An efficient, stereoselective synthesis of (−)-bulgecinine from (S)-aspartic acid

Hexafluoroacetone as Protecting and Activating Reagent: New Routes to Amino, Hydroxy, and Mercapto Acids and Their Application for Peptide and Glyco- and Depsipeptide Modification

“…The diazoketone derived from HFA[Asp(Cl)] and ethyl diazoacetate when treated with dirhodium tetraacetate forms a cyclic enol, which tautomerizes during purification to give the ketone. The trans-stereoisomer is formed exclusively. , The stereoselectivity can be explained by the concave shape of the bicyclic HFA-prolines. Thus, substituents such as the ethyl carboxylate prefer the exo position to avoid steric strain, giving the trans -dicarboxylate (Scheme ).…”

“…Reduction of the 4-oxo group with Na[BH 3 CN] proceeds stereoselectively because the hydride transfer occurs preferentially from the Re face (de = 88%). Subsequent cleavage of the lactone and reduction of the ethyl carboxylate in the 5-position provides efficient access to bulgecinine [( 2S, 4 S ,5 R )-4-hydroxy-5-hydroxymethylproline] (Scheme ). , Bulgecinine is the aglycone of bulgecines, a class of glycopeptide antibiotics attracting growing interest due to their synergistic effect on the antibacterial activities of β-lactams. −

44

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“…The trans-stereoisomer is formed exclusively. 120,121 The stereoselectivity can be explained by the concave shape of the bicyclic HFA-prolines. Thus, substituents such as the ethyl carboxylate prefer the exo position to avoid steric strain, giving the trans-dicarboxylate (Scheme 43).…”

“…Subsequent cleavage of the lactone and reduction of the ethyl carboxylate in the 5-position provides efficient access to bulgecinine [(2S,4S,5R)-4-hydroxy-5-hydroxymethylproline] (Scheme 44). 120,121 Bulgecinine is the aglycone of bulgecines, a class of glycopeptide antibiotics attracting growing interest due to their synergistic effect on the antibacterial activities of β-lactams. [122][123][124]…”

Hydrosilylation with Bis(alkynyl)(1,5-cyclooctadiene)platinum Catalysts: A Density Functional Study of the Initial Activation

“…A second approach is the encapsulation of the platinum catalyst in a finely divided material such as a thermoplastic silicone resin, an organic thermoplast, or a cyclodextrine, ,, which does not release the catalyst until a certain temperature is reached; problems with this approach include the large size of the capsules (preventing the formation of stable suspensions) and the sudden release of locally high catalyst doses (resulting in inhomogeneous cross-linking). A third method is the design of new classes of platinum catalysts that show essentially no hydrosilylation activity at ambient temperatures and high curing rates above a tunable kick-off temperature, thus combining pot lives of several months with high curing rates . One of the best examples for such catalysts are bis(alkynyl)(1,5-cyclooctadiene)platinum complexes (COD)Pt(CCR) 2…”