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

CONCLUSION 725
those containing functional groups) over Raney nickel are shown in Scheme
3.284 (402).
Evidently, an increase in steric hindrance around the reduced fragment requires
the presence of ammonium ßuoride in the reaction mixture. It should be noted that
potassium ßuoride has no effect. It is highly probable that ammonium ßuoride
is required for slow elimination of HF, which gradually desilylates the nitroso
acetal fragment thus facilitating its reduction. As can be seen from Scheme 3.284,
many reduction products are derivatives of unnatural amino acids. Since the initial
nitroso acetals can be prepared by silylation of simple acyclic AN, possibilities
have been opened for the synthesis of unnatural amino acids from available AN.
3.5.8.2. Reduction of the Oximino Fragment in Substituted 5,6-Dihydro-
4H-Oxazines Catalytic hydrogenation of substituted dihydro-4H -oxazines
(552), as well as their reduction with sodium cyanoborohydride (553), were studied
in sufÞcient detail and were used in several total syntheses. However, the use
of silylation of six-membered cyclic nitronates enables the synthesis of previously
unknown dihydrooxazines containing functionalized substituents at the C-3 and
C-4 atoms from easily available precursors.
New selected possibilities created by reduction of new types of oxazines (603)
in the synthesis of polyfunctional products (473, 554) are presented in Scheme
3.285. Coupling the reduction of the C=N bond with NaBH3CN followed by
hydrogenation of the N–O bond or a one-step catalytic hydrogenation, and the
double-bond transfer from the C(3) to the C(4) position, enables the synthesis
and detection of 14 types of reduction products. In some cases, reduction is
stereoselective.
3.6. CONCLUSION
To conclude, silylation of AN and their derivatives made it possible to substantially
extend the reactivity of AN. The reactions of AN with nucleophilic,
electrophilic, and radical reagents at both the α- andβ-carbon atoms are summarized
in Chart 3.24.
New Synthones:
a N(O-)2
g
b
b
•
b
N-OH
N-OX
N-OSi
N-OH
g
NO2 b a
H
New Products and Intermediates:
Nitrosoacetales
a-Nitroso-alkenes
Conjugated ene-oximes
Conjugated ene-nitriles
Functionalized nitrones
Chart 3.24 New reactivities of aliphatic nitro compounds realized via their silylation.