224
hydrolyzed followed by spontaneous lactonization to 1186. The spiro-lactone 1186
was transformed to palmosalide C (1187) in several steps.
This same group further studied the methodology to construct a bakkane skeleton with a variety of functional groups (Scheme 25) [338]. The cyclohexene ketal
1188 was used as the starting material, and 1190 was prepared via 1189 by a similar
route described above. Formation of the cyclopentane ring (from 1190 to 1191) and
its lactonization afforded 1192. Homogynolide B (1193) was synthesized in several
steps from 1192. Starting from (+)-carvone (1108), (−)-homogynolide A (1195),
having an angeloyloxy group at C-2α, was prepared via 1194.
It is interesting that palmosalide C (1187) has the C-8 carbonyl group in the
lower side of a spiro γ-lactone while homogynolides A (1195) and B (1193) have
this in the upper side [275].
An intramolecular Diels-Alder reaction was employed by Back et al. in the synthesis of a hydrindane skeleton (Scheme 26) [339]. The benzyloxy ketone 1196 was
O
Cl
Cl
CH 2 OH
CH 2 OH
CO 2 CH 3
CO 2 Bu t
O
O
O
O
O
O
1187 (palmosalide C)
7
8
11
1
CH 2 Cl
SO 3 C 2 H 5
1182
1183
1184
1185
1186
1181
Scheme 24 Synthesis scheme for palmosalide C (1187) by Greene’s group
Scheme 25 Synthesis scheme for (−)-homogynolide A (1195) and (±)-homogynolide B (1193)
by Greene’s group
M. Tori and C. Kuroda