conditions: a) NaOMe (1 equiv), MeOH, 0 °C, 6 h; b) TBAF (4 equiv), THF, RT, 12 h; c) TEMPO (0.1 equiv), BAIB (2.5 equiv), CH2Cl2:H2O = 2:1, RT, 2 h.
The last step will be a global deprotection step by hydrogenation since all functional groups are protected by benzylic esters and can therefore be cleaved, also reducing the double bond
c
b
CONCLUSION
77 derived from the HWE olefination step. Ancorinosides do not possess a methyl substitution at the C-7 and hence no stereoselective hydrogenation is necessary during synthesis.
With this natural product in hand, biochemical evaluation can again be carried out, not only to find similarities or differences between ancorinoside B and epicoccamide D, but also eventually find additional information concerning the mode of action of the type-1 matrix metalloproteinase (MT1-MMP) inhibition which was found[120] in the case of the ancorinoside B. It is assumed that tetramic acids derived from different amino acids strongly differ in their biological activity.
Since ancorinosides have a tetramic acid moiety derived from N-methyl D-aspartic acid (NMDA), it might also be worth to test[121] for occurring activating behaviour of nerve cells.
NMDA is a known agonist for receptors bound in the postsynaptic membrane regulating ion in- and efflux normally in a voltage-dependent manner.[122] The function of this receptor is directly connected to the process of learning and disfunction of the receptor leading to depression, including several other mood disorders[303], or is involved in Alzheimer’s disease[304].
V.3. Contribution to virgineone total synthesis
The protected side chain of virgineone was synthesized in nine steps with a poor overall yield of 2.2%. This yield issue was caused by the problematic selective protection of the 1,2-diol gained from Sharpless dihydroxylation. It might be worth performing the synthesis, to this point, anew, since at least the selective pivaloyl esterification should give yields better than 32%. If this particular route still returns poor yields it might be useful to protect the starting material for this synthesis (dec-9-en-1-ol) with a more stable triisopropylsilyl (TIPS) group, which can possibly sustain trityl deprotection and still maintaining orthogonality to the THP protective group.
If these changes to the synthesis of the virgineone side chain prove successful the remaining steps for total synthesis can be then derived from epicoccamide D total synthesis. The only three differences compared to these synthetic routes are the missing N-methylation, the usage of an accordingly protected L-tyrosine derivative, and the required THP deprotection and oxidation late in synthesis. The oxidation step needs to be carried out at any point after the HWE olefination step to avoid side reactions and possibly before the actual Lacey-Dieckman cyclisation step in order to circumvent purification problems which might occur once the
CONCLUSION
78 corresponding tetramic acid is in place. A suitably protected tyrosine derivative (172) for aminolysis is shown in Figure V.1. as well as the step where THP deprotection and oxidation seems most reasonable (173).
A B
172 173
Figure V.1. Suggested tyrosine derivative and educt for oxidation. A) O-acetyl protected tyrosine derivative usable for the aminolysis reaction during the virgineone total synthesis. B) Key step and educt for the oxidation process to get the desired ketone which also arises in the natural product.
The acetyl protective group might be a clever choice since the conditions necessary to perform the Lacey-Dieckman cyclisation are conform to the normal standard conditions for removing an acetyl-group. It is also worth noting that the 2-hydroxyl group on the sugar moiety again needs to be protected right after the epimerisation has been performed.
With all these tools in hand a virgineone total synthesis should be reachable. If all these methods fail there is still the possibility to combine the knowledge of establishing the β-glycosidic linkage between mannose and its side chain with the published virgineone aglycon synthesis by Yajima et al.[124] It might be possible to use their methodology to install the long oxidized fatty acid by cutting it into two pieces with one bearing the ketone from a retrosynthetic view and later connect both parts by cross metathesis. The required glycoside 174 is shown in Figure V.2. besides the second reaction partner 175 for cross metatesis.
A B
174 175
Figure V.2. Possible educts for cross metathesis. A) Necessary glycoside synthesized by the epicoccamide methodology. B) 3-acyl tetramic acid bearing also a terminal double bond for cross metathesis.
If a cross metathesis is performed to bring both the glycoside and the tetramic acid together, a C-2 protection of the sugar moiety might not be necessary. It is obvious that the cross metathesis is best performed right before the Lacey-Dieckman cyclisation step in order to have the product easily purified and avoid catching of the cross metathesis catalyst by the
CONCLUSION
79 tetramic acid. It might also be necessary to build up the tetramic acid moiety by a HWE-, Lacey-Dieckman- and aminolysis reaction sequence in order to have access to both stereoisomers at the C-7 methyl group.
