Rearrangement cycloisomerization

Keywords Cascade Skeletal rearrangement Cycloisomerization Cycloaddition Carbenoid... 

Representative procedure for the gold-catalyzed Gosteli-Claisen rearrangement/ cycloisomerization of alkoxycarbonyl-substituted propargyl vinyl ethers. Isopropyl 4,5-dimethyl-3-(4-nitrophenyl)juran-2-carboxylat ... 

The skeletal rearrangements are cycloisomerization processes which involve carbon-carbon bond cleavage. These reactions have witnessed a tremendous development in the last decade, and this chemistry has been recently reviewed.283 This section will be devoted to 7T-Lewis acid-catalyzed processes and will not deal, for instance, with genuine enyne metathesis processes involving carbene complex-catalyzed processes pioneered by Katz284 and intensely used nowadays with Ru-based catalysts.285 By the catalysis of 7r-Lewis acids, all these reactions generally start with a metal-promoted electrophilic activation of the alkyne moiety, a process well known for organoplatinum... 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

The low stability of these unbranched polyenynes, however, prevented any exploitation of their biological activity (Scheme 18.4). Whereas mycomycin (7) readily rearranges to the dienetriyne isomycomycin (10) [16b], the corresponding marasin (8) is cycloisomerized to the tetrahydrofuran analog isomarasin (11) [17a],... 

Under optimized conditions, cycloisomerizations of a number of functionalized hept-l-en-6-ynes took place in good-to-excellent yields (Table 9.3). Heteroatom substitution was tolerated both within the tether and on its periphery. Alkynyl silanes and selenides underwent rearrangement to provide cyclized products in moderate yield (entries 6 and 7). One example of seven-membered ring formation was reported (entry 5). Surprisingly, though, substitution was not tolerated on the alkene moiety of the reacting enyne. The authors surmize that steric congestion retards the desired [2 + 2]-cycloaddition reaction to the point that side reactions, such as alkyne dimerization, become dominant. 

Although a number of competing pathways have been shown to be involved301 in the thermal cycloisomerization of hexa-l,3-dien-5-ynes (255), isobenzenes (256) have been established302 as intermediates in their thermal rearrangement to arenes. On solvolysis, the diethylphosphate ester of l-(2-ethynylphenyl)-4-trimethylsilyl-4-(trimethylsilyl-oxy)pent-2-yn-l-ol has been found303 to afford 5-(2-ethynylphenyl)-3-trimethylsilyl-... 

Mild Ni(0)-catalysed rearrangements of l-acyl-2-vinylcyclopropanes to substituted dihydrofurans have been developed.86 The room temperature isomerizations afford dihydrofuran products in high yield. A highly substituted, stereochemically defined cyclopropane has been employed in the rearrangement to evaluate the reaction mechanism. The Cu(II)-catalysed cycloisomerization of tertiary 5-en-l-yn-3-ols with a 1,2-alkyl shift affords stereoselectively tri- and tetra-cyclic compounds of high molecular complexity (Scheme 29).87 A proposed mechanism has been outlined in which... 

Intramolecular 2 + 2 + 2-cycloisomerizations of cyclic triynes and enediynes have been reported with RhCl(CO)(PPh3)2.126 The transition metal-catalysed rearrangement of alk-5-ynals to /-alkynyl ketones and cyclopent-l-enyl ketones was developed using [Rh(P(OPh)3)2]BF4 or Cu(OTf)2 as a catalyst and the effect of substituents on the partition to products was elaborated (Scheme 84).127... 

In the presence of appropriate ruthenium catalyst precursors, diallyl and allyl homoallyl ethers do not lead to the expected metathesis or cycloisomerization products, but undergo first isomerization to form allyl vinyl ethers, and then a Claisen rearrangement which gives unsaturated aide-... 

Propargyl naphthyl ethers can be rearranged to to naphthofurans in shorter reaction times when exposed to microwave irradiation (Scheme 89) <1996JCM338>. These alkynes first undergo Claisen rearrangement to form an allene intermediate which in turn spontaneously cycloisomerizes to the benzofuran product. 

From the 5-silyl derivatives of o-alkynylarylthiols, cycloisomerization leads to 3-silyl-benzothiophenes. The gold salt is found to be highly effective in promoting Meyer-Schuster rearrangement. ... 

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