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

R
CR 1 R 2
Me
N R
O
1
R2 R
REACTIONS OF NITRILE OXIDES 31
Me CO2Me
65 66 67
N
O
CH 2CH 2CO 2Me
Bicyclopropylidene smoothly undergoes 1,3-dipolar cycloaddition to nitrile
oxides to give rather stable bisspirocyclopropaneisoxazolines 68 (Scheme 1.19).
Formation of side product 69 was observed for 68 (R = Ph). The yield of 69
depended on temperature and duration of the reaction rising from 5% (THF,
66 ◦ C, 7h) to 14% (PhH, 80 ◦ C, 14h). Two routes were suggested for the side
reaction: (a) rearrangement of 68 to 70 followed by reaction of the latter with a
second molecule of PhCNO to give 69, and (b) reaction with a second molecule
of PhCNO to give 70 with subsequent rearrangement of the latter to 69 (234).
On the basis of previously published data (235), concerning thermal rearrangement
of 68 (R = Ph and mesityl) to furo[3,2-c]pyridine derivatives, reactions of
mesitonitrile oxide and triphenylacetonitrile oxides were carried out (o-Cl2C6H4,
170 ◦ C, 5 days) leading to compounds 72 (R = 2,4,6-Me3C6H2, Ph3C) in 7% and
21% yields, respectively (Scheme 1.20) (234).
Facial selectivity in 1,3-dipolar cycloadditions to cis-3,4-dimethylcyclobutene
(73) (Scheme 1.21) was studied. Only phenylglyoxylo- and pyruvonitrile oxides
lacked facial selectivities (anti:syn = 1:1). All other nitrile oxides formed preferably
anti-74. Theanti/syn ratio increased from 60:40 (R = p-O2NC6H4) and
65:35 (R = Ph) to 87:13 and 92:8 for bulky tert-Bu and mesityl substituents,
respectively. The transition-state structure of the cycloaddition of formonitrile
oxide was determined using both HF/6–31G* and B3LYP/6–31G* methods. The
RCNO +
R = Ph, Me, 2,4,6-Me3C6H2, Ph3C
R
N
Ph
O
68
R = Ph
O
N
Ph
O
O
N N
O
N
Ph
70 71
Scheme 1.19
N
69
O