Synthesis of 2,4-disubstituted piperidines via radical cyclization: unexpected enhancement in diastereoselectivity with tris(trimethylsilyl)silane

A novel approach to 2,4-disubstituted piperidines is reported, involving the radical cyclization of 7-substituted-6-aza-8-bromooct-2-enoates. Cyclization with tributyltin hydride affords the trans piperidines with trans/cis diastereomeric ratios ranging typically from 3:1 to 6:1. Cyclization with tris(trimethylsilyl)silane affords the same products with diastereomeric ratios of up to 99:1 in certain cases. The enhancement in diastereoselectivity results from the selective rearrangement of the minor stereoisomer through a cascade process involving radical cyclization to the piperidine radical, 1,5-radical translocation, and attack of the translocated radical onto the sulfonamide with extrusion of SO2 in a Smiles-type rearrangement. Slower trapping of the piperidine radical by tris(trimethylsilyl)silane compared to tributyltin hydride accounts for the occurrence of the rearrangement cascade in the former case.     

A new general synthesis of functionally substituted pyrazolo[1,5-a]pyrimidines

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NMR investigation of photoinduced chiral inversion in (R)/(S)-naproxen–(S)-tryptophan linked system

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Capturing polycyclic aromatic sulfur heterocycles in electron donor–acceptor complexes

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Pentafluoroperbenzoic acid as the efficient reagent for Baeyer–Villiger oxidation of cyclic ketones

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Unusual examples of methane hydrate nucleation in bilayer water–oil systems

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Synthesis and photophysical studies of novel 4-aryl substituted 2-phenyl-, 2-(fluoren-2-yl)- and 2-cymantrenylquinazolines

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Reaction of (2-methoxyprop-2-yl)diphenylphosphine oxide with alkyl bromides

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Enzyme self-inactivation is a main limitation of the preparation of eicosanoids

Synthesis of prostanoids is accompanied by various processes reducing the product yield. These processes are also known to affect syntheses of thromboxane (TX) and 12(S)-hydroxy-5(Z),8(Z),10(E),14(Z)-eicosatetraenoic acid (12-HETE). Partially purified preparations of TX synthase and prostaglandin (PG) synthase were used to optimize TX synthesis with respect to concentrations of the enzymes and eicosapolyenoic acid (EPA). Conditions for the maximum product yield and the minimum consumption of enzymes were determined. Consumption of the TX synthase was large owing to its inactivation during the reaction and the nonenzymatic destruction of the intermediate product PG-endoperoxide. Separate addition of PG and TX synthases increased the product yield by preventing EPA sorption on ballast proteins. Microsomal 12-lipoxygenase (12-LO) was also shown to be inactivated during the reaction, and this process was the major limitation of 12-HETE synthesis. Lipoxygenase reaction in the presence of some reducing agents led to a considerable increase of the 12-HETE yield, supposedly by preventing further oxidation of the 12-LO reaction product 12-hydroperoxy derivative of eicosatetraenoic acid. The possibility of using human blood platelet microsomes for preparation of some derivatives of EPAs is discussed.