Total synthesis and biological evaluation of fluorinated cryptophycins

Cryptophycins are cytotoxic natural products that exhibit considerable activities even against multi-drug-resistant tumor cell lines. As fluorinated pharmaceuticals have become more and more important during the past decades, fluorine-functionalized cryptophycins were synthesized and evaluated in cell-based cytotoxicity assays. The unit A trifluoromethyl-modified cryptophycin proved to be highly active against KB-3-1 cells and exhibited an IC50 value in the low picomolar range. However, the replacement of the 3-chloro-4-methoxyphenyl-substituent in unit B by a pentafluorophenyl moiety resulted in a significant loss of activity.

For the chiral analytical HPLC a Thermo Separation Products system equipped with a UV-6000 detector, a P-4000 pump and a Daicel Chiralpak 10 µm column (C18; 250 × 4.60 mm) was used. A flow rate of 1 mL min −1 using an eluent consisting of hexane/isopropanol (9/1) was employed (method M4).

General procedure GP2 -Macrolactamization
After addition of absolute piperidine (5.0 equiv) at room temperature to a solution of the acyclic depsipeptide (1.0 equiv in absolute DMF, 30 mL/mmol), the reaction mixture was stirred overnight in the dark, and the conversion was monitored by TLC.
Subsequently, the reaction mixture was diluted with ethyl acetate (300 mL/mmol) and washed with H 2 O (400 mL/mmol). The aqueous phase was extracted with ethyl acetate (3 × 200 mL/mmol) and the combined organic layers were washed with brine (100 mL/mmol), dried over MgSO 4 and evaporated to dryness in vacuum. The residue was purified by flash chromatography.

General procedure GP3 -Cleavage of the acetonide
Trifluoroacetic acid (10 mL/mmol) and water (5 drops) were added at 0 °C to a solution of the acetonide (1.0 equiv) in absolute dichloromethane (10 mL/mmol). The reaction mixture was stirred at this temperature while the reaction progress was monitored by TLC. After complete conversion (2-3 h) the solvent was removed in vacuum without heating the bath, the residue was dissolved in EtOAc (500 mL/mmol), and saturated NaHCO 3 solution (750 mL/mmol) was added. After phase separation the aqueous layer was extracted with EtOAc (3 × 250 mL/mmol).
The combined organic layers were dried over MgSO 4 , filtered and evaporated to dryness in vacuum. The resulting diol was dried under high vacuum and subjected to further reactions without purification.

General procedure GP4 -Synthesis of the cyclic orthoformate
The diol resulting from GP3 and PPTS (2.5 equiv) were dried overnight under high vacuum. Under argon atmosphere absolute dichloromethane (30 mL/mmol) and absolute trimethyl orthoformate (10 mL/mmol) were added. The reaction mixture was 6 stirred for two hours at room temperature while the reaction progress was monitored by TLC or analytical RP-HPLC. The reaction mixture was then filtered through a thin plug of silica gel, which was washed with EtOAc/CH 2 Cl 2 (300 mL, 1:1 v/v). The combined filtrates were evaporated to dryness in vacuum. The resulting cyclic orthoformate was subjected to further conversions after drying under high vacuum without purification.

General procedure GP5 -Synthesis of the bromohydrin formate
The orthoformate (1.0 equiv) was dissolved in absolute CH 2 Cl 2 (15 mL/mmol) and a 0.5 M solution of acetyl bromide in absolute CH 2 Cl 2 (2.5 equiv acetyl bromide) was added. The resulting solution was stirred for four hours at room temperature while the reaction progress was monitored by TLC or analytical RP-HPLC. Subsequently, the reaction mixture was diluted with absolute CH 2 Cl 2 (10 mL) and then poured into an ice-cold mixture of saturated NaHCO 3 solution and ice water (50 mL, 1:1, v/v). The mixture was washed with absolute CH 2 Cl 2 and after phase separation the aqueous layer was extracted with CH 2 Cl 2 (3 × 20 mL