Cyclization skeletal rearrangements

Hypothetical (carbene)gold(i) structures of intermediates and reaction coordinates have been calculated (B3LYP/ 6-31G and LAN2DZ levels) for (H3P)Au+-catalyzed cyclization reactions of terminal enynes. The endocyclic skeletal rearrangement reactions were found to proceed exclusively via cyclopropylcarbene complexes.240... 

Open chain hydrocarbons, skeletal rearrangements, 29 298-302 aromatic selectivities, 29 302 atomization, 29 298-302 cyclization, 29 298-302 Open sequence in reaction mechanisms, 32 ... 

A common feature of any cyclization reaction is that a new intramolecular C—C bond is produced that would not have been formed in the absence of the catalyst. Those reactions in which one ring closure step is sufficient to explain the formation of a given cyclic product will be called simple cyclization processes, although their mechanism is, as a rule, complex. We shall distinguish those cases in which any additional skeletal rearrangement step(s) is (are) required to explain the process. Some specific varieties of hydrocarbon ring closure processes are not included. A recent excellent review deals with the formation of a second ring in an alkyl-substituted aromatic compound (12). Dehydrocyclodimerization reactions have also to be omitted—all the more since it is doubtful whether a metallic function itself is able to catalyze this process (13). 

B. Cyclization of Open Chain Hydrocarbons WITH Skeletal Rearrangement... 

The appearance of very small amounts of methylcyclopentane from di-methylbutanes (75) indicates subsequent C5 cyclization of skeletal rearrangement products like those reported by Muller and Gault (93). This reaction has not been observed over palladium. 

Ring-closing metathesis of an enyne, which has double and triple bonds in the molecule, is a remarkable reaction which is useful in synthetic organic chemistry. In enyne metathesis, the double bond is cleaved and carbon-carbon bond formation occurs between the double and triple bonds. The cleaved alkylidene part is moved to the alkyne carbon. Thus, the cyclized compound formed in this reaction has a diene moiety [Eq. (6.77)]. The reaction is also called skeletal rearrangement and is induced by Pt, Pd, Ga, and Ru catalysts ... 

Serratose, et al., have succeeded in converting readily available lactone 361 to semi-bullvalene. The scheme, which involves no skeletal rearrangement, is based on diazoketone cyclization chemistry within an oxygenated cyclopentenyl derivative... 

The last synthesis to evolve which is due to Ito and his coworkers is interesting in that it relies on a stereospecific skeletal rearrangement of a bicyclo[2.2.2]octane system which in turn was prepared by Diels-Alder methodology (Scheme XLVIII) Heating of a toluene solution of cyclopentene 1,2-dicarboxylic anhydride and 4-methylcyclohexa-l,4-dienyl methyl ether in the presence of a catalytic quantity of p-toluenesulfonic acid afforded 589. Demethylation was followed by reduction and cyclization to sulfide 590. Desulfurization set the stage for peracid oxidation and arrival at 591. Chromatography of this intermediate on alumina induced isomerization to keto alcohol 592. Jones oxidation afforded diketone 593 which had earlier been transformed into gymnomitrol. 

Allylic esters equilibrated under very mild conditions (2 h, 25 C) in the presence of 2-4% PdCfe catalyst. These rearrangements were not complicated by skeletal rearrangements, cyclizations or elimination,... 

Selective activation of alkyne functions of enynes to give products either of alkoxy-cyclization or of exo- and endo-skeletal rearrangement can be achieved by using alkynophilic cationic gold(I) complexes. The endocyclic cyclization catalysed by gold(I) proceeds via a mechanism different from those known for Pd(II), Hg(II), or Rh(I) catalysts.118... 

