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The Application Research of α-oxo Gold Carbene in the Syntiesis of Heterocycles and Heteroatom Compounds

Author: HeWeiMin
Tutor: XiangJianNan
School: Hunan University
Course: Organic Synthesis
Keywords: gold alkyne α-oxo gold carbene oxetan-3-one chiral azetidin-3-one 2,5-disubstituted oxazole chloro/bromom ethyl ketones
CLC: O626
Type: PhD thesis
Year: 2012
Downloads: 38
Quote: 0
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Heterocycles and heteroatom compounds are the molecules possessing one or more heteroatom(s) such as nitro, oxygen, sulfur, halogen and so on. They are not only found in alkaloids, nucleic acids, antibiotics, and other biologically active molecules but also the essential components of many drugs. Efficient, atom-economic, and selective constructions of heterocyle compounds from readily available starting materials under mild conditions remain an important task in synthetic chemistry.Gold-mediated homogeneous catalysisis is one of the hot topics in the organic chemistry. Gold compounds are rather unique soft Lewis acids which have been termed by Furstner as π-acid. The π-acid can activate π bonds for reactions with arange of nucleophiles, leading to sigmatropic rearrangements, migrations, and cycloisomerizations. A particularly important class of nucleophiles is an oxidant that possesses a nucleophilic oxygen and can formally deliver the oxygen atom during the reaction, and this type of gold-catalyzed alkyne oxidation has been proposed to generate a reactive a-oxo gold carbene intermediate, which would undergo an array of versatile transformations. Studies in this area use tethered oxidants such as sulfoxide, nitrone, epoxide, nitro, amine N-oxide and pyridine/quinoline N-oxides as external oxidants to achieve intermolecular alkyne oxidation.Recent years, we also have some studies in reactive a-oxo gold carbene intermediate, as shown in the following4parts:(1) A general solution for the synthesis of various oxetan-3-ones has been developed. This reaction uses readily available propargylic alcohols as substrates and proceeds without the exclusion of moisture or air ("open flask"). Notably, oxetan-3-one, a highly valuable substrate for drug discovery, can be prepared in one step from propargyl alcohol in a fairly good yield. Mechanistically, reactive a-oxo gold carbenes are generated as intermediates through intermolecular alkyne oxidation and subsequent intramolecular O-H insertion. This safe and efficient generation of gold carbenes offers a potentially general entry into a-oxo metal carbene chemistry without using hazardous diazo ketones. (2) A practical and flexible synthesis of chiral azetidin-3-ones has been developed. The key reaction is a gold-catalyzed oxidative cyclization of chiral N-propargylsulfonamides. Mechanistically, reactive a-oxo gold carbenes are generated as intermediates through intermolecular alkyne oxidation and subsequent intramolecular N-H insertion. The use of tert-butylsulfonyl as the protecting group takes advantage of the chiral tert-butylsulfinimine chemistry and avoids additional unnecessary deprotection and protection steps. Moreover, the Bus group can be easily removed from the azetidine ring under acidic conditions.(3) The first efficient intermolecular reaction of gold carbene intermediates generated via gold-catalyzed alkyne oxidation has been realized using nitriles as both the reacting partner and the reaction solvent, offering a generally efficient synthesis of2,5-disubstituted oxazoles with broad substrate scope. The overall reaction is a [2+2+1] annulation of a terminal alkyne, a nitrile, and an oxygen atom from an oxidant. The reaction conditions are exceptionally mild, and a range of functional groups are easily tolerated. With complex and/or expensive nitriles, only3equiv could be sufficient to achieve serviceable yields in the absence of any solvent and using only1mol%BrettPhosAuNTf2as the catalyst. (4) We have reported a surprising halide abstraction by a-gold carbenes generated via gold-catalyzed intermolecular oxidation of terminal alkynes. Synthetically useful chloro/bromom ethyl ketones can be prepared in one-step from terminal alkynes. The strong electrophilicity of the gold carbene intermediates reveals that gold in general is less effective in back bonding than rhodium but perhaps similar to Ag.

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