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

REACTIONS OF NITRILE OXIDES 71
(R 6 = Me, Ph) was induced by TiCl4 at –78 ◦ in CH2Cl2. A nitrile oxide, generated
by subsequent addition of Et3N in THF, underwent intramolecular addition
to give 8% to 92% of compounds 333 as diastereoisomeric mixtures.
R 1
R 2
O N
(CH 2) n−1
R 3 R 4
333
R 5
CH2
R 1 = H, Me, Ph, CH:CH2; R 2 = H, Me; n = 2, 3;
R 3 = H, Me, Ph; R 4 , R 5 = H, Me
A study of Lewis acid-promoted reactions of 1-nitroalka-1,5-(or 1,6-)dienes
with allylic stannanes shows that, in the presence of TiCl4, 1-nitrohexa-1,5-diene
reacts smoothly with allyltrimethylstannane to give a diastereoisomeric
mixture of 6-allyl-3a,4,5,6-tetrahydro-3H -cyclopent[c]isoxazoles, while the reaction,
using AlCl3 as catalyst, leads to allylated cyclohexanone oxime derivatives
in good yield. A similar reaction of 1-nitrohepta-1,6-diene, however, gives
a bicyclic isoxazoline, irrespective of the Lewis acids employed. In the latter
case, nitrile oxides derived from 1-nitroalka-1,6-dienes undergo a stepwise
cycloaddition, as shown by the lack of stereospeciÞcity in the reactions of
(1E,6Z )-1-nitro-7-phenylhepta-1,6-diene and (1E,6Z )-1-nitroocta-1,6-diene
(402).
Certain speciÞc steric effects are operative on intramolecular nitrile oxide—
oleÞn cycloadditions. These effects are governed by both ring size and character
of substituents. Thus, cycloadditions to the exomethylene group are successful
with substituted methylenecyclohexanones 334 (m = 1, 2; n = 2) and gave tricyclic
335 (m = 1, 2), but do not occur with methylenecyclopentanones 334
(m = 1, 2, 3; n = 1). Activation energies calculated by molecular mechanics
are consistent with these results. Cleavage of 335 (m = 2) by Raney Ni gives
cis-decalone 336 (403).
(CH2)n
CH2
CH2(CH2)mCNO
O N
(CH 2)m
O
HOCH2
334 335 336
Substituent effects on intramolecular dipolar cycloadditions can be illustrated
by the gem-dicarboalkoxy effect (404). This effect (rel. rate > 20) has been
H