Bicyclic 3a//-cyclopentene[8 annulcnc-l,4-(5//,9a//)-dioncs undergo three types of acid-induced transannular reactions (1) Michael addition (5-exo-trig or 6-exo-trig) leading to the tetracyclic diones, (2) 3 + 2-cycloaddition followed by a novel sequential skeletal rearrangement to 2-naphthalenone derivatives, (3) ipso-Friedel-Crafts alkylation accompanied by the rearomatization and the loss of water (Scheme 25). The factors that control the reaction mode of these transannular cyclizations are discussed... 

Skeletal rearrangement reactions over Pt single crystals have been studied for methyl cyclopentane, 2- and 3-methylpentane350 and for n-hexane.3sl One conclusion351 is that whereas aromatization reactions are very sensitive to surface structure [Pt(l 11)> Pt(100)], isomerization, Cs-cyclization, and hydrogenolysis reactions display little dependence on structure. Temperature and H2 pressure are more important in affecting the selectivity. 

Polysubstituted 1,3-oxazolidines were prepared in a one-pot diversity oriented four-component reaction (4-MCR), comprising two linked domino processes. Thus, domino synthesis of enol ethers 9 was followed by a sequential amine addition-cyclization sequence [74]. While strong microwave irradiation (900 W) of silica-gel absorbed conjugated alkynoates 9 and amines afforded tetrasubstituted pyrroles (via the skeletal rearrangement of 1,3-oxazolidines, see Sect. 2.1 and Scheme 5) [24], the use of milder microwave conditions (160 W power, 90 min) furnished 1,3-oxazolidines. Under these mild conditions the 1,3-oxazolidines did not rearrange to pyrroles and with respect to diastereoselectivity, the 1,3-oxazolidines were obtained as mixtures of syn/anti isomers. Overall, the formation of one C-C bond, one C-0 bond, two C - N bonds and a ring in this MCR required less than 3 hours and utilized simple and commercially available reagents (Scheme 26). 

Transition metals catalyze a very wide variety of hydrocarbon reactions ranging from hydrogenation of olefins and exchange of paraffins with deuterium at lower temperatures to skeletal rearrangement, cyclization, hydrogenolysis, cracking, and carbiding under more severe conditions. Because of this flexibility in... 

Sesquiterpenoids (Sq) are Cj5 compounds formed by the assembly of three isoprenoid units. They are found in many living systems but particularly in higher plants. There are a large number of sesquiterpenoid carbon skeletons, which arise from the common precursor, farnesyl diphosphate, by various modes cyclization followed, in many cases, by skeletal rearrangement. 

Endo-skeletal rearrangements also take place with 1,6-enynes, bnt the proposed mechanism is just a variation of the exo-single-cleavage rearrangement. Formed by endo cyclization, bicyclo[4.1.0]hept-4-ene derivatives arise in some cyclizations of 1,6-enynes by proton loss and protodemetalation ofthe endo cyclopropylcarbene. " That is the case from 1,6-enynes tethered as sulfonamides and in the intramolecular cyclization of 1,6-enol ethers with alkynes (equation 40). ... 

However, it should be mentioned that Gould and Halley recently presented an alternative mechanism for PD 116198 involving a skeletal rearrangement [96]. We will later return to discussion of the problem of linear vs angular cyclization mode. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

Vysotskii, Y. B., Bryantsev, V. S. Quantum-Chemical Treatment of Cyclization and Recyclization Reactions XXV. Skeletal Rearrangements of Radicals, and Lowest Triplet-State Systems. Russ. J. Org. Chem. 2002, 38,1244-1251. 

Dihydrobenzofurans and 2,2-dimethylchromans. Cyclization with skeletal rearrangement is observed when aryl allyl ethers are heated with Mo(CO)j in toluene, for example, aryl prenyl ethers give 2,2-dimethylchromans. ... 

A considerable amount of work has been devoted to the skeletal rearrangements of substituted aromatics, especially of alkyl-substituted benzenes. The first of these reactions that have been investigated are the cyclizations of diphenylmethane to fluorene and of dibenzyl to phenanthrene (757) (Scheme 77). 

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