CHEM01200604012 Dibakar Goswami - Homi Bhabha National ...

tartrate, Ti(O-i-Pr) 4 , CH 2 Cl 2 ; (vi) Me 3 Al, hexane, 25 o C; (vii) (a) CH 3 C(OMe) 3 , Me 3 SiCl,
CH 2 Cl 2 , 0 o C, (b) K 2 CO 3 , MeOH, 25 o C; (viii) KCN, LiClO 4 , CH 3 CN, 70 o C; (ix) t-
BuMe 2 SiOTf, 2,6-lutidine, CH 2 Cl 2 , 0
o C; (x) DIBAL-H, toluene, -78
o C; (xi)
trimethylphosphonoacetate, tetramethylguanidine, THF, 30 o C.
Scheme IV.2.1
In a recent approach, 155b the methyl ester of the acid 123 was subjected to ADH
reaction to furnish a γ-lactone, which on base-catalyzed α-methylation afforded the lactone
124. This on acetalization gave 125. Conversion of 125 to the corresponding aldehyde
followed by allylation gave 126 in 98% de. A cross-metathesis between 126 and t-butyl
acrylate yielded the unit-A precursor 127 in 42% yield and 95% de (Scheme IV.2.2).
vii
O
O
CO 2 H i-iii O
iv CO 2 Me v, vi
C 6 H 5
O
OH
123 124 125
O
O
O OH Ot-Bu
O
126
O
OH
127
(i) MeI, Cs 2 CO 3 , acetone, reflux; (ii) AD-mix, t-BuOH-H 2 O, 0 o C; (iii) LDA, MeI, THF, -
78 o C; (iv) Amberlyst-15, MeOH, C(CH 3 ) 2 (OMe) 2 , 25 o C; (v) DIBAL-H, CH 2 Cl 2 , -78 o C;
(vi) CH 2 =CHCH 2 Sn(n-Bu) 3 , MgBr 2 .Et 2 O, CH 2 Cl 2 , -78 o C; (vii) t-butylacrylate, Grubbs 2 nd
generation, CH 2 Cl 2 , 25 o C.
150