44
B. Schmidt
For the synthesis of mantidactolide A ((4R,9S)-126) (+)-β-citronellene ((S)-129)
and (S)-propylene oxide ((S)-67) were used as enantiomerically pure starting materials. (+)-β-Citronellene ((S)-129) was converted into the carboxylic acid (S)-130 in
three steps and the alcohol 131 was obtained by Cu-mediated epoxide opening of
(S)-67. Steglich esterification gave diene 132, which underwent RCM in the presence of second-generation Grubbs’ catalyst to furnish selectively the (Z)-configured
product. Hexafluorobenzene was added in this step to enhance the activity of the
metathesis catalyst [148]. Hydrogenation of the double-bond gave mantidactolide A
((4R,9S)-126). Interestingly, specimens of M. femoralis collected at other locations
in Madagascar did not contain (S)-124 and (4R,9S)-126 as volatile pheromones,
but a cocktail of branched alkanols and 8-methyl-9-decanolide (128), for which
the name mantidactolide B was proposed and a relative (8R*,9R*)-configuration
was assigned tentatively. The identification of mantidactolide B was accomplished
through a strategy similar to that outlined above for mantidactolide A [146].
The femoral gland extract of two specimens of the Madagascan frog Gephyromantis luteus was investigated recently [149]. GC-MS showed that just two
compounds were present, and due to the relatively large amount of material (ca.
500 μg per frog) NMR spectroscopic analysis of the mixture was possible. One
compound was identified as the known sesquiterpene frogolide (133) [150]. For the
other compound a ten-membered lactone structure 134 with a 9-ethyl substituent and
two methyl groups at C-4 and C-8 were tentatively assigned, based on the mass spectrometric fragmentation pattern and H,H-COSY and HMBC correlations observed
in the NMR-spectra obtained from the mixture (Chart 5).
To verify the constitutional structure 134 proposed for the novel natural product
and to elucidate the relative and absolute configuration, a stereoselective synthesis
was undertaken [149]. Considering the structure of the semiochemicals previously
Chart 5 Components of the femoral gland extract from Gephyromantis luteus
B. Schmidt
For the synthesis of mantidactolide A ((4R,9S)-126) (+)-β-citronellene ((S)-129)
and (S)-propylene oxide ((S)-67) were used as enantiomerically pure starting materials. (+)-β-Citronellene ((S)-129) was converted into the carboxylic acid (S)-130 in
three steps and the alcohol 131 was obtained by Cu-mediated epoxide opening of
(S)-67. Steglich esterification gave diene 132, which underwent RCM in the presence of second-generation Grubbs’ catalyst to furnish selectively the (Z)-configured
product. Hexafluorobenzene was added in this step to enhance the activity of the
metathesis catalyst [148]. Hydrogenation of the double-bond gave mantidactolide A
((4R,9S)-126). Interestingly, specimens of M. femoralis collected at other locations
in Madagascar did not contain (S)-124 and (4R,9S)-126 as volatile pheromones,
but a cocktail of branched alkanols and 8-methyl-9-decanolide (128), for which
the name mantidactolide B was proposed and a relative (8R*,9R*)-configuration
was assigned tentatively. The identification of mantidactolide B was accomplished
through a strategy similar to that outlined above for mantidactolide A [146].
The femoral gland extract of two specimens of the Madagascan frog Gephyromantis luteus was investigated recently [149]. GC-MS showed that just two
compounds were present, and due to the relatively large amount of material (ca.
500 μg per frog) NMR spectroscopic analysis of the mixture was possible. One
compound was identified as the known sesquiterpene frogolide (133) [150]. For the
other compound a ten-membered lactone structure 134 with a 9-ethyl substituent and
two methyl groups at C-4 and C-8 were tentatively assigned, based on the mass spectrometric fragmentation pattern and H,H-COSY and HMBC correlations observed
in the NMR-spectra obtained from the mixture (Chart 5).
To verify the constitutional structure 134 proposed for the novel natural product
and to elucidate the relative and absolute configuration, a stereoselective synthesis
was undertaken [149]. Considering the structure of the semiochemicals previously
Chart 5 Components of the femoral gland extract from Gephyromantis luteus