Synthetic approach to microsclerodermins: construction of three building blocks

As an approach to the construction of microsclerodermins ( 1 ), the three building blocks 2 , 3 , and 4 were efficiently prepared from the carboxylic acids 6 , 16 , and 28 .


Introduction
Microsclerodermins have been isolated from the lithistid marine sponge Microscleroderma sp. by Faulkner and co-workers. 1 They are a family of complex cyclic peptides that display potent anti-fungal and anti-proliferative activities.One of the representative peptides are microsclerodermins A and B whose structures have been determined to be 1a and 1b, respectively, as shown in Fig. 1.Our continuing interests in the synthesis of biologically active aquatic natural peptides 2 led to synthesize micro-sclerodermins.Toward this end, we have already accomplished the synthesis of four requisite building blocks: 3 the tryptophan derivative 2, 3d the hydroxypyrrolidinone equivalent 3, 3d 4-amino-3-hydroxybutanoic acid (GABOB) derivative 4, 3d and AMMTD 5, 3a-c,e shown in Fig. 1.We now report the details of the synthesis of 2-4.3d

Results and Discussion
We first synthesized the tryptophan derivative 2. We have already developed 4 the method for the asymmetric synthesis of α-amino acids by alkylation of the imine 10 derived from glycine ester and optically active 2-hydroxy-3-pinanone (HyPN, 9).The alkylating agent 8 to carry out this method was prepared from 2-indolecarboxylic acid (6) by esterification with iodomethane, Mannich reaction, and then quaternization, as shown in Scheme 1.The corresponding bromo derivative was also prepared, but it was found to be labile and the chemical yield was variable.The imine 10 derived from (-)-HyPN (9) 4a,b was lithiated with lithium diisopropylamide (LDA) and the resulting enolate was alkylated with 8 to give the alkylated imine 11, which was directly treated under weak acidic conditions to remove the chiral auxiliary 9.The resulting amine was converted to the corresponding tert-butoxycarbonyl (Boc) derivative 2 with Boc 2 O.The enantiomeric excess of 2 was 87%, and it raised to 91% by recrystalization.

Scheme 1
The absolute configuration of the tryptophan derivative 2 should be (R), 4 but it was further unambiguously confirmed by transformation to the known homoserine derivative 15, as shown in Scheme 2. Thus the tryptophan derivative 2 with 87% ee was oxidized with ruthenium tetroxide to give the aspartic acid derivative 13 which was purified as the methyl ester 14.Although the compound 14 was known, 5 its specific rotation was not reported.Therefore, the compound 14 was further transformed to the homoserine derivative 15 in three steps, shown in Scheme 2. The homoserine derivative 15 thus obtained was completely identical with 15 derived from (S)-aspartic acid according to the known method, 6 but the sign of their specific rotations were opposite to each other: [α] D 25 +32.3 (c 1.09,EtOH) for 15 from 2, +37.1 (calcd.for 15 with 100% ee), reported 6 [α] D 25 -37.5 (c 1, EtOH).These results clearly indicate that the absolute configuration of the tryptophan derivative 2 is (R), as expected.

Scheme 2
The requisite pyrrolidinone derivative 3a was prepared from N-Boc-(S)-aspartic acid 4-benzyl ester (16), as shown in Scheme 3. Homologation of 16 was achieved by treatment with 1,1'-carbonyldiimidazole (CDI) and then the magnesium salt of malonic acid half ester to give the β-keto ester 17.After catalytic hydrogenolytic removal of the benzyl function, the resulting acid 18 was treated with ethyl chlorocarbonate-triethylamine and then aqueous ammonia.The products were revealed to be a diastereoisomeric mixture of the cyclic hemiaminals 3a and epi-3a in a ratio of 3:1. 8

Scheme 3
The hemiaminal 3a easily underwent the dehydration under both acidic and basic conditions to give theα,β-unsaturated ester 19, shown in Scheme 4.

Scheme 4
These results suggest that the synthesis of microsclerodermins could proceed via theγ-lactone 21, which will give the keto-amide 20b, equivalent to the hemiaminal 20a, by alkaline hydrolysis, oxidation of the hydroxyl group, and amidation, as shown in Scheme 5.

Scheme 5
Thus, the γ -lactone trimethylsilylethyl (TMSE) ester 24 was prepared from 16 by transformation to the imidazolide, homologation with TMSE acetate, 9 catalytic removal of the benzyl function, reduction with sodium borohydride, and lactonization with 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC), as shown in Scheme 6. Attempted removal of the TMSE group with tetrabutylammonium fluoride (TBAF) did not give the γ-lactone 3b, but resulted in the formation of the α,β-unsaturated dicarboxylic acid 25. 10 To prepare 3b in more efficient way, the β-keto ester 17 was first transformed to the hydroxy dicarboxylic acid 26, which underwent the lactonization with EDC to give 3b, as shown in Scheme 6.The lactone 3b was fully characterized by its transformation to the sarcosine derivative 27.Thus, we could succeed in the efficient synthesis of 3b in shorter steps.

Scheme 6
Finally, the synthesis of the required GABOB derivative 4 started from dimethyl (R)-malate, which was regioselectively reduced according to the known method, 11 as shown in Scheme 7. The
To a stirred solution of the crude amine (9.9 mmol) in dioxane (30 ml) was added Boc Aqueous KHSO 4 (1M, 5 ml) was added to the reaction mixture and the aqueous layer was extracted with EtOAc (10 ml x 3).The organic layer was washed with saturated brine (10 ml), dried over Na 2 SO 4 , filtered, and concentrated in vacuo to give the crude carboxylic acid 13 which was used for the next step without further purification.

(3S,4RS)-3-N-(tert-Butoxycarbonyl)amino-4-[(trimethylsilyl)ethyl]methyl-butyrolactone (24).
To a stirred solution of 22 (96 mg, 0.206 mmol) in EtOAc (1 ml) was added 5% Pd/C (40 mg) under argon atmosphere at room temperature.The mixture was stirred for 12 h under hydrogen atmosphere at room temperature, and filtered through a pad of celite, concentrated in vacuo to give the carboxylic acid 23 as a colorless oil, which was used for the next step without further purification.
To a stirred solution of the crude carboxylic acid 23 (0.206 mmol) in EtOH (1 ml) was added NaBH 4 (13 mg, 0.343 mmol) at 0°C.The mixture was stirred for 15 min at 0°C.The reaction mixture was quenched with 1M aqueous KHSO 4 (3 ml), extracted with CHCl 3 (10 ml x 3), dried over Na 2 SO 4 , filtered, and concentrated in vacuo to give the crude hydroxy acid as a colorless oil, which was used for the next step without further purification.
To a stirred solution of the crude alcohol (0.325 mmol) in THF (1 ml) was added dropwise 1N aqueous NaOH (0.9 ml, 0.9 mmol) at 0°C.The mixture was stirred for 5 h at 0°C, then transferred to a separating funnel with water (50 ml).The aqueous layer was washed with CHCl 3 ( Boc-Lactone 27.To a stirred solution of 26 (345 mg, 1.25 mmol) in CH 2 Cl 2 (13 ml) and DMSO (13 ml) were added EDC•HCl (358 mg, 1.88 mmol) and DMAP (16 mg, 0.13 mmol) at 0°C.The mixture was stirred for 17 h at room temperature.After dilution with ether (100 ml), the mixture was washed with 1M aqueous KHSO 4 (30 ml), saturated brine (30 ml), dried over MgSO 4 , filtered, and concentrated in vacuo to give the crude lactone carboxylic acid (134 mg).The aqueous layer was salted out and extracted with ether (10 ml x 10).The combined organic solution was dried over MgSO 4 , filtered, and concentrated in vacuo to give the crude 3b (312 mg) as a colorless amorphous solid, which was used for the next step without further purification.(IR ν max (CHCl 3 ) 1786 cm -1 (lactone C=O) ).
To a stirred solution of Boc-MeGly-OTce (577 mg, 1.8 mmol) in CHCl 3 (4 ml) was added TFA (2 ml) at room temperature.The mixture was stirred for 1 h at room temperature., then concentrated in vacuo to give the light yellow residue.Toluene was added to the whole and the mixture was concentrated in vacuo.This work-up was repeated three times to remove the excess of TFA completely.The crude TFA salt was used for the next step without further purification.
The residue was purified by column chromatography (silica gel BW-200, 25 g, hexane : EtOAc = Methyl (R)-3,4-dihydroxybutanoate (29).In a 300 ml, three necked, round-bottled flask mounted with a reflux condenser and thermometer was placed a solution of (R)-dimethyl malate (28) (5 ml, 37.7 mmol) in THF (80 ml).To this was added 10.0-10.2M borane-methylsulfide complex (3.9 ml, 39.0-39.8mmol) dropwise at room temperature under stirring during 30 min.The solution was stirred at room temperature until evolution of hydrogen gas ceased (ca.30 min).Then, the flask was cooled with a water-ice bath and stirring was continued for 30 min.To the solution was added NaBH 4 (72 mg, 1.90 mmol) in one portion (exothermic) under vigorous stirring at 0°C.When the exothermic reaction subsided, the water bath was removed and the reaction was stirred at room temperature for 30 min.MeOH (13 ml) and p-TsOH (360 mg, 1.9 mmol) were added to the reaction mixture, and the resulting slightly cloudy solution was stirred for 30 min at room temperature, followed by concentration to give a colorless gum.This was dissolved in benzene (40 ml) and MeOH (40 ml), and the resulting solution was concentrated again: this operation was repeated.To the residue was added benzene (60 ml) and the solution was concentrated, which was repeated to eliminate MeOH and B(OMe) 3 as thoroughly as possible to give a clear, colorless gum.