Silver-catalyzed cyclization

In more sophisticated approaches, [3,3]-sigmatropic rearrangements have been incorporated in cascade reactions in which each step could be catalyzed by silver salts. Usually, the silver promoted [3,3]-sigmatropic shift is combined with a silver-catalyzed cyclization,68 as outlined in the general scheme below for alkynyl compounds (Scheme 3.44). 

Hiyama et al.69 showed that monoacetylated 1,4-butynediols nicely afforded acetoxy allenols on silver-catalyzed rearrangement. These compounds were ideal substrates for silver-catalyzed cyclization, so the overall sequence could directly be performed in one pot, leading to substituted 2,5-dihydrofurans in high overall yields (Table 3.5). In this process, 5-10 mol% of silver perchlorate or tetrafluoroborate was used in refluxing benzene. 

From a mechanistic perspective, the reaction is similar to those described above. Silver-catalyzed cyclization of the ketone to the allene gave a cyclic oxonium intermediate. A [1,2]- or [1,5]-alkyl shift modified the sigma skeleton leading to an alkylsilver intermediate, which on elimination gave a trisubstituted furan. 

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). 

The corresponding nitrogen-containing heterocycles are accessible by silver-catalyzed cyclization of allenic amines, amides, or oximes.334 Thus, Dieter and Yu335 have reported the efficient transformation of various a-aminoallenes 397 or 399 into mono- or bicyclic 3-pyrrolines 398 or 400 in the presence of AgN03 (Scheme 116). Unfortunately, a rather high catalyst loading was required. 

The silver-catalyzed cyclization of A -substituted, 1-substitnted l-amino-4,5-dienes gave the corresponding 2,5-disubstituted pyrrolidine derivatives 6 in good yield. The intramolecular reaction was performed using silver tetrafluoroborate in dichloromethane at 20 °C. High 1,3-asymmetric induction was observed with the .s-2,5-disubstituted pyrrolidine derivative essentially the only product observed86,87- 216 218. -Substitution proved necessary for asymmetric induction. In fact, when the free amine underwent cyclization, only an equimolar mixture of cis- and franx-diastereomers was recovered. 

The allenic oxime (135) undergoes a silver-catalyzed cyclization to form the nitrone (136) (Equation (10)) which may be trapped as the dipolar adduct (137) with vV-methylmaleimide <91JCS(Pl)659>. Azepinonitrones have also been prepared by the spontaneous intramolecular cyclization of alky-nylhydroxylamines, prepared by the reduction of the corresponding oximes <92TL742l, 92TL7425>. 

Dalla, V. and Pale, P. 1999. Silver-catalyzed cyclization of acetylenic alcohols and acids A remarkable accelerating effect of a propargylic C-O bond. New J. Ghent. 23 ... 

Pale, P. and Chnche, J. 2000. Silver-catalyzed cyclization of acetylenic alcohols Synthesis of functionalized 2-methylene-oxolanes. Eur. J. Org. Chem. 2000 1019-1025. 

Silver-catalyzed cyclization—stannylation of o-alkynylanihnes carrying N-electron-withdrawing groups with 2-tributylstannylfuran provided a straightforward route to (3-indolyl)stannanes (13CC11794). 

Thiol-mediated cyclizations of Y-hydroxy-a,p-unsaturated trifluoromethyl ketones 121 was utilized to the preparation of trifluoromethylfuran 122 [93], a,p-Unsaturated trifluoromethyl ketones 123 were transformed into 3-alkylidene-4-yn-2-ols 124 which underwent silver-catalyzed cyclization into furans 125 [94],... 

The Wu group reported an interesting synthesis of 1-amino- and 1-aryloxyiso-quinolines 166 through the addition of an amine/phenol in the presence of PyBrop (Scheme 19.41) [73], After silver-catalyzed cyclization of 2-aIkynylbenzaldoximes 34 and intermolecular nucleophilic addition and deprotonation, the substituted isoquinolines 166 would be generated with the release of phosphoramide mediated by PyBrop. 

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