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

O
N
O
249a-g
+
R′
250a,b
R
O
N
O
REACTIVITY OF NITRONATES 585
R
R′
(exo)
O
N
R′ O
R
(endo)
For 249 R = Me (a), Ph (b), CO 2Me (c), MeCO (d), NO2 (e), CN (f), H (g).
For 250 R = CO 2Me (a), Ph (b).
Scheme 3.168
N
O
N
O
R
O
251-cis
R
O
R′
R′
251′-trans
The pioneering study (337) was performed more than 25 years ago and,
unfortunately, the principal results of this study were not published in reviewed
journals. Six-membered cyclic nitronates (249a–g) and dipolarophiles (250a–d)
(Scheme 3.168) were investigated. This appropriate choice of compounds made
it possible to eliminate problems associated with stereoisomerism of nitronates.
Cycloaddition reactions produce nitroso acetals (251) in high yields. In all
cases (R = H, Me, Ph, CO2Me, COMe, CN, NO2; R’=CO2Me, Ph, CN, CH2Cl),
the regioselectivity was very high, that is, the substituent R ′ in the Þve-membered
ring is always attached to the carbon atom bonded to the oxygen atom. The stereoselectivity
of the reaction substantially depend on the nature of the substituent
and was determined by the attack of the oleÞn on the dipole (see Scheme 3.168
and Table 3.16).
Generally, the exo approach of oleÞns to nitronates dominated in these reactions.
For most of nitroso acetals (251), the relative conÞgurations of substituents
were determined by NMR, on the assumption that the six-membered ring adopts
a chair conformation and the fused Þve-membered ring has an envelop conformation
with the nitrogen atom deviating from the plane through the other four
atoms of the ring. For one nitroso acetal (251 g) (R=CN, R ′ =C6H4-p-Br, (cis)),
the validity of the assumptions was conÞrmed by X-ray analysis. The resulting
stereoisomers differ in the related orientation of the substituents R and R ′ .
Selected data on the conÞgurations of stereoisomers of nitroso acetals are given
in Table 3.16.
The kinetic and activation parameters for a number of (249 + 250) reactions
were determined by spectrophotometry and NMR (see Table 3.17) (337).