ring opening to deliver the unsaturated seven-membered ring lactone in high
yields (Scheme 70) [100].
5.5.2 Ring Expansion of Oxacyclohexane-2-Carboxaldehydes
The organo-catalysed rearrangement of oxacycloalkane-2-carboxaldehydes has
been successfully achieved by treatment with N-heterocyclic carbene (NHC)
under basic conditions. While the reaction proceeded well with furan derivatives,
the extension to the synthesis of seven-membered ring lactones was more problematic in terms of time and yields. For example, treatment of 190 with carbene 191
(10 mol%) led to lactone 192 (48 %) after 10 days (Scheme 71) [101].
MeO
B
O
O
O
O
O
OH
Br
BnO
BnO
MeO
O
O
OH
MeO
HO
HO
OMe
Graphislactone D, 185
Pd(OAc) 2
Cs 2 CO 3
S-Phos
1)
2) H 2 , Pd/C
183
184
65%
Scheme 69 Access to graphislactone D by a tandem Suzuki coupling/macrolactonisation
O
OMe
Ph
O
Ph
BF 3
MeO-BF 3
n-Bu
OTIPS
Bu
O
MeO-BF 2
186'
187'
O
n-Bu
O
BF 3
Ph
O
O
n-Bu
Ph
186
187
189
188
n93%
-
+
+
-
-
Scheme 70 Access to an unsaturated lactone by an electrocyclic ring opening of an oxetenium
intermediate
Synthesis of Seven-Membered Ring Ethers and Lactones
315