A Guidebook to Mechanism in Organic Chemistry

Carbonium Ions, Electron-deficient Nand O Atoms

determines which of them in fact migrates. Thus it is found, in prac­
tice, to be always the anti-R group that rearranges:

R R' HO R'

V V C C

|| || (i.e.R'-CO-NHR)wi/>>

. N N
\ ' \
OH R
Confirmation of this fact requires an initial, unambiguous assign­
ment of configuration to a pair of oximes. This was effected as fol­
lows: working with the pair of oximes (XXVI) and (XXVII), it was
shown that one of them was converted into the cyclic isoxazole
(XXVIII) on treatment with alkali even in the cold, while the other
was but little attacked even under very much more vigorous condi­
tions. The oxime undergoing ready cyclisation was, on this basis,
assigned the configuration (XXVI) in which the oxime OH group
and the nuclear bromine atom are close together and the one resisting
cyclisation, the configuration (XXVII), in which these groups are
far apart and correspondingly unlikely «to interacfwith each other:

OaN

:X ©OH

Br HO'

(XXVI)

cold

(XXVIII)

^^xBr xOH

(XXVII)

Subsequently, configuration may be assigned to other pairs of
ketoximes by correlation of their physical constants with those of
pairs of oximes whose configuration has already been established.
Once it had been clearly demonstrated that it was always the anti-B.
group that migrated in the Beckmann reaction, however, the product
obtained by such transformation of a given oxime has normally been
used to establish the configuration of that'oxime. Thus, as expected,
(XXVI) is found to yield only a substituted N-methylbenzamide,
while (XXVII) yields only a substituted acetanilide.
That a mere, direct interchange of R and OH has npt taken place