from acrylate 220 in 70 % yield. The resulting lactone 221 was then subjected to
reductive acetylation conditions and alkylated with silyl enol ether 222 in the
presence of zinc chloride to afford the 2,6-trans THP intermediate 223.
TBDPSO
O
O
OAc
O
TBDPSO
O
O
OAc
O
A
B
A
G-I
Ti(i-OPr) 4
CH 2 Cl 2
73%
215
216
Scheme 58 Class 2 RCM strategy to the A ring fragment of the phorboxazoles [106]
O
O
O
OBn
MOMO
O
O
O
OBn
MOMO
1) DIBAL-H, CH 2 Cl 2
then CSA, EtOH (86%)
2) Allyltrimethylsilane
TMSOTf, CH 2 Cl 2 (85%)
O
O
OBn
MOMO
A
B
A
B
B
G-I
Ti(i-OPr) 4
CH 2 Cl 2
94%
217
218
219
Scheme 59 Class 2 RCM strategy to the B ring fragment of the phorboxazoles [107]
O
OTBS
O
O
OTBS
O
1) DIBAL-H, CH 2 Cl 2
then Ac 2 O, Py, DMAP (99%)
2) ZnCl 2
CH 2 Cl 2 (89%)
O
OTBS
O
OTMS
G-II
CH 2 Cl 2 , then
Pd/C, H 2
70%
220
221
223
222
B
B
Scheme 60 Class 2 RCM/reductive acetylation/alkylation sequence to B ring fragment of
leucascandrolide A [108]
80
M.A. Perry et al.