Silver-catalyzed synthesis

The silver-catalyzed synthesis of methylene butyrolactones has been known for a long time 39 the field was revived by Genet more recently.40 Scheme 12.21 shows that the silver(I) catalysis required 10 mol% of catalyst and 110°C, but then gave a very good yield of product. Scheme 12.22 displays the corresponding investigation of Genet and coworkers the different conditions and catalysts tested are shown in Table 12.15. 

Silver-Catalyzed Synthesis of Five-Membered Heterocycles... 

Dihydroquinolines are obtained via Cu- or Ag-cata-lyzed A -coupling/cyclization when using 2-alkynylbenz-aldehydes, amines, and ketones, which combines metal and enamine catalysis, as demonstrated by Wu et al. [119]. 2-Alkynylbenzaldehyde derivatives have also been used as the additional nucleophile carrier in the silver-catalyzed synthesis of naphthyridines and thienopyridines recently reported by Verma et al. [120], A variety of naphthyridines and thienopyridines were prepared via dual activation using L-proline as the organocatalyst, whereas for other... 

The aldehyde component has also been replaced by diethyl formylphosphonate hydrate in the silver-catalyzed synthesis of V-PMP-protected a-aminopropargylphospho-nates [139] and by 2-oxoacetic acid [140] or glyoxylic acid [141] in copper-catalyzed microwave-assisted decarboxyla-tive three-component couplings. In addition, glyoxylic acid has been coupled with amines and alkynes in the gold-catalyzed synthesis of butenolides (see Scheme 3.48) [121]. 

SCHEME 8 Proof of concept. Silver-catalyzed synthesis of a chroman spiroacetal similar to... 

SCHEME 52 Silver-catalyzed synthesis of quinoxaline and benziinldazoles in aqueous medium. 

SCHEME 3.18 Silver-catalyzed synthesis of propargylamines through a multicomponent... 

Liu W, Jiang H, Huang L (2010) One-pot silver-catalyzed and PIDA-mediated sequential reactions Synthesis of polysubstituted pyrroles directly from alkynoates and amines. Org Lett 12(2) 312—315... 

Several reviews on the synthesis of aziridines have been published in the previous year. These publications include a review on the silver catalyzed addition of nitrenes (among other intermediates such as carbene) across a double bond <06EJOC4313> a review on sulfur ylide addition to imines to form aziridines <06SL181> a review on nitrogen addition across double bonds <06ACR194> a general review on functionalization of a,p-unsaturated esters with some discussion of aziridination <06TA1465>... 

Marshall and co-workers used the silver-catalyzed version of this cycloisomerization as the final step in the synthesis of (-)-kallolide B from precursor 115 (Scheme 15.33) [51, 54]. Again, the reaction is stereospecific, as has also been demonstrated in the synthesis of kallolide A [55] and other examples [77]. 

Application of silver-catalyzed cydization is a key step in the synthesis of clavepic-tines A and B, a synthesis which also established the absolute configuration for these compounds. With regard to the allene unit and the heterocycle, enantiomeri-cally pure precursor 129 was prepared and then cyclized to the quinolizidine 130 with AgN03 in a diastereoselective manner (Scheme 15.40) [89, 90]. The synthesis was conducted with an inseparable 1 1 mixture of diastereomers at C-14 from the diastereomerically pure allene with regard to the axial chirality of the allene a 7 1 mixture of diastereomers (at C-10) was formed. 

Minisci-type substitution is one of the most useful reactions for the synthesis of alkyl- and acyl-substituted heteroaromatics. The acyl radicals are formed by the redox decomposition from aldehyde and /-butyl hydroperoxide or by silver-catalyzed decarboxylation of a a-keto acid with persulfate. Synthesis of acylpyrazines 70 as ant pheromones are achieved by this methodology using trialkyl-substituted pyrazines 69 with the acyl radicals generated from aldehydes or a-keto acids (Equation 10) <1996J(P1)2345>. The latter radicals are highly effective for the acylation. Homolytic alkylation of 6-chloro-2-cyanopyrazine 71 is performed by silver-catalyzed decarboxylation of alkanoic acids to provide 5-alkyl-substituted pyrazines 72 (Scheme 18) <1996CCC1109>. 

A critical step in a synthesis of the benzoquinone antibiotic sarubicin A (5) is addition of a methyl nucleophile to the aldehyde group of 2. Reaction with CH3MgBr is very slow at low temperatures and the yield and stereoselectivity are low. In contrast, methyltriisopropoxytitanium reacts with 2 to give the desired triol 3 in 80% yield. Oxidative cyclization of 3 to 4 was carried out in two steps benzylic bromination followed by silver-catalyzed ring closure.5... 

Scheme 6. Silver-catalyzed lactonization in Marshall s synthesis of (—)-Kallolide B (17).

As a part of a program directed toward the synthesis of the potent topisomerase I inhibitors, the lamellarins (e.g., 153 and 154), Porco has reported the silver triflate-catalyzed tandem cycloisomerization-azomethine ylide cycloaddition of 155 (Scheme 2.42).75 The postulated mechanism of this intriguing and highly efficient process is shown in Scheme 2.43. Silver-catalyzed addition of the imine nitrogen to the alkyne results, on subsequent deprotonation, in the formation of an azomethine ylide 160. This ylide participates in [3+2] cycloaddition with the alkyne component leading to formation of a dehydropyrrole 161. Finally, oxidation by adventitious oxygen leads to formation of the product 162. 

Silver-catalyzed cyclization reactions of allenic carbonyl compounds were applied for the total synthesis of various natural products, including rubifolide,331 kallolide A and B,332,332a and deoxypukalide.333 In their total synthesis of the enantiomer of rubifolide 396, Marshall and Sehon331 took advantage of the silver-catalyzed cycloisomerization of the allenic ketone 393 to the furan 394, as well as of the analogous reaction of the allenic carboxylate 395 to the target molecule 396 (Scheme 115). 

Silver-catalyzed silylene transfer was reported as a general method for the synthesis of silaziridines <07OL3773>. Despite the sensitivity of silaziridines to water and air, these intriguing compounds could be isolated in good yield. 

Cyclohexene silacyclopropane 47 undergoes silver-catalyzed silylene transfer, acting as an efficient method for silacyclopropane 48 synthesis (Equation 5) <2004JOC4007>. Kinetic studies of the transfer reaction suggested a possible mechanism for silver-mediated silylene transfer with a kinetic order of one for 47 <2004JA9993>. 

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