Ring forming process

If the attacking radical contains an adequately placed radical acceptor functionality, the possibility of a radical cycloaddition is provided, offering a procedure to construct cyclic products from acyclic precursors. For this type of ring-forming process, in which two molecular fragments are united with the formation of two new bonds, the term annulation has been adopted (Scheme 3.3). 

Three-membered ring-forming processes involving X-CH2-CH2-F or CH2-C(Y)-CH2F (X = CH2, O, or S and Y = O or S) in the gas phase have been treated by the ab initio MO method with a 6-31+G basis set." When electron correlation effects were considered, the activation (AG ) and reaction (AG°) free energies were lowered by about lOkcal mol indicating the importance of electron correlation in these reactions. The contribution of entropy of activation -TAS ) at 298 K to AG is very small the reactions are enthalpy controlled. 

Multiple Ring Forming Processes 11.11.7.1 Cascade Cyclization/Hydrosilylation of Trienes... 

This chapter describes three different transition metal catalyzed reactions having one common feature. They are all examples of ring forming processes, leading to distinctly different product classes and following different mechanisms. 

Free radicals were for a long time believed to be too reactive and indiscriminate in their reactivity to be harnessed usefully for synthesis. However, because of increased understanding of the nature of their reactivity, synthetic chemists have learned to tame these highly reactive intermediates to such an extent that reactions involving free radical intermediates are now considered to be quite useful in synthesis, particularly with respect to carbon-carbon bond forming and carbocyclic ring forming processes. 

The abbreviations to describe the ring forming processes were established by Baldwin in 1976.10... 

A different synthetic approach to l-sila-2,4-diaza-3-oxocyclopentanes utilized the reaction of silenes 68 with ureas and afforded the heterocycles 69 in moderate yields (50-65%) (Equation 6). It was proposed that the reaction proceeds in two steps with the ring-forming process being the nucleophilic attack of the nitrogen atom at the carbon center <20030M1314>. 

This selective a-alkenylation method was also used for the preparation of a number of other a-(l-alkenyl)-/3-ketoesters, for example, in the direct synthesis of the CP-263,114 core system by a bicyclic ring-forming process, starting from readily available /3-ketoesters (Scheme 10). Treatment of the vinylstannane by lead tetraacetate followed directly by exposure to the methyl /3-ketoester led to the derived ketone 65 in 51% yield.84... 

Finally, bicyclo[3.3.0]octenones such as 129 have been found capable of [2+2] photocycloaddition to simple alkenes, the ring forming process occurring with high stereoselectivity from the less hindered surface.200 ... 

An electrocyclic reaction involves the concerted formation of a n-bond between the two ends of a conjugated rr-system (ring forming process), or the reverse reaction (ring opening process) in which the o-bond is broken to produce a conjugated system. 

Molander et al. have reported on the dianionic [4 + 3] annulation reaction of 1,4-dicarbonyl compounds with 3-iodo-2-[(trimethylsilyl)methyl]propene (4a X = I, Y = CHiSiMea) x)moted by tin(II) fluoride The extraordinary efficiency of ttiis novel stereoselective seven-member ring forming process, coupled with the simplicity of the procedure, promises to provide an expedient route to innumerable cyclic organic molecules (Scheme 9). 

An extensive study of known cyclization reactions led J. E. Baldwin to formulate a set of empirical rules to predict the relative ease of ring-forming processes. These... 

The elegant route to (-)-punctatin A (125) developed by Paquette nicely illustrates the synthetic utility of photochemically mediated intramolecular C—H insertion (Scheme 3). The excellent stereoselectivity observed in the four-membered ring forming process (123) — (124) was, in fact, predicted. The intermediate 1,4-diradical could cyclize to the equatorially fused rran.r-cyclobutane, as illustrated. The alternative cyclization mode, leading to the c/5-fused cyclobutane, has a bad steric interaction with the angular alkoxymethyl group, and so is less favored. 

With this method a new bicyclic ring-forming process has been developed that results in direct construction of the CP-263,114 core system from a readily available /i-keto ester (Scheme 13.16) [44]. 

A particularly effective strategy for the design of new heterocyclic libraries employs a tandem N-acyliminium ion cyclization/nucleophilic addition for ring-forming processes [108] (Scheme 19). The described methodology provides access to bi-, tri-, and tetracyclic derivatives of l-acyl-3-oxopiperazines. The bicyclic variants in particular represent an interesting probe for a constrained type I p-turn motif with potential for combinatorial diversification. 

Synthetic ways to dihydropyridines can be classified into ring-forming processes and ring transformations, liie first general approach is best represented by the versatile Hantzsch synthesis. This method conveniently affords 1,4-dihydro- or 3,4-dihydro-, but not 1,2-dihydro-pyridines. The second approach may be illustrated by partial reduction of pyridines or pyridinium salts and nucleophilic addition of organometallic reagents to pyridines and pyridinium salts leading to 1,2- or l,4-dihydropyridines . 

Several new synthetic approaches to the quinine system from mero-quinene have a common quinuclidine ring-forming process 17, 20, 21). This process involves the intramolecular addition of the secondary amine function to the double bond conjugated to the quinoline ring... 

A study was carried out to get insight into the reaction of the ring-forming process. Palladacycle formation was found to be faster with para (with respect to the Pd-C bond) electron-releasing substituents than with the electron-with-drawing ones [22]. The process thus corresponds to an electrophilic aromatic substitution (Eq. 12). 

Up until now we haven t made any special presentations concerning syntheses of rings, because the ring-forming processes you ve seen so far were nothing more than intramolecular versions of ordinary reactions, such as... 

A novel strategy towards the total synthesis of cyclopeptide alkaloids is based upon the hitherto unknown ring-forming process in which formation of aryl-alkyl ether bond is the crucial step (168). 

Dysprosium(III) catalysis in organic synthesis (Friedel—Crafts alkylation, Mannich-type, Diels—Alder, and Povarov reactions) 12T2015. Enamides and related functional groups as nucleophihc components in ring-forming processes catalyzed by electrophihc metal salts 12SL814. 

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