Menthyl
P-(bromomethyl)-N-tert-butylphosphonamidate
5 has been prepared from
(1R,2S,5R)-(-)-menthol and the
S
P
diastereoisomer has been isolated. This
rearranges with methoxide [Me
3
(PhCH
2
)N
+
MeO
-
in THF–MeOH] to give only the
S
P
diastereoisomer of menthyl methyl
(tert-butylamino)methylphosphonate 6 and very largely (95%) the
S
P
diastereoisomer of menthyl methyl
N-tert-butyl-N-methylphosphoramidate 7. The
configurations of these products show that when the (postulated)
azaphosphiridine oxide intermediate 16 suffers ring opening by
methoxide, the P–N bond is cleaved (to give 6) with inversion of
configuration but the P–C bond is cleaved (to give 7) with
predominant retention. These contrasting stereochemistries suggest that
nucleophilic attack on the P
![[double bond, length as m-dash]](https://www.rsc.org/images/entities/char_e001.gif)
O group of the azaphosphiridine
oxide generates a five-coordinate intermediate (not merely a transition
state) that exists long enough to undergo
pseudorotation.