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

368 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
(−) 682 LiAlH4/THF 65ºC, 1.5 h
O
BH 3 SMe2, THF
65ºC, 1 h
81%
N
O
H
OH
CO 2Me
O
(+) 685
N
O
684
H
OH
1. CH 3SO 2Cl
CH2Cl 2, rt, 15 h
2. DBU
20ºC, 0,5 h
81%
OH
O
HCl/CH 3OH
20ºC, 1 h
HO
H
Cl− Scheme 2.294
N
O
2. HCl, CH3OH
20ºC, 1h
82%
H
(+) 686
+
N
OH
HO
H
Cl −
N
CO 2Me
1. DIBAL-H
CH2Cl 2, 0ºC, 0.5 h
H OH
(−) 688 HCl
+
OH
H
OH
(−) 687 HCl
undergo facile, rearrangements and provide ready access to a variety of novel
heterocyclic systems in synthetically useful yields.
2.8.2.1. Intramolecular Reactions Intramolecular 1,3-dipolar cycloadditions of
nitrones with alkynes are illustrated in Schemes 2.296, 2.297, 2.298 (818, 819).
Heating of N -methyl-α-[2-(3-phenyl-2-propynyl-1-oxy)benzylidenyl]nitrone
(707) in toluene at 120 ◦ C for 2 hours gave a single rearranged product, the structure
of which has been assigned as 3-benzoyl-4-N -methylamino-2H -1-benzopyran
(710). In these reactions, intramolecular dipolar cycloaddition produces
4-isoxazoline (708) which undergoes preferential N–O bond scission affording
diradical (709). Subsequent rearrangement, by way of a 1,2-hydrogen shift ultimately
gives benzopyran (710). Alternatively, the isomerization of (708) to(710)
could follow an acid catalyzed path.
OH