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

588 NITRONATES
R 2 O
R 1
OR 1
O N
H
R
O
3
N O
R
O
3
N
H
OBz
O
R
O
3
+
+
H
OBz
249 i,j 250a,i-m 252 252′
R1 – OBz, R 2 – Bu or
R 1
OBz
OBz
OBz
OBz
OBz
OBz
OBz
OBz
R 2
X
X
X
X
X
X
X
X
R 3
CO 2Me
CO2Me
CO2Bu t
COMe
COCH2OTDS
COCH2OBu
CHO
CH2OH
Ph
(X)
Olefin
250a
250a
250i
250d
250j
250k
250l
250m
Product
252a + 252a′
252b + 252b′
252c + 252c′
252d + 252d′
252e + 252e′
252f + 252f′
252g + 252g′
252h + 252h′
Yield,%
* by NMR
** converted by reduction with NaBH4 into 252h + 252h′ in 62%
Scheme 3.169
79
86
91
88
90
89
**
81
7,6/1
6,5/1
11,4/1
6,2/1*
5/1*
5,7/1
1/2
1,3/2
OR 1
d.r 252/252′
fashion. The degree of stereoselectivity depends on the nature of the substituent
R 3 (Scheme 3.169) and varies in a wide range (from 11.4/1 to 1/2), the exo
approach of oleÞn to nitronate being dominant as a rule.
The reactions of vicinally disubstituted oleÞns (253a–g) (Scheme 3.170) occur
nonregioselectively and produce two regioisomers of nitroso acetals ((254) and
(255), respectively). Each regioisomer can have two stereoisomers, which differ
in the mutual arrangement of the CO2R ′ group and the substituents in the
six-membered ring.
For the substituents (R 2 and R 3 ), the contribution of the head-to-tail approach
giving rise to nitroso acetals (255) and (255 ′ ) increases in the series:
O > C > Si > H. In the authors’ opinion (162) , the FMO theory does not explain
all facts and does not have predictive ability for the estimation of the regioselectivity
of cycloaddition of 1,2-disubstituted dipolarophiles, because this problem
is strongly related to electronic perturbations of dipolarophiles. The parameters,
which are generally used to account for the inßuence of substituents, (422–425)
also cannot approximate the regularities of regioselectivity of [3 + 2]-cycloaddition
observed for 1,2-disubstituted oleÞns. In the authors’ opinion (162), the
so-called polarity index (P), which has been recently suggested for explaining of