Silver-catalyzed cycloisomerization

The latter mode of reaction has even been reported to proceed in presence of sil-ver(I) ions [127], which is not easy to understand in the context of Marshall s silver-catalyzed cycloisomerization of allenyl ketones (see Chapter 15). 

Alkyn-l-ols, silver-catalyzed cycloisomerizations, 9, 561 Alkynones, stannylcupration, 9, 374... 

Allenic Alder-ene reactions, characteristics, 10, 584 Allenic carbonyl compounds reductive cyclization, 10, 524 silver-catalyzed cycloisomerizations, 9, 559 Allenic carboxylic acids, silver-catalyzed cycloisomerizations, 9, 559... 

Our interest in this chapter is in silver-catalyzed cycloisomerization reactions. Therefore, we shall present different silver-catalyzed cycloisomerization reactions as a function of the nucleophilic and electrophilic moiety. Cycloisomerization reactions including the classical ene-yne cycloisomerization (with X = CHR, Scheme 5.1), and the related heterocyclization reactions with heteroatoms embedded in unsaturated systems (X = NR, O Scheme 5.1) belong to the same reaction family. In addition, the alkynyl part can be exchanged for an allene unit. Internal or external nucleophiles (Nu) can then stabilize, through cascade reactions, the positive charge created.24... 

This overview of silver-catalyzed cycloisomerization reactions shows the great flexibility of the method and its usefulness in reaching the core of numerous heterocycles. 

The same products are accessible by silver-catalyzed cycloisomerization of allenic ketones. Marshall and Bartley327 used AgNOs/silica gel in hexane to convert the allenic ketones 384 into the furans 386 with excellent yields (Scheme 112). Deuterium labeling experiments were interpreted in terms of the intermediate 385 which seems to arise from the coordination of silver catalyst to the allenic double bond distal to the carbonyl group. Again, gold precatalysts can be used with much lower catalyst loadings than their silver counterparts (see Section 9.12.4.3). 

The silver-catalyzed cycloisomerization of allenic carboxylic acids to butenolides was also reported by Marshall and co-workers.329 Starting from the enantiomerically enriched propargyl mesylate 390, palladium-catalyzed hydroxycarbonylation led to the chiral allenecarboxylates 391 which afforded the butenolides 392 upon treatment with silver nitrate (Scheme 114). Unfortunately, partial racemization could not be avoided in this two-step sequence. In a related study, Ma and Shi330 have shown that the combination of Pd(PPh3)4 and Ag2C03 promotes the cyclization of allenecarboxylates to the corresponding butenolides, accompanied by the introduction of aryl or alkenyl groups at C3. 

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). 

A highly e fi o-selective gold- and silver-catalyzed cycloisomerization of 1,4-diynes has been developed (Wilckens et al. 2010). By employing electron-rich phosphines and A-heterocyclic carbenes as ligands, a number of 1,4-diynes could be cyclized in a desymmetrizing fashion to form dihydrodioxepines and enantiomeri-cally enriched tetrahydrooxazepines (Scheme 11.11). 

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