Ring contraction rearrangement

The anodic oxidation of 7-MeO-CHT in MeOH results in the formation of benzalde-hyde dimethyl acetal through a ring contracting rearrangement, whereas 3-MeO-CHT and 1-MeO-CHT are prepared by thermal rearrangement of 7-MeO-CHT and afford 7,7-diMeO-CHT in 83% and 85% yields, respectively, upon the anodic oxidation. The hydrolysis of 7,7-diMeO-CHT in 5% aqueous H2SO4 gives tropone in 85% yield (equation 17). 

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). 

Paquette has used the chloroketal derivatives of thietane 1,1-dioxide as an especially well-suited model for the theoretical study of the thermally induced intramolecular six-electron ring contraction rearrangement, which is pointed out in Eq. (54) for the ketals of 2-bromocyclobutanone that give cyclopropylcarboxylates. Desulfurization of thietanes to cyclopropane did not prove too successful. ... 

Such syntheses can occur by ring expansion, ring contraction, rearrangement, ring fission (with or without subsequent elaboration), fragmentation with subsequent elaboration, or combination with a second synthon followed by other reactions. 

The ring contraction rearrangement of alicyclic a-diketones is much more valuable from the synthetic viewpoint e.g. equation 4). There are several routes available for the synthesis of a-diketones from monoketones. These include direct oxidation using selenium dioxide permanganate or osmium tetrox-ide addition across the double bond of an enone followed by base-promoted elimination of water bromi-nation to give the dibromo ketone followed by hydrolysis condensation with 4-nitroso-/V,/V-dimethylaniline followed by acidic hydrolysis — but there are many others. The bromination-hydrolysis route has been extensively studied by Wallach." The conversion of menthone into 1-hydroxy-... 

New amine functionalization reactions have been applied to the functionalization of benzodiazepines, for example, twofold methylation of 2,3,4,5-tetrahydro-lff-benzo[e][l,4]diazepine was achieved using carbon dioxide and phenylsilane with ruthenium catalysis (13AGE9568). