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

576 NITRONATES
NO2
R
O
N
241
O
R′
NO2
O N O
242
CO 2Me
HCl
R′
CF 3CO 2H HO
Scheme 3.160
O N
N
R′
OH
O
R
Me
H
Me
R′
H
CO 2Me
CO 2Me
CO2Me ~100%
Both N–O bonds in this group of nitroso acetals are cleaved with 20% sulfuric
acid. By contrast, gaseous hydrochloric acid selectively reacts at one N–O
bond and cleaves the ring containing the most electronegative substituent at the
C-5 atom. This reaction pathway can be attributed to higher stabilization of the
negative charge on the corresponding oxygen atom.
However, treatment of nitroso acetal (242) with CF3CO2H leads to selective
N–O bond cleavage in the unsubstituted six-membered ring (49), in spite of
the fact that the Þve-membered ring contains the –N–O–C(CO2Me)- fragment,
which efÞciently stabilizes the negative charge. This result can be attributed to the
anomeric effect, because the pseudo-axial position (related to the Þve-membered
cycle) is preferable for the oxygen atom, involved in the six-membered ring (for
more details, see Section 3.4.3.4.4.).
Very interesting rearrangements of fused bicyclic acetals (243), which occur
by the action of boron trißuoride etherate, were found and studied in detail by
Chlenov et al. (401) (Scheme 3.161).
In each particular case, the rearrangement pathway depends on the nature of
the substituents R and R ′ . All of these rearrangements involve the N–O bond
cleavage in the six-membered ring, but differ in the modes of stabilization of the
resulting formal charges on the oxygen and nitrogen atoms.
Rearrangements of type (a) lead to binding of the nitrogen atom to the C-3
atom and the proton transfer from the latter atom to oxygen. Rearrangements of
type (b) are accompanied by migration of the substituent from C-6 to the nitrogen
atom, the oxygen atom taking the place of this substituent. Rearrangements of
type (c) involve the N–(C-8) and O-(C-6) bond formations accompanied by
elimination of the substituent from C-6. Rearrangements of type (d) involve
the formation of the double bond between the nitrogen atom and the C-6 atom
with the simultaneous migration of the substituent from C-6 to the oxygen atom.
Finally, rearrangements of type (e) are characterized by the formation of bonds
between nitrogen and C-5 and between oxygen and C-6.
In the author’s opinion, the main reaction pathway for nitroso acetals (243)
is determined by the ease of N–O bond cleavage in the O–N–O fragment.