Magnus
Sellstedt
a,
Melanie
Schwalfenberg
a,
Slava
Ziegler
a,
Andrey P.
Antonchick
a and
Herbert
Waldmann
*ab
aMax-Planck-Institute für Molekulare Physiologie, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany. E-mail: herbert.waldmann@mpi-dortmund.mpg.de
bTechnische Universität Dortmund, Otto-Hahn-Strasse 6, 44221 Dortmund, Germany
First published on 18th November 2015
Due to their enhanced metabolic needs many cancers need a sufficient supply of glucose, and novel inhibitors of glucose import are in high demand. Cytochalasin B (CB) is a potent natural glucose import inhibitor which also impairs the actin cytoskeleton leading to undesired toxicity. With a view to identifying selective glucose import inhibitors we have developed an enantioselective trienamine catalyzed synthesis of a CB-inspired compound collection. Biological analysis revealed that indeed actin impairment can be distinguished from glucose import inhibition and led to the identification of the first selective glucose import inhibitor based on the basic structural architecture of cytochalasin B.
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Fig. 1 Structure of cytochalasin B (CB) with the semi-saturated isoindolinone motif highlighted and the outline of the synthesis strategy employing enantioselective trienamine organocatalysis. |
For the synthesis of cytochalasin B and other cytochalasans an inter- or intramolecular Diels–Alder reaction between a diene- or a triene part and an α,β-unsaturated amide has been employed as a key step.6 Recently, trienamine catalysis has emerged as a powerful method to steer the steric course of asymmetric Diels–Alder reactions,7 and we decided to employ this method in the preparation of a cytochalasin inspired compound library. Chen and Jorgensen and co-workers have described that linear dienals, which in the presence of proline-derived catalysts form asymmetric trienamines, react with highly activated dienophiles such as cyanoacrylates to form Diels–Alder adducts in high enantiomeric excesses.8
However, weaker dienophiles, such as maleimides, did not yield the expected products. Subsequently, maleimides have successfully been reacted with branched aryl-9 and methyl-10 dienones by using cinchinoa alkaloid derived catalysts. However, the trienamine catalysed reaction between dienals and maleimides which would yield the semi-saturated isoindolinone core of the cytochalasans (Fig. 1) has to the best of our knowledge not yet been reported.
Entry | Cat. | Solvent | Acid | Temp. [°C] | Time [h] | Yield [%] | eeb [%] |
---|---|---|---|---|---|---|---|
a Reaction conditions: 1a (0.15 mmol), 2a (0.165 mmol), catalyst (0.03 mmol), acid (0.03 mmol), solvent (1.5 mL). b The ee was determined by chiral HPLC using a Chiralpak IA column. c 40 mol% acid. d n.r. = no reaction. OFBA = 2-fluorobenzoic acid, BA = benzoic acid, TMS = trimethylsilyl, TES = triethylsilyl, TBS = tert-butyldimethylsilyl, TFA = trifluoroacetic acid. | |||||||
1 | A | CHCl3 | TFAc | 40 | 24 | n.r.d | — |
2 | B | CHCl3 | OFBA | 40 | 90 | 21 | −28 |
3 | C1 | CHCl3 | OFBA | 40 | 18 | 78 | 34 |
4 | C2 | CHCl3 | OFBA | 40 | 17 | 85 | 44 |
5 | C3 | CHCl3 | OFBA | 40 | 24 | 74 | 46 |
6 | C4 | CHCl3 | OFBA | 40 | 40 | 41 | 78 |
7 | C5 | CHCl3 | OFBA | 40 | 64 | 68 | 81 |
8 | C6 | CHCl3 | OFBA | 40 | 20 | 62 | 24 |
9 | C7 | CHCl3 | OFBA | 40 | 21 | 68 | 6 |
10 | C8 | CHCl3 | OFBA | 40 | 22 | 65 | 82 |
11 | C8 | PhMe | OFBA | 40 | 19 | 61 | 58 |
12 | C8 | EtOH | OFBA | 40 | 41 | 50 | 76 |
13 | C8 | DME | OFBA | 40 | 20 | 45 | 45 |
14 | C8 | EtOAc | OFBA | 40 | 16 | 59 | 49 |
15 | C8 | CHCl3 | BA | 40 | 20 | 85 | 82 |
16 | C8 | CHCl3 | AcOH | 40 | 23 | 62 | 80 |
17 | C8 | CHCl3 | OFBA | −10 | 120 | 79 | 83 |
To explore the scope of this reaction, various dienals and maleimides were subjected to trienamine catalysis with catalyst C8 (Table 2). The alkyl substituted dienals 2b–d11 were more unstable than the phenyl substituted dienal 2a, and a slightly higher excess (1.5 equiv.) of these dienals was used. The reaction was run at room temperature and the resulting aldehydes were directly treated with a Wittig-reagent to avoid isolation of the somewhat sensitive products. The resulting α,β-unsaturated esters 4–7 were formed with 72–92% ee (Table 2).
Entry | Prod. | R1, R2 | Time 1st step (h) | Yield (%) | ee (%) |
Endo![]() ![]() |
E![]() ![]() |
---|---|---|---|---|---|---|---|
1 | 4a | H, H | 22 | 37 | 78 | 18![]() ![]() |
9![]() ![]() |
2 | 4b | Bn, H | 22 | 71 | 72 | 19![]() ![]() |
19![]() ![]() |
3 | 4c | Ph, H | 40 | 75 | 92 | 11![]() ![]() |
>20![]() ![]() |
4 | 5a | Me, H | 24 | 34 | 81 | 9![]() ![]() |
>20![]() ![]() |
5 | 5b | Bn, H | 45 | 25 | 73 | >20![]() ![]() |
>20![]() ![]() |
6 | 5c | Ph, H | 45 | 46 | 73 | >20![]() ![]() |
>20![]() ![]() |
7 | 6a | Bn, H | 70 | 58 | 85 | >20![]() ![]() |
14![]() ![]() |
8 | 6b | Ph, H | 47 | 78 | 85 | >20![]() ![]() |
15![]() ![]() |
9 | 7a | Me, H | 21 | 71 | 76 | >20![]() ![]() |
>20![]() ![]() |
10 | 7b | Bn, H | 21 | 58 | 81 | >20![]() ![]() |
>20![]() ![]() |
11 | 7c | Ph, H | 21 | 57 | 79 | >20![]() ![]() |
>20![]() ![]() |
12 | 7d | Ph, Me | 120 | 25 | 82 | >20![]() ![]() |
>20![]() ![]() |
The reaction tolerated a variety of maleimides and dienals. However, linear dienals such as hexadienal failed to give any notable conversion under these conditions, and when a high concentration of hexadienal was used, the maleimide instead underwent a Diels–Alder reaction with the dienal rather than the trienamine. This is in agreement with the same observation for aryl dienones.9 Generally the products were formed in good yields, but compounds 5, which resulted from the least stable dienal, 2b, were obtained in lower yields. Also the 2-methyl substituted maleimide gave a slower reaction and lower yield. The endo-selectivity was good to excellent in all cases.
The prepared α,β-unsaturated esters are themselves interesting as reactants to construct new, more complex natural product-like heterocycles, as exemplified by the synthesis of compound 8 by means of a subsequent highly diastereoselective intramolecular Michael addition (Scheme 1). The configuration of the newly formed stereocenter was determined by NOE measurements. Compound 8 has structural similarities to cytotoxic natural products such as quadrone16 and suberosenol A.17
For the synthesis of a CB-inspired compound collection we envisioned reduction of the intermediate aldehyde formed in the cycloaddition reaction to avoid potential reactivity problems. For establishment of a suitable synthesis sequence initially the cycloadducts were synthesized as racemates in a thermal non-enantioselective Diels–Alder reaction as shown in Scheme 2. To this end, the dienes were equipped either with a methyl ether (9a) or an allyl ether (9b) which would enable subsequent ring closure by means of ring closing metathesis. The obtained imides 10 were then selectively reduced with DIBAL,18 activated as sulfones, and then coupled with Grignard-reagents using a zinc-mediated reaction19 to produce 11a–g with an additional substituent in the pyrrolidine ring by analogy to the structure of CB. Imides with unprotected N-H did not provide the expected products, but the corresponding TBS-protected imides conveniently gave the substituted N-H amides 11f–g without a separate deprotection step. Boc-protection of these two amides was necessary for further reactions. To further approximate the CB structure, α-hydroxylation of the amide is necessary. For α-hydroxylation of related compounds the Davis oxaziridine has previously been employed,20 and 12b–c were synthesized using similar conditions. To prepare compounds 12a and 12d, however, the reaction with dioxygen18 was more effective. The tertiary alcohols 12 proved unreactive to coupling with acids using coupling reagents such as carbodiimides,20 PyBOP, or HBTU. The alcohols were also unreactive to acid chlorides and 4-DMAP and required deprotonation with sodium hydride to react with acid chlorides. Finally metathesis of 13d–e successfully afforded the macrocyclic compounds 14a–b21 with the same ring-size as CB. Several of the synthesized compounds were also prepared asymmetrically using trienamine catalysis, followed by reduction with sodium borohydride, and then either methylation with TMS-diazomethane, or allylation with tert-butyl-allyl carbonate22 (Scheme 3).
![]() | ||
Scheme 2 Synthesis of a CB inspired compound collection. PMB = para-methoxybenzyl, TMSE = 2-trimethylsilylethyl, Boc = tert-butoxycarbonyl. |
The compounds 4–8 and 10–15 were then investigated for inhibition of glucose import in the human colon cancer cell line HCT116 using a 2-deoxyglucose-based assay.23 Compounds which showed significant inhibition at the initially employed concentration of 180 μM were also analysed at 90 μM and 45 μM. The results revealed that compounds which embody the partially saturated isoindolinone core of CB but lack the medium-sized lactone ring can inhibit glucose import as was in particular observed for 11c and 12c (Fig. 2A). The absolute configuration of these cycloadducts is only of minor importance. These compounds carry only a substituent in the cyclohexane ring and a benzyl group in the pyrrolidine. Analogous bicyclic compounds with an additional substituent α to the carbonyl group (13) were not active. Notably, compound 14a which contains the macrocyclic ring was active in the glucose import assay. Bicyclic compound 12c and tricyclic CB analog 14a were finally investigated for possible influence on the actin cytoskeleton (Fig. 2B). Very gratifyingly, at 180 μM concentration both compounds did not impair the actin cytoskeleton in cells.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ob02272j |
This journal is © The Royal Society of Chemistry 2016 |