It is imaginable, and indeed beneficial to perform this total synthesis in collaboration with Prof. Arata Yajima (personal communication). That might combine different approaches in a helpful way and can be important for future projects.
After successful total synthesis of virgineone biochemical tests and comparisons can be performed to eventually find a target where virgineone interacts with the calmodulin-dependent stress response system[125] and potentially explain its antifungal behaviour[125].
V.4. N-Glycosylation for aurantoside G and J synthesis
The first trial of chemical N-glycosylation using a tetramic acid derivative which was not 3-acylated and a trichloroacetimidate derived Schmidt donor yielded 42% of a selective 4-O-glycosylation product. From the current point of view other donors might react in a similar way since no other products or side reactions were observed. This reaction was not useful in terms of the planned aurantoside total synthesis but can potentially be used to synthesize tetramic acid 4-O-glycosides and investigate their biological behaviour. The sugar moiety of this new compound class might either be cleaved once transported into a cell or retained and partly involved into the biological function. Both compounds are worth to test for potential antibacterial, antifungal or cytotoxic properties.
The desired N-glycosylation was successful albeit with poor yields, using a readily available BF2-complex of a 3-acyl tetramic acid with a Schmidt donor. Both BF3•OEt2 and TMSOTf were tested as promoter for this type of reaction. Since both activating reagents gave comparable yields below 10% the actual route for direct N-glycosylation of a tetramic acid was discarded and an approach using the Fukayama-Mitsunobu reaction to gain access to the desired N-glycoside was explored. The direct glycosylation tested might still be a reasonable approach for this type of reaction. Literature suggests changing the solvent for a direct N-glycosylation utilizing a Schmidt type donor from CH2Cl2 to nitromethane as shown by Takahashi et al.[284] In their studies this change increased the yield from 46% to 98%. To optimize this reaction might still be useful: It can be used to build up a small compound library of N-glycosylated tetramic acids fast and eventually get some insight into structure-function relationships (SAR) of this compound class.
CONCLUSION
80 The tested approach using the mentioned Mitsunobu type reaction yielded a N-glycoside of an amino acid in excellent yields (94%) but was not very stereoselective (α:β 1:3 or 1:4 respectively) since the participating group effect cannot play a role in this reaction as can be seen from the mechanism[282]. This reaction opened a possible approach for the aurantoside total synthesis and after altering electronic properties of the donor as well as some changes in the reaction sequence proved to be a successful advancement. The full synthetic record of this approach can be seen in the Master thesis of M. Petermichl.[49]
V.5. Stereoinduction by tetramic acid boron complexes
Substrate controlled asymmetric hydrogenation was not observed in terms of the applied homogeneous catalysis as shown by the results for epicoccamide synthesis using the rhodium based Et-DUPHOS catalyst. Utilizing palladium on charcoal on the same substrate (the BF2-complex) revealed a detectable stereoinduction (42%).
In this hetereogeneous hydrogenation the stereoinduction is definitely substrate controlled.
This is because not only the catalyst bears no stereogenic information, but also an increase of diastereomeric excess during some trials using different substrates was discovered. Whereas a non-complexed 3-acyl tetramic acid had a barely detectable stereoinduction of 4% possibly due to the elevated methyl group standing over the tetramic acid plane[98], the corresponding BF2-chelate complex had already a stereoinduction of 42%. This induction was increased by changing the chelate from a difluoroboron to a diethylboron complex. This complex showed a diastereomeric excess of 60% during hetereogeneous catalytic hydrogenation, but was not stable during the reaction revealing only 35% conversion until the tetramic acid complex was hydrolyzed which additionally induced deactivation of the catalyst. The evidence of deactivated catalyst was supported by the fact that hydrolyzed tetramic acid was barely (and unselectively) hydrogenated and a high amount of educt was found during chiral HPLC analysis. A fast hydrolysis rate is explainable by the fact that the used diethylboron complex is by far not as Lewis-acidic as the BF2 derivative and therefore not strongly bound and complexed by the tetramic acid.
To circumvent this fast hydrolysis rate during hydrogenation a more Lewis-acidic, therefore more stable and maybe even larger boron complex, can be supposed. A boron complex derived from a dialky boronic acid ester can be synthesized and used in the hetereogeneous hydrogenation step. Synthesis from literature of dialkyl or diaryl boronic acid ester fluorides
CONCLUSION
81 was performed by A. Meller et al.[305] utilizing a corresponding alcoholate (e.g.
2,2-dimethylpropanol or 2,6-di-tbutylphenol) lithium salt together with BF3•OEt2. A second approach in literature is the synthesis of various large alkyl boronic acid esters published by Medrano et al.[306] as shown below.
176 177 178