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Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

The review covers the knowledge on photoremovable protectingngroups and includes all relevant chromophores studied in thentime period of 2000–2012; the most relevant earlier works arenalso discussed.

Versatile and Expeditious Synthesis of Aurones via AuI-Catalyzed Cyclization

Aurones are conveniently formed in a three-step procedure including a goldI-catalyzed cyclization of 2-(1-hydroxyprop-2-ynyl)phenols as a highly regio- and stereoselective key step. A wide diversity of derivatives can be obtained starting from substituted salicylaldehydes. Synthesis of natural 4,6,3,4-tetramethoxyaurone and structure revision of two natural products (dalmaisione D and 4-chloroaurone) were achieved.

Mechanistic Studies and Improvement of Coinage Metal-Catalyzed Transformation of Alkynyloxiranes to Furans: An Alcohol Addition−Cyclization−Elimination Cascade

In the presence of alcohol Ag or Au salts or complexes catalyze the conversion of alkynyloxiranes to substituted furans. Both catalysts are effective, and a large furan diversity can be obtained in high yield with one or the other catalyst. Mechanistic studies revealed that a cascade pathway and not the sometimes reported direct intramolecular nucleophilic addition of oxirane oxygen atom to intermediate acetylene-metal pi-complex occurs. Under the defined conditions, the intermediate formation of epoxide opening products has been identified. Depending on the catalyst, one or both of the latter cyclized to dihydrofurans, and further elimination of the alcohol led to the corresponding furans. These results highlight the duality between oxophilicity and alkynophilicity of Ag or Au salts.

Silver(I)-Catalyzed Cascade: Direct Access to Furans from Alkynyloxiranes

Functionalized furans are conveniently formed by a new silver(I)-catalyzed reaction of alk-1-ynyl oxiranes in the presence of p-toluenesulfonic acid and methanol. Evidence supported a cascade mechanism.

Gold(I)-catalyzed cycloisomerization of β-alkynylpropiolactones to substituted α-pyrones.

Substituted α-pyrones were straightforwardly synthesized in good to excellent yields by a new gold(I)-catalyzed rearrangement of β-alkynylpropiolactones.

Gold(I)-catalyzed rearrangement of N-aryl 2-alkynylazetidines to pyrrolo[1,2-a]indoles.

Various N-aryl-2-alkynylazetidines were very efficiently converted to pyrrolo[1,2-a]indoles with gold catalysts, especially the 2-biphenyl-dicyclohexylphosphino-gold(I) hexafluoroantimonate, in dichloromethane at room temperature. Additionally, two formal syntheses of bioactive non-natural compounds, i.e. 7-methoxymitosene and an 5-HT(2C) receptor agonist, have been achieved.

Gold(I)/(III)-catalyzed rearrangement of divinyl ketones and acyloxyalkynyloxiranes into cyclopentenones.

Multifaceted gold(I/III) catalysts with their carbophilic and oxophilic characters catalyzed very efficiently the formation of hydroxylated cyclopentenones from simple divinyl ketones or acyloxyalkynyloxiranes. The Nazarov reaction is rapidly performed in dichloroethane with 5 mol % of the simple gold(III) trichloride salt at 70 °C, while the rearrangement of alkynyloxiranes requires 5 mol % of a more stable NHC gold(III) triflimidate complex.

Inorganic–Organic Heteropolyacid–Gold(I) Hybrids: Structures and Catalytic Applications

Gold(I)-polyoxometalate hybrid complexes 1-4 ([PPh3AuMeCN]xH4-x SiW12O40, x=1-4) were synthesized and characterized. The structure of the primary gold(I)-polyoxometalate 1 (x=1) was fully ascertained by XRD, FTIR, (31)P and (29)Si magic-angle spinning (MAS) NMR, mass spectroscopy, and SEM-energy dispersive X-ray spectroscopy (EDX) techniques. Moreover, this complex exhibited better catalytic activity and selectivity compared with standard, homogeneous, gold catalysts in the new rearrangement of propargylic gem-diesters.

Coinage metals-catalyzed cascade reactions of aryl alkynylaziridines: silver(I)-single vs gold(I)-double cyclizations.

Alkynylaziridines carrying an aryl group could be efficiently converted into aminoallenylidene isochromans, isoquinolines, or tetrahydronaphtalenes with silver(I) salts and into 1-azaspiro[4.5]decane derivatives with gold(I) complexes. Mechanistic investigations revealed that both Ag- and Au-catalyzed reactions involved a Friedel-Crafts type intramolecular reaction leading to an allene and that Au also rapidly promoted a second intramolecular cyclization of the aminoallene intermediate to the corresponding spiro derivative. Stereochemical investigations suggested an anti-SN(2)-type pathway for the first cyclization leading to a stereodefined allene, which could then be cyclized to the corresponding stereodefined spiro product. These results highlight the duality between oxo- or azaphilicity and alkynophilicity of Ag and Au as well as their complementarity in terms of reactivity.

Silver(I)-catalyzed deprotection of p-methoxybenzyl ethers: a mild and chemoselective method.

The p-methoxybenzyl protecting group (PMB) on various alcohols and an acid was efficiently and selectively cleaved by the action of a catalytic amount of silver(I) hexafluoroantimonate combined with 0.5 equiv of 1,3,5-trimethoxybenzene in dichloromethane at 40 °C.