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

NO 2
O
Me R 2
R 1 PhSH
Et 3N cat.
MeCN, 25°C
MeCH(NO2)C(R 2 )C(O)R'
SPh 17a
SYNTHESIS OF NITRONATES 451
Bu t OK
THF; −78°C
PhS
HO
R 1
R 2
O N
R1 Me Me
R
Et Et
2 Me Et Prn yield 5c 54 67
dr 93/7 70/30
Me
75
66/34
60 NO2CH2C(SPh)R
92/8
2COR1 17b O N
PhS
HO
Et
Me CH2CH2CO2Me O
5d
Scheme 3.20
In addition, cyclopropanes (13b) can be subjected to isomerization to form
target nitronates (5b) (see Section 3.2.1.2.1.2)
In spite of the obvious simplicity, this approach has not been examined in practice
until recent years. In the very recent past, French researchers demonstrated
that this approach can be used for the synthesis of Þve-membered nitronates
5c,d starting at least from nitro ketones (17a) containing the PhS substituent at
C-2. These ketones have been generated from the corresponding β-nitroenones
(Scheme 3.20) (69).
Taking into account that nitronates (5c,d) have hemiacetal’s fragment at the
C-5 atom, it is necessary to perform their generation under very mild conditions
(THF, −78 ◦ C). Nitronate (5d) cannot be isolated in individual state. However,
the latter compound can be detected by spectroscopic methods. Nitro ketones
(17b) containing the –CH2NO2 fragment are recovered after attempts to perform
cyclization, which can be attributed to evidence that cyclization is reversible.
Finally, the role of the PhS-fragment in nitro ketone subjected to cyclization
remains unclear. In other words, an interesting process presented in Schemes
3.19 and 3.20 requires a more detailed study.
In 1981, it was demonstrated (70) that anions of nitro compounds can be
involved in C,C-coupling with allyl acetates at the allylic carbon atom with
the use of metal complex catalysis. For many years, this observation did not
come to the attention of chemists interested in the synthesis of cyclic nitronates.
However, Trost demonstrated (71) that this process can be used in the synthesis
of Þve-membered cyclic nitronates from oleÞns (18) containing two acyl groups
in the different allylic positions (Scheme 3.21).
Evidently, this interesting reaction starts with coordination of Pd by the allylic
fragment at the site of attachment of one of the acyloxy groups. Then the anion of
the nitro compound is involved in C,C-coupling with oleÞn (18) followed by successive
ionization of the nitro fragment. Finally, Pd is mediated in O,C-coupling
5c
Me
O