Concise and Efficient Total Syntheses of Virenamides A and D

Concise total syntheses of linear thiazole-containing peptides virenamides A ( 1 ) and D ( 4 ), isolated from Australian ascidian Diplosoma virens have been accomplished from Boc-L-valine ( 6 ) in 7 steps. A cyclization between thioamide and bromoacetaldehyde was applied to form thiazole ring as a key step.


INTRODUCTION
Marine natural products are a rich source of novel peptides, many of which show high levels of biological activity [1][2][3]. For example, the thiazole-containing cyclic peptide largazole exhibits extremely potent antiproliferative activity against a number of cancer cell-lines including MDA-MB-231 mammary cells (GI50 7.7 nM), U2OS fibroblastic osteosarcoma cells (GI50 55 nM), HT29 colon cells (GI50 12 nM), and IMR-32 neuroblastoma cells (GI50 16 nM) and linear thiazole-containing peptide dolastatin 10 is one of the most potent antineoplastic agents [4][5]. Over a decade ago, Bowden et. al isolated five cytotoxic linear peptides virenamides A-E (1-5) from Australian ascidian Diplosoma virens and assigned their structures by extensive NMR experiments [6][7]. In 1999, Moody et. al had reported a stereoselective synthesis of virenamide B (2) in which an elegant diastereoselective addition of 2-lithiothiazole to oxime ether was applied to construct the thiazole ring in excellent yield and diastereomeric excess [8]. For a long term concern, we initiated the total synthesis of virenamides in order to investigate the potential bioactivity of the derivatives. Herein, we report the concise efficient total syntheses of virenamides A (1) and D (4) from accessible starting material Boc-L-valine (6), which would provide enough product for further biological studies.

Fig. 1 Chemical structures of virenamides A-E (1-5)
Our initial retrosynthetic analysis of 1 and 4 is outlined in Scheme 1. We envisaged that the thiazole ring of 1 and 4 could be constructed through a cyclization between thioamide 9 and bromoacetaldehyde 10. Thioamide 9 in turn could be obtained from a cheap starting material Boc-L-valine (6).

RESULTS AND DISCUSSION
In the present work we report the first total synthesis of virenamides A and D based on N-(tert-butoxycarbonyl)-Lphenylalanyl-N-[(S)-(-)-1-(thiazole-2-yl)-2-methyl-propyl]-L-valinamide (14), which was prepared from (S)-(-)-N-(tertbutoxycarbonyl)-1-(2-thiazolyl)-2-methylpropylamine (11) [13], as key intermediate. As illustrated in Scheme 1, the cyclization of thiamide 9 [9][10][11][12] with bromoacetaldehyde 10 to form thiazole is the key step for synthesis of Virenamides A and D and is crucial as it is prone to epimerization at the α-stereogenic center. We optimized the reaction conditions for this conversion. As can be seen from Table 1, when bromoacetaldehyde 10 effected the reaction (Entries 1, 2, no base), the deprotection of Boc group of 11 was observed because of the acidic condition where the simultaneous release of HBr during the cyclization. Thus, we treated the cyclization reaction mixture with Boc2O/TEA to get 11 in moderate yield, but HBr in the reaction mixture resulted in almost entirely racemization of the product [13]. Based on what the literature described [13], we tried several inorganic bases and organic bases to form thiazoline intermediate, which was used for next step without purification. Dehydration of thiazoline intermediate afforded 11 in different yields and ee values. Although inorganic base could give high yield in general (Entries 4, 5), it was found that organic base yield much better ee value (Entries 7-9). DIPEA was proved to be the best acid trapper, which gave a yield of 82% with 94.5% ee (Entry 7) according to chiral-HPLC analysis [14].
Having synthesized the key intermediate 11, the next step was to prepare intermediate 14.
We should prepare dipeptide 13 firstly, which was through a two-step sequence. Boc-(L)-Phenylalanine was reacted with L-valine methyl ester to give dipeptide ester in 95% yield, which was saponificated with 1M NaOH/THF to give dipeptide 13 [15] in 94% yield. Then the synthesis of 14 was achieved in two steps including removal of Boc group from 11 to provide amine hydrochloride 12 [8], Coupling of 12 with dipeptide 13 with ClCOOBu-i/NMM to give tripeptide 14 in 54% yield [16]. Likewise, removal of Boc group from 14 with AcCl in MeOH smoothly provided amine hydrochloride 15 in almost quantitative yield, which was used for next step without further purification. The structures of compound 1 and 4 were determined from spectroscopic as well as optical rotation analytical data, which were consistent with those described for the ntatural products [6][7]. The 1 H NMR spectrum of compound 1 revealed a double signal at δ 3.08 ppm (J 6.6 Hz) due to =CH protons, a triple signal at δ 5.13 ppm (J 6.7 Hz) due to CH2 protons, two single signals at δ 1.55 and δ 1.69 due to C22. The high resolution mass spectrum of compound  3-6, 8, 9) or determined by chiral-HPLC (entries 2, 7). c Isolated yields after flash column chromatography. d The starting material was completely consumed after 36 h. e The starting material was not completely consumed after 36 h.

CONCLUSIONS
In summary, we have developed a very concise route for the first total syntheses of virenamide A and D starting from Boc-L-valine in 7-steps (overall yields: 26% for virenamide A; 22% for virenamide D). Syntheses of these natural products and their derivatives in large scale could be realized by this route, which facilitates further biological experiments. Studies towards the structure modifications of these natural products for further pharmacological investigation are ongoing.