Pyridine

2,4,6,8-tetrahydro-2,4,6,8-tetramethylcyclotetrasiloxane (D₄^H) was purchased from ABCR (Karlsruhe, Germany) and used as supplied. Tetrahydrofuran (THF) and toluene were supplied by Avantor Performance Materials (Gliwice, Poland) and were distilled from sodium-benzophenone prior to use. Pyridine was supplied by ChemPur (Piekary Śląskie, Poland) and distilled from calcium hydride before use. Pd/C (5% activated), Celite^®, chlorodimethylsilane (98%) (Me₂SiHCl) were supplied by Sigma-Aldrich (St. Louis, MO, USA). All other materials (anhydrous magnesium sulphate, calcium hydride) were purchased from Avantor Performance Materials.

Betulin > 95% (Natchem), succinic anhydride 99% (Sigma-Aldrich, St. Louis, MO, USA), pyridine (min. 99.5%, Chempur, Karlsruhe, Germany), acetic anhydride (POCh S.A., Gliwice, Poland), dicarboxylic derivatives of poly(ethylene glycol) (with Mn = 250 and Mn = 600) (ACROS Organics, Fair Lawn, NJ, USA), DMSO (Chempur), trichloroacetic acid (Chempur), acetic acid (Chempur), Tris buffer, sulforhodamine, poly(vinyl alcohol) (M_w = 88,000 g/mol, 88% hydrolyzed) (ACROS Organics), and methylene chloride (Chempur) were used as supplied.
KB, HeLa, MCF-7, and Hep-G2 cells were obtained from the European Collection of Cell Culture (ECACC) supplied by Sigma Aldrich. A-549, U-87 and HDF cells were purchased from the American Type Culture Collection (ATCC) through LGC Standards (Lomianki, Poland). KB, Hep-G2, and U-87 cells were cultured in EMEM medium, while HeLa cells were grown in RPMI 1640 medium, A-549 cells in F-12K medium, MCF-7 cells in DMEM medium, and HDF cells in Fibroblasts Growth Medium. Each medium was supplemented with 10% fetal bovine serum, 1% l-glutamine and 1% penicillin/streptomycin solution. All of the cultures were maintained at 37 °C in a humidified atmosphere containing 5% CO₂.