J. Org. Chem. 1985,50, 910-912

910
Scheme H I a
H
N+

C02Et

7c. d

H
(CHz),,

s

0

Sc, d

COzEt

8

( I c , n = l ; d , n = 2.

to pyrazolinones 6a,b together with ylides 5a,b as minor products. Bridged 2-pyrazolin-bones 6a,b, whose structures are supported by 'H and 13C NMR spectroscopy, show two strong infrared absorption bands in the 17001800-cm-' range. The first band (1740 cm-') is assigned to the ester carbonyl while we attribute the second one
(1765 cm-' for 6a and 1760 cm-l for 6b) to the amide carbonyl. These abnormal high values for an amide band reveal, in accordance with Bredt's rule, an important inhibition of the N-C=O resonance in these N-bridgehead lactams." On the other hand, ' NMR spectra of the crude maH terials obtained from diesters 4c,d at 390-400 "C showed the characteristic signals of a terminal vinyl group suggesting that an elimination reaction follows or competes with the [1,4] migration. Flash column chromatography of the mixture resulting from the thermolysis of the piperidine derivative 4c afforded besides the pyrazolinone
6c12an isomeric N-alkenyl compound whose spectroscopic datal3support the hydroxy-bpyrazole structure 7c. The analogous elimination product 7d formed in lower yield
(15% estimation on the basis of 'H NMR spectra) could not be isolated.
Pyrazolinone 6c submitted to flow pyrolysis at 400 O
C
was recovered unchanged. Thus hydroxy-bpyrazoles 7c,d result from an intramolecular elimination reaction of ylides
5c,d (Scheme 111),either by a six-center mechanism involving the negative charge delocalized on the carbonyl oxygen (path a) or by a Cope-type elimination14 (path b) and subsequent isomerization (8 7) to the stable tautomer in aprotic media.ls We believe that the elimination takes place according to the latter pathway since crystallographic analyses of analogous ylidic