Page 19 - Volume 3, Issue 2, July 2010
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© Benaki Phytopathological Institute
Mosquito oviposition aggregation pheromone
49
tion of
86
with cyclopentanone in the pres-
ence of 30 mol% of
L
-proline gave
syn
- and
anti
-adducts (75:25 ratio). The
syn
-adduct
87
was then treated with Raney-Ni to give
the desired
β-
hydroxy
ketone which upon
oxidation and acetylation led to molecule
1a
(Figure 29)
(4 steps, 35.5% yield)
.
2.6. Chemo – enzymatic synthesis
A chemo-enzymatic synthesis of (–)-(5
R
,
6
S
)-6-acetoxy-5-hexadecanolide was pub-
lished by Schick (Henkel
et al.,
1995). The
methyl ester of
racemic
5,6-dihydroxyhexa-
decanoic acid was prepared by a Wittig re-
action of (4-carboxybutyl)-triphenylphos-
phonium bromide with undecanal followed
by esterification with methanol and
cis
di-
hydroxylation with osmium tetroxide. After
conversion of the dihydroxy ester
90
and
91
into 6-hydroxy-hexadecanolide by lacton-
ization, (–)-(5
R
,6
S
)-6-acetoxy-5-hexadecan-
olide was obtained by an enantioselective
lipase-catalyzed acetylation with vinyl ace-
tate (Figure 30)
(6 steps, 11% yield)
.
2.7. Chemo – microbial synthesis
Oehlschlager explored the role of bak-
er’s yeast as a chiral reagent presenting a
multi-step formal synthesis of (–)-(5
R
,6
S
)-
6-acetoxy-5-hexadecanolide (Ramaswamy
and Oehlschlager, 1991). His approach to
the synthesis involved the elaboration of
the chiral diol
94
(Figure 31). Diol
94
was
prepared by baker’s yeast reduction of hy-
droxyketone
93
in 47% yield and an opti-
cal purity of 98.5%. The primary alcohol was
converted enzymatically to butyrate where-
as the secondary alcohol was THP protect-
ed. Regeneration of the primary alcohol,
oxidation and addition of
n
-decylmagne-
sium bromide afforded an
erythro
:
threo
mix-
ture (45:55). Conversion of this mixture into
erythro
diol was achieved through Swern
oxidation, removal of the THP group and
Figure 27
Figure 28
