Cyclic ring-contraction product

Though the reaction is not general, certain cyclic ketones can be photolyzed to give ring-contracted products.In the example above, the cyclobutanone (55) was photolyzed to give (56), where BZ is benzoyl. This reaction was used to synthesize tetra-r rf-butyltetrahedrane (57). 

In a reaction similar to 17-37, certain sulfones, both cyclic and acyclic, extrude SO2 on heating or photolysis to give ring-contracted products." An example is the... 

In a reaction similar to 7-47, certain sulfones, both cyclic and acylic,461 extrude S02 on heating or photolysis to give ring-contracted products.462 An example is the preparation of naphtho(f )cyclobutene shown above.463 In a different kind of reaction, five-membered cyclic sulfones can be converted to cyclobutenes by treatment with butyllithium followed by LiAIH4,464 e.g.,... 

The acid-catalysed hydrolysis of die cyclic diazotiiiolactone, 4-diazoisochroman-3-one (159) to a ring-contracted product involves protonation of the diazo carbon in the slow step.133 A variable-temperature NMR sriidy has been undertaken to study the reaction of the pyrazine (160) giving (161a R = OH, Me, H). Spectral evidence has been obtained supporting the involvement of the dihydroimidazopyrazine ring system (161b).134 Work by Edward and co-workers on the alkaline hydrolysis of strychnine and some of its derivatives was dealt with earlier.21... 

Many of the above-described examples highlight the diastereoselectivity of the solid-state reaction. Cyclic ketones with cis- or trans-a,a -substituents give the ring-contraction product with the same relative stereochemistry. Examples include 2-indanones 157 and 159 (Scheme 2.38), cyclohexanones 162 and 164—167 in Scheme 2.39, and ketones 177 and 179 in Scheme 2.43. The diastereospedfidty and enantiospecificity of linear ketones was also tested with the pentasubstituted acetone derivative 187 in its racemic and enantiomerically pure forms (Scheme 2.45). Notably, both samples reacted efficiently and in a completely diastereo-specific manner when reactions were carried out to 60% conversion. Sample of ( I )-(2R,4S)-187 gave the corresponding combination product in 100% ee and... 

Ring contractions. This azide can function in 1,3-dipolar cycloaddition to enamines of cyclic ketones (1). The resulting AMriazolines (a) have not been isolated since they undergo loss of nitrogen with ring contraction to form the ring-contracted products (2). These can be converted into cycloalkanoic acids (3). ... 

This reaction has general application for preparing benzilic acids and their esters which have important biological activities. The formed benzilic acid can be used as reducing reagent to transform Qf, -unsaturated ketones into saturated ketones.In addition, the benzilic acid rearrangement on cyclic of-diketones will lead to ring contracted products 2ii, 2kk, 2nn... 

At high pressure ( kbar) the normally more sluggish i-butyldimethylsilyl enol ethers of cyclic ketones react in good yield to afford the ring contracted product (eq 3). ... 

Another advantage of sterically demanding trisyl azide is the selective azidation of cyclic 8-ketoesters (eq 32). Thus, the desired azidation product A was slowly formed as a single product in either THF or MeCN. In contrast, more reactive sulfonyl azides such as 4-methoxybenzenesulfonyl azide, 4-nitrobenzenesulfonyl azide, mesyl azide, triflyl azide, and tosyl azide provided not only the azidation product A but also a ring-contraction product B, a ring-opened diazo-transfer product C, and a mixture of ring-opened olefins D. The amounts of by-products B, C, and D were dependent upon the azide reagent. 

Gould and coworkers have extensively studied the biosyntheses of the kinamycins, and this work was recently reviewed [5a]. Feeding studies established that the carbo-cyclic skeletons of the kinamycins are constructed from 10 equivalents of 5-acetyl coenzyme A, and the pathway shown in Scheme 3.4 was proposed. The pathway begins with formation of the natural product dehydrorabelomycin (29). A novel ring contraction then occurs to form the cyclopentadienone 30. Feeding studies with /V-15-ammonium sulfate established that the diazo functional group is then installed... 

The photolysis of cyclic diazo ketones in hydroxylic solvents leads to ring contracted carboxylic acid derivatives via this ketocarbene -> ketene rearrangement. Examples of such reactions are given in (2.24)239) and (2.25) 240). In this last example a photoequilibrium between the diazo ketone and its valence isomer, a diazirine, has been observed, both products then eliminating nitrogen to afford the cyclobutane carboxylic acid. 

The thallium trinitrate-mediated ring contraction of frani-decal-2-ones has opened up a new route to the hydrindane system, and fluorinative ring contraction of cyclic alkenes to afford difluorocycloalkanes has been induced by iodotoluene difluoride and EtsN-HF. A possible mechanism is shown in Scheme 78. The double bond of the cyclohexene ring is attacked by iodotoluene difluoride activated by HF from the axial direction, followed by the addition of a fluoride ion from the trans direction. Reductive elimination of iodotoluene from the resulting adduct, ring contraction and the addition of the fluoride ion to the carbocation stabilized by fluorine then take place to give the ring-contracted difluorinated product. 

The pioneering work on enantioselective [2,3]-Wittig rearrangement was carried out by Marshall and Lebreton in the ring-contracting rearrangement of a 13-membered cyclic ether using lithium bis(l-phenylethyl) amide (63) as a chiral base (equation 34). Upon treatment with a (S,S)-63 (3 equivalents) in THF at —70 to —15 °C, ether 64 afforded the enantioenriched [2,3]-product 65 in 82% yield with 69% ee. The reaction was applied in the synthesis of (+)-aristolactone (66). 

The photolysis of cyclic polysilanes results in ring contraction with concomitant extrusion of a silylene fragment. Although the formation of two reactive intermediates potentially complicates mechanisms for product formation, it has provided a useful method for the synthesis of both unstable and stable disilenes... 

Valence tautomerization to the bicyclic azetine (47 R1 = OEt, R2 = Me) has been observed in the photosensitized (PlfcCO or PhCOMe) ring contraction of the otherwise photostable 2-ethoxy-4,5-dihydro-3.H-azepine (46 R4 = OEt, R2 = Me) (71JOC1934). In contrast, sensitized photolysis of the dimethylamino derivative (46 R1 = NMe2, R2 = H) gave only polymers. Unsensitized photolysis in pyrex yields several cyclic products, all of which are attributable to the intermediacy of an unstable 7-azabicyclo[3.2.0]hepta-3,6-diene (47 R1 = NMe2, R2 = H) (73CC327). 

Other ring contractions that give azapentalene derivatives include the rearrangement of a fused oxazine to a pyrazolo[l,5-a]indol-4-one with acetic anhydride,165 and an interesting photochemical contraction of a cyclic lactone, which produces an isomeric fused indolone by a mechanism thought to involve the first meta photo-Fries rearrangement [Eq. (19)].166 The product was used as an intermediate for the synthesis of mitomycin antibiotics (Section VI,A). 

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