
containing at least two substituents at the C-4,C-5, and C-6 atoms have
stereoisomers, which can as a rule be separated by liquid chromatography. Several
strategies for diastereo- and enantioselective synthesis of cyclic nitronates
are known. These strategies will be discussed in detail in Sections 3.4.4 and 3.4.5.
For unknown reasons, unsubstituted cyclic nitronates are much less stable than
their substituted analogs. However, six-membered nitronate are stable (see entry
20 in Table 3.8). Apparently, fragmentation of these compounds is associated
with initial cleavage of the weak endocyclic N–O bond.
Chart 3.3 shows selected physical characteristics of rare strained fourmembered
as well as seven-membered cyclic nitronates.