Abstract
Starting from 2-(triphenylsilyl)ethanol a new oxycarbonyl protecting group cleavable by fluoride ion induced Peterson-elimination has been developed. Known 2-(triphenylsilyl)ethanol has been prepared from commercially available triphenylvinyl-silane by a hydroboration-oxidation sequence using the sterically hindered borane reagent 9-BBN. The silyl alcohol was subsequently transformed into its chloroformate, imidazolylcarboxylic acid ester and p-nitrophenyl carbonate and used in standard protocols for the formation of carbamates and carbonates. The Tpseoc group proved to be highly resistant against acidic conditions applied in removal of tert-butyl esters and the t-Boc-group. It also withstood catalytic hydrogenation, treatment with morpholine, methylhydrazine and Pd-reagents/allyl-scavanger combinations, conditions required to cleave Cbz-, Fmoc-, phthalimide- and Alloc-groups. The Tpseoc-group is cleaved upon treatment with TBAF/CsF at 0 °C or r.t. with cleavage times reaching from <10 min. to 24 h. Its orthogonality, ease of cleavage and UV-detectability makes the Tpseoc-group a promising alternative to other widely used silicon based amine protecting groups like the Teoc- and SES-groups.
Highlights
Masking potentially reactive sites of a polyfunctionalized organic molecule with appropriate protecting groups is a fundamental process in modern synthetic organic chemistry
During our investigation of the synthesis of glycopeptide mimetics we were prospecting for an amine protecting group which should be cleavable by fluoride ions under mild conditions, but at the same time resistant enough to survive the acidic conditions applied to cleave the Boc-group
Despite the estimated favorable properties of the 2-(triphenylsilyl)ethyl moiety mentioned, we found no further evidence in the literature of its use in any kind of protecting strategy, leading to the opinion that an investigation of an amine protecting group based on the oxycarbonyl derivative of
Summary
Masking potentially reactive sites of a polyfunctionalized organic molecule with appropriate protecting groups is a fundamental process in modern synthetic organic chemistry. Protection of the amine moiety in amino acids underwent a constant evolution from simple amides to more sophisticated carbamates resulting in the triumvirate of amine protecting groups consisting of the Boc- [2], Cbz- [3] and Fmoc-group [4]. Those three justify their superiority by the ease of installation, mild cleavage conditions, the excellent orthogonality among each other and not the least, the vast experience gained by the community of synthetic organic chemists since their introduction in the 1930s and 70s. By far the most popular among them is the Teoc-group [5], based on the
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