Intramolecular quenching

Although enol ethers have received moderate notice as nucleophiles to quench intramolecular iminium ions, silyl enol ethers have been given scant attention. The first report of a silyl enol ether participating in an intramolecular Mannich reaction is found in Oppolzer and coworkers synthesis of ( )-vincamine (Scheme 14). 2 Dihydro-3-carboline (25) and silyl enol ether (26) were mixed in DMF, then warmed to 70 C for 64 h in the presence of diisopropylethylamine to provide a 1 1 mixture of cis and trans tetracyclic aldehydes (27) in 74% yield. 

Interestingly, inadiation of 5-ew-bromo-5-eniio-benzyloxy-7-oxabicyclo[2.2.1] heptan-2-one in the presence of EtsN neither leads to the corresponding cyclohexenol derivative nor to the reduction of the ketone into a mixture of exo-and eniio-7-oxanorboman-2-ols. Only the product of hydrogenolysis of the bromide (6-eniio-benzyloxa-7-oxabicyclo[2.2.1]heptan-2-one) is formed and isolated in 60 % yield. The radical intermediate, formed by single-electron transfer (SET) from EtsN to the bromide and subsequent C-Br bond cleavage, can be quenched intramolecularly as exemplified with the next reaction [265] (Scheme 36). 

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

A modification of the K-R reaction was introduced by Mozingo. This method involved reacting an o-hydroxyacetophenone with an ester in the presence of metallic sodium to form a 1,3-diketone. Treatment of the diketone with an acid then delivered the chromone via an intramolecular cyclization reaction. This method was applied to the preparation of 2-ethylchromone (21). 0-hydroxyarylketone 22 was allowed to react with ethyl propionate (23) in the presence of sodium metal.The resulting sodium enolate was then quenched with acetic acid to deliver the 1,3-diketone 24. Upon heating 24 in glacial acetic acid and hydrochloric acid, 2-ethylchromone (21) was delivered in 70-75% overall yield. 

Intramolecular cyclization of 2-phenysulfonylmethyl lactam 3 took place upon reaction with lithium hexamethyldisilazan via generating its a-sulfonyl carbanion to give a cyclized postulated intermediate that can be quenched with trimethylchlorosilane to afford the stable silyl ketal 4. The later ketal was desulfonylated by Raney-Ni and desilylated through treatment with tetrabutyl ammonium fluoride (BU4NF) to afford the carbacephem 5 (94M71) (Scheme 1). 

The enolate of the 1,4-adduct, obtained after the stereoselective Michael addition step, as discussed in the previous sections, may be quenched in situ with various electrophiles. The fact that additional stereogenic centers may be formed via such tandem Michael addition/quench-ing procedures, giving products with high diastereoselectivity in many cases, extends the scope of these methods substantially. Furthermore these procedures occasionally offer the possibility of reversing the syn/anti diastereoselection. In the next sections pertinent examples of diastereoselective inter- and intramolecular quenching reactions will be discussed. 

The reactions of the lithium enolates of substituted 2-cyclohexenones and 2-cyclopentenones with ( )-l-nitropropene give a mixture of syn- and ami-products3. The lithium enolate of 3,5,5-trimethyl-2-cyclohexenone gives a mixture of the syn- and //-3.5,5-trimethyl-6-(l-methyl-2-nitroethyl)-2-cyclohexcnoncs in modest diastereoselection when the reaction mixture is quenched with acetic acid after. 30 minutes at —78 =C. When the reaction mixture is heated to reflux, tricyclic products are obtained resulting from intramolecular Michael addition of the intermediate nitronate ion to the enone moiety. 

Radical Diels-Alder reactions have been used mainly to synthesize polycyclic molecules. These reactions, like those that involve cations and anions as components, proceed quickly but generally do not give high yields. Thus, the tricyclic enone 14 is the result of an intramolecular Diels-Alder reaction of quenched vinyl radical intermediate 13 obtained by treating the iododienynone 12 with n-tributyltin hydride/2,2 -azobisisobutyronitrile (AIBN) [28] (Equation 1.11). 

Extrusion or elimination of small stable species such as CO2 Intramolecular energy transfer Quenching involving translational or vibrational excitation of another molecule... 

State decarbonylation reaction in total synthesis was reported recently in the case of natnral prodnct (+)-herbetenolide, which farther illustrates the exquisite control that the solid state may exert on the chemical behavior of the otherwise highly promiscuous reactive intermediates. As word or caution, it should be mentioned that intramolecular quenching effects known to act in solution can also affect that reaction in the solid state. Recently reported examples include the well-known intramolecular P-phenyl and electron transfer quenching. ... 

Marme N, Knemeyer JP, Sauer M, Wolfram J (2003) Inter- and intramolecular fluorescence quenching of organic dyes by tryptophan. Bioconjug Chem 14 1133-1139... 

Johansson MK, Cook RM (2003) Intramolecular dimers a new design strategy for fluorescence-quenched probes. Chemistry 9 3466-3471... 

Oncul S, Demchenko AP (2006) The effects of thermal quenching on the excited-state intramolecular proton transfer reaction in 3-hydroxyflavones. Spectrochim Acta A Mol Biomol Spectrosc 65 179-183... 

The pioneering work Forster and Hoffmann [28] on the viscosity dependence of the fluorescence quantum yield of triphenylmethane dyes (TPM) has set the foundation for several reports in these dyes (Fig. 12). It was found that both an ability to twist around the carbocationic center and the donor-acceptor properties are important [66], Specifically, a strong intramolecular quenching is observed for 34 that is virtually absent (two orders of magnitude slower quenching rate) in the bridged... 

Rettig W, Klock A (1985) Intramolecular fluorescence quenching in aminocoumarines. Identification of an excited state with full charge separation. Can J Chem 63(7) 1649-1653... 

Vogel M, Rettig W (1985) Efficient intramolecular fluorescence quenching in triphenyl-methane dyes involving excited states with charge separation and twisted conformations. Ber Bunsenges 89(9) 962-968... 

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