Monocarbamates, derived from (S)-2-(dibenzylamino)butane-1,4-diol, and the influence of the second O-protecting group on the regioselectivity of deprotonation application to the synthesis of the boletus toxin (2S,4S)-gamma-hydroxy-norvaline

[摘要]：Differentially protected 1-O- and 4-O-monocarbamates, derived from (S)-2-(dibenzylamino)butane-1,4-diol are prepared and investigated with respect to their capability of being deprotonated and forming the corresponding lithium carbanions. In the 1-O-trityl and methyl 4-O-monocarbamates 6a and 6b the pro-S-4-H is removed by sec-butyllithium/(-)-sparteine with high diastereoselectivity. The 1-O-(2-methoxyethyl) 4-O-monocarbamate (23e) undergoes a highly selective, substrate-controlled abstraction of the pro-S-4-H without addition of any diamine. On the other hand, the 4-O-methyl 1-O-monocarbamate 7a reacts with sec-butyllithium in diethyl ether with essentially complete stereoselectivity and forms the bicyclic chelate 33 complex with (S)-configuration at the lithiated C-1 atom. Trapping by means of iodomethane, CO2, and other electrophiles proceed with complete stereoretention. The method is applied for the synthesis of Boletus toxin (2S,4S)-gamma-hydroxy-norvaline in the form of the lactone hydrochloride. Further, evidence was found that the 2-(dimethylamino) group in the 1-O-TBDMS 4-O-monocarbamate 16 induces a highly substrate-controlled deprotonation in the (4S)-position.

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