PROGRESS OF STUDIES ON THE NATURAL CEMBRANOIDS FROM THE SOFT CORAL SPECIES OF SARCOPHYTON GENUS - Medicinal Chemistry of Bioactive Natural Products - page 287

OAc OAc

O a b

c

O O

OH OH Br

122

233 234

d e

CN

CHO

O O

Br Br

235

236

f

O O

Ph

H

Ph

OH

O

O

N

B

237 181 OMe

238

Reagents and Conditions: (a) Ti(O-i-Pr) 4 , L-( þ )-DET, t-BuO 2 H, CH 2 Cl 2 , 40 C, 95%; (b) i. MsCl,
Et 3 N, CH 2 Cl 2 , 10 C 0 C; then LiBr, acetone, 50 C, 82%; ii. K 2 CO 3 , MeOH, 23 C; then MnO 2 ,
Hexane, 23 C, 91%; (c) LiN(SiMe 3 ) 2 /(Me) 2 CHCH(CN)P(O)(OEt) 2 , toluene, 78 C; then 234, 90%;
(d) DIBAL-H, Hexane, 78 C; then 10% aq. oxalic acid, 0 C, 88%; (e) i. Me 3 SiCN, cat. KCN/18-C-6,
THF; ii. LiN(SiMe 3 ) 2 , THF, reflux; then TBAF, 85% from 236; (f) BH 3 Me 2 S, 238 (10 mol%), toluene,
0 C, 88%.

Scheme 6-27

Chemical conversion of pseudoplexaurol (182) to 14-deoxycrassin (239) was
realized by a Lewis acid-catalyzed lactonization process (Scheme 6-32).

6.4.4 Total Synthesis of Cembranolides

Sarcophytonin B (67) was first isolated from Sarcophyton spp. by Kobayashi
and Hirase 27 in 1990. cis- and trans-Cembranolides were isolated from Sinularia
mayi by Uchio et al. (Scheme 6-33). 126 and from Lobophytum michaelae by Coll
et al. 76 Cembranolides were assumed to be logical biosynthetic precursors of sar-
cophytonin B. Several syntheses of the cembranolides and sarcophytonin B (67)
have been reported.
( )-cis-Cembranolide was synthesized via Cr(II)-mediated intramolecular
macrocyclization of o-formyl-b-methoxycarbonylallyl halides, which were pre-
pared from geranylgeraniol (Scheme 6-34). 127,128