Chemistry Reference
In-Depth Information
exo
-type
Ar
1
-6/7
endo
-type
Ar
1
-6
O
O
n
N
N
n
n
N
O
O
O
O
Ph
Ph
n
Yield (%)
339
338
340
1 65
341
2 80
SCHEME 10.53.
determining the success of nitrenium ion spirocyclizations (Table 10.5). While
Kikugawa found considerable success in accessing these synthetically important
targets through the solvolysis of
O-
alkyl chlorohydroxamate (entry 1), attempts to
employ the iodine(III)-mediated oxidation of hydroxamates were initially
unpromising: brief exposure of
to PIFA at 65
C, for example, led to spirodienone
342
343
in low yields (entry 2). Economic considerations notwithstand-
ing, Kikugawa has found that use of trifluoroethanol dramatically improves the yield
of
and oxime
344
343
, an observation ascribed to the polarity of this nonnucleophilic solvent
(entry 3).
127
Koser's reagent (HTIB) has also been employed in this context, as
illustrated by the formation of
N-
phthalimido spirolactams (entry 4).
120
Wardrop
et al.
have also investigated the PIFA-mediated spirocyclization of
O-
alkyl hydroxamates
and found that use of methanol in combination with
CH
2
Cl
2
offers an economical and operationally simple solvent system with which
345
TABLE 10.5. Iodine(III)-Mediated Spirocyclization of O-Alkyl Arylhydroxamates and
Related Compounds. Effect of Amide Substitution on Product Yield and Distribution
OMe
O
NOMe
X
OMe
X
HN
N
N
n
n
n
O
O
O
342
343
344
Entry
X
Conditions
Yield of
343
(%)
t
-BuOCl; Ag(I)/TFA, 0
C, 20min
1
OMe
83
34 (31)
a
PIFA/CHCl
3
,65
C, 3min
2
OMe
PIFA/TFE, 0
C, 15min
3
OMe
80
4
NPhth
HIBT/TFE, rt, 3 h
77
a
Yield of
344
.
