a) BEt2F•pyridine[296] (5 equiv), CH2Cl2, RT, 12 h, 65%; b) Pd/C (5%, 10 wt%), 1 bar H2, CH2Cl2, RT, 4 h, 34%.
This conversion was carried out in the same manner as for the auxiliary of the epicoccamide D total synthesis[32]. The free 3-acyl tetramic acid 97 was transferred into its BEt2-complex by the use of BEt2F•pyridine derived from B. Wrackmeyer’s group[296]. This procedure yielded the desired BEt2-complex 167 in 65% yield.
The complex was subjected to identical conditions of hydrogenation (see Table IV.2.) as the BF2-complex. This boron-dialkyl complex of a 3-acyl tetramic acid was not stable under standard hydrogenation conditions using palladium on charcoal. Since the complex decomposed rapidly during reaction, the hydrogenation stopped after roughly 35%
conversion. The resulting product mixture contained the educt 97 and the reduced diethylboron complex of the 3-acyl tetramic acid 168. This mixture was analyzed again by chiral HPLC which revealed a diastereomeric excess of about 60%. Exact numbers were not accessible since the educt eluted in the middle of the two possible diastereomers and without baseline separation.
These preliminary results looked promising and additional work needs to be carried out at least with more stable boron complexes.
CONCLUSION
74
V. CONCLUSION
V.1. Total synthesis of epicoccamide D
The secondary metabolite epicoccamide D, derived from fungal sources, was synthesized for the first time. The overall yield was 17% over 19 steps in the longest linear sequence starting from D-glucose. These high yielding steps included a C-2 glucose epimerisation reaction, a HWE olefination and a Lacey-Dieckman cyclisation as key steps of the total synthesis.
Another key step was the stereoselective hydrogenation of the corresponding 3-acyl tetramic acid BF2-complex, which not only enabled access to all four possible configurations in terms of the unknown configuration of the stereocenters on the tetramic acid moiety, but also allowed the assignment of the absolute configuration of the natural product. An auxiliary technique was applied comparing NMR spectra and optical rotation of a synthetic tetramic acid derivative prepared by two different protocols. The auxiliaries were chosen to have similar substitutions as the natural product. The auxiliary was, on one hand, derived from the same methodology applied for the epicoccamide D total synthesis, and on the other hand prepared by an acylation method having a known absolute configuration[28,29]. Subsequent experiments showed that the same catalyst employed for both the auxiliary and the natural product synthesis had identical stereoinduction. Applying the other enantiomer of the catalyst (R,R instead of S,S-DUPHOS derived catalyst) revealed full inversion of the formed stereocenter (no match-mismatch), and showed that the stereocontrol in this homogeneous hydrogenation was only accomplished by the chiral catalyst with no observed substrate control. This hydrogenation step, using a cheap rhodium based chiral catalyst revealed the absolute configuration of natural epicoccamide D to be (5S,7S). The suggested absolute configuration was later confirmed by the Yajima group.[123] The hydrogenation step was achieved by employing a known[30,98,134]
BF2-chelate complex forming procedure.
The described method establishing the C-7 stereocenter complements other routes with similar synthetic C-7-branched targets using the Evans auxiliary technique[207]. After this selective alkylation a 4-O to C-3 acylation rearrangement protocol[123,205], developed by Yoshii et al.[113], and improved by Yoda et al.[31,205], was carried out. This method can also referred to be a direct C-3 acylation[297]. Additionally this total synthesis clears the way for the synthesis of members from the large glycotetramate family including ancorinosides and virgineone.
CONCLUSION
75 It is also worth noting that the access to N-methylated amino acid methyl esters was improved by direct usage of the Boc-protected amino acid and a two steps N-methylation esterification procedure. The second step liberated enough HCl to remove the Boc-group in situ without purification in between as it is usually described in literature[157,158].
The shown total synthesis of epicoccamide D was of a modular character and allows access to additional natural derivatives and synthetic ones inspired by nature. Synthesizing such compounds in future might deliver an insight into the importance of the β-configurated glycosidic linkage, in which improvements, in terms of biological activities of another sugar moiety, can be made. Potentially other biological functions can be revealed by changing the alanine derived tetramic acid moiety to other (natural) amino acids. All this information might show an overall structure-activity relationship. Preliminary work has been carried out together with D. Linder[298] finding access to α-branched epicoccamide derivatives. Having an adequate amount of material, not only of natural occurring epiccoccamide D, but also of other non-natural derivatives, in hand will allow extensive biochemical assays to be performed in order to find the mode of action of epicoccamides, and eventually reveal their biological target to elucidate their antimicrobial and cytotoxic effects.
To the best of our knowledge the published[32] epicoccamide D total synthesis was the first total synthesis of a naturally occurring glycosylated tetramic acid beside a synthesis by Pronin et al.[43]. In their synthetic approach no common hexose or pentose was attached to the 3-acyl fatty acid residue of a tetramic acid rather than a hexose named L-rhodinose[299,300]
(2,3,6-trideoxy-L-galactose) which only occurs in gram-negative bacteria[301].
V.2. Total synthesis of the ancorinoside B diglycoside
The synthesis of the ancorinoside B diglycoside using either a readily C-6 oxidized acceptor for the first glycosylation or performing a convenient oxidation stept right before the second one failed. Many different donor-acceptor combinations were tested and temperature, promoter load and type of promoter (TMSOTf vs. BF3•OEt2) were changed with no positive result.
The diglycoside Gal-β(1-4)GlcU (part of ancorinoside B and C) was synthesized with 7.6%
overall yield over 10 steps, starting from D-glucose. The total amount of steps was 16 to gain the desired diglycoside. This synthesis utilized a new glycosylation acceptor (or donor in case
CONCLUSION methods and the diglycoside already in hand. The only difference compared to epicoccamides might be a problem arising from two occuring esters, one from aspartic acid side chain and one amino acid methyl ester, close to each other during Lacey-Dieckmann cyclisation. In respect to this fact the necessary remaining steps of the total synthesis are shown below starting from the fully protected diglycoside. The remaining steps start with regard to the later oxidation of the C-5 tetramic acid side chain to install the aspartic acid functionality from the synthesis of a fully protected N-methyl homoserine[212,302] methyl ester derivative which can be derived from affordable D-methionine.
Additionally another phosphonate needs to be synthesized since ancorinosides do not bear a methyl group on the side chain. This can be accomplished using the method from Ley et al.[56]
but starting with bromoacetyl bromide rather than bromopropionly bromide.
With all that material in hand, the total synthesis of ancorinoside B can be commenced in future with similar steps as for the epicoccamide D synthesis. The primary TBS group on the side chain will be deprotected, oxidized and transferred into the corresponding β-ketoamide via HWE olefination and aminolysis. The synthesis will be finalized by removal of the silyl protective group on the amino acid side chain from the above mentioned homoserine derivative after the mentioned cyclisation followed by oxidation.
107 R = CH2OTBS 169 R = CH2OTBS
170 R = CH2OH 171 R = COOH
Scheme V.1. Suggested cyclisation, deprotection and oxidation step of the aminolysis product 107 to