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

706 NITRONATES
Table 3.36 The synthesis of 3-halomethyl substituted 5,6-dihydro-4H-oxazines
(517).
518→ 518→516→
(516, 517+517’ 517+517’ Ratio
Entry 517) (518) R 1 R 2 R 3 R 4 R 5 X Yield % Yield % (517:517”)
1 a a H An a H OMe Me Br 54 69 12:1
2 a b H An H OMe Me Cl 66 75 9:1
3 b c H An H OEt H Br 65 75 17:1
4 b d H An H OEt H Cl 68 79 only 517d
5 c e H An H H OEt Br 58 64 6:1
6 d f H An H Ph H Cl 65 68 15:1
7 e g H An H H Ph Cl 67 67 10:1
8 f h H An -(CH2)4- H Br 60 59 only 517h
9 f i H An -(CH2)4- H Cl 51 53 only 517i
10 g j H An H OMe Me Br 63 78 11:1
11 h k H Ph H Me Me Br 53 61 -
12 h l H Ph H Me Me Cl 45 48 -
13 i m H OBz H Me Me Cl 63 72 -
14 j n Me An H OMe Me Cl 21 34 only 517j
a An 4-MeOC6H4.
A convenient procedure for catalytic carbonylation of bromooxazines (520)
giving rise to previously unknown methoxycarbonylmethyl-substituted oxazines
(525) in good yields (538) deserves note, all the more because this is the Þrst
example of catalytic C,C-coupling reactions of ethers of α-halo substituted
oximes.
Functionalized 5,6-dihydro-4H -oxazines are direct precursors of a series of
useful products, for example, of proline derivatives (539), unnatural amino acids
(540), and some alkaloids (541).
The possibility of synthesizing unnatural amino acids from the resulting functionalized
oxazines was exempliÞed by the selective reduction of product (521a)
to amino acid (527) (473).
3.5.5. Silylation of β-Functionalized Aliphatic Nitro Compounds as a
Procedure for the Synthesis of β-Functionalized α-Nitrosoalkenes
In Sections 3.5.3. and 3.5.4, it was demonstrated that silylation of AN sometimes
afford conjugated nitrosoalkenes as unstable intermediates with various
reactivities (503). The generation of scarcely known α-nitroso alkenes, containing
EWG groups in the β position, by elimination of trimethylsilanol from the
corresponding SENAs is of particular interest (see Scheme 3.224).
3.5.5.1. Selected Transformations of β-Functionalized α-Nitroso Alkenes The
reactivity of β-EWG-substituted α-nitrosoalkenes is similar to that of nitroso