SCHEME 1.

Reagents and conditions: Butyric anhydride, triethylamine, anhydrous pyridine, rt, 48 h. all reactions were followed by TLC carried out on Merk silica gel 60 F254 plates with a fluorescent indicator on the plates were visualized with UV light (254 nm). Preparative chromatographic purifications were performed using a silica gel column (Kieselgel 60). Solutions were concentrated with a Buchi R‐114 rotary evaporator at low pressure. Melting points, determined using a Buchi melting point B‐540 instrument, are uncorrected and represent values obtained on chromatographically purified material. RP‐HPLC preparative purification was performed on a Shimadzu prominence LC‐20 AP system equipped with a multiwavelength prominence SPD‐20A UV–vis detector on a Phenomenex Kinetex XB‐C18 column (5 mm, 21.2 × 250 mm) employing the following solvents: A: 100% acetonitrile in 0.1% TFA, B: 100% H₂O in 0.1% TFA. The operational conditions were as follows: Linear‐gradient of 5%–70% acetonitrile + 0.1% TFA in 30 min, using a flow rate of 30 ml·min^‐1. The purity of the products was assessed by analytical RP‐HPLC using a Phenomenex Kinetex XB‐C18 column (5 μm, 4.6 × 250 mm). The column was connected to a Rheodyne model 7725 injector, a Shimadzu‐10 ADsp HPLC system, a Shimadzu SPD‐20 a/SPD‐20 AV UV–vis detector set to 254 nm. Mass spectra were performed on LTQ Orbitrap XL™ Fourier transform mass spectrometer (FTMS) equipped with an ESI ION MAX™ source (Thermo fisher, San José, USA)—Negative mode. ¹H‐NMR spectrum was recorded on Varian mercury plus 400 MHz instrument. Chemical shifts are reported in parts per million. The following abbreviations are used to describe peak patterns when appropriate: S (singlet), d (doublet), t (triplet), m (multiplet), bs (broad singlet)