1. A convenient incorporation of conformationally constrained 5,5-dimethylproline into the ribonuclease A 89-124 sequence by condensation of synthetic peptide fragments
V Cerovský, E Welker, H A Scheraga J Pept Res. 2003 Mar;61(3):140-51. doi: 10.1034/j.1399-3011.2003.00041.x.
The presence of l-5,5-dimethylproline (dmP) within an amino acid sequence results in the formation of an X-dmP peptide bond predominantly locked in a cis conformation. However, the common use of this unnatural amino acid has been hampered by the difficulty of the economical incorporation of the dmP residue into longer peptide segments due to the steric hindrance imposed by the dimethyl moieties. Here, we describe synthesis of the C-terminal 36-residue peptide, corresponding to the 89-124 sequence of bovine pancreatic ribonuclease A (RNase A), in which dmP is incorporated as a substitute for Pro93. The peptide was assembled by condensation of protected 5- and 31-residue peptide fragments, which were synthesized by solid-phase peptide methodology using fluorenylmethyloxycarbonyl chemistry. We focused on optimizing the synthesis of the Fmoc-Ser(tBu)-Ser(tBu)-Lys(Boc)-Tyr(tBu)-dmP-OH pentapeptide (residues 89-93) with efficient acylation of the sterically hindered dmP residue. In a comparative study, the reagent O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate was found to be superior to bromo-tris-pyrrolidino-phosphonium hexafluorophosphate and tetramethylfluoroformamidinium hexafluorophosphate for the synthesis of the -Tyr(tBu)-dmP- peptide bond in solution as well as on a resin.
2. Stereoselective Polymer-Supported Synthesis of Morpholine- and Thiomorpholine-3-carboxylic Acid Derivatives
Petra Králová, Veronika Fülöpová, Michal Maloň, Tereza Volná, Igor Popa, Miroslav Soural ACS Comb Sci. 2017 Mar 13;19(3):173-180. doi: 10.1021/acscombsci.6b00178. Epub 2017 Jan 30.
Herein we report the polymer-supported synthesis of 3,4-dihydro-2H-1,4-oxazine-3-carboxylic acid derivatives using immobilized Fmoc-Ser(tBu)-OH and Fmoc-Thr(tBu)-OH as the starting materials. After the solid-phase-synthesis of N-alkyl-N-sulfonyl/acyl intermediates, the target dihydrooxazines were obtained using trifluoroacetic acid-mediated cleavage from the resin. This approach was also studied for the preparation of dihydrothiazines from immobilized Fmoc-Cys(Trt)-OH. Inclusion of triethylsilane in the cleavage cocktail resulted in the stereoselective formation of the corresponding morpholine/thiomorpholine-3-carboxylic acids. Stereochemical studies revealed the specific configuration of the newly formed stereocenter and also the formation of stable N-acylmorpholine rotamers.
3. Polymer-Supported Stereoselective Synthesis of Benzoxazino[4,3-b][1,2,5]thiadiazepinone 6,6-dioxides
Petra Králová, Michal Maloň, Tereza Volná, Veronika Ručilová, Miroslav Soural ACS Comb Sci. 2017 Oct 9;19(10):670-674. doi: 10.1021/acscombsci.7b00115. Epub 2017 Aug 31.
Herein, we report the stereoselective synthesis of trisubstituted benzoxazino[4,3-b][1,2,5]thiadiazepinone 6,6-dioxides from polymer-supported Fmoc-Ser(tBu)-OH and Fmoc-Thr(tBu)-OH. After the solid-phase synthesis of N-alkylated-N-sulfonylated intermediates using various 2-nitrobenzenesulfonyl chlorides and bromoketones, the target compounds were obtained via trifluoroacetic acid (TFA)-mediated cleavage from the resin, followed by cyclization of the diazepinone scaffold. Except for the threonine-based intermediates, the inclusion of triethylsilane (TES) in the cleavage cocktail yielded a specific configuration of the newly formed C3 chiral center. The final cyclization resulted in minor or no inversion of the C12a stereocenter configuration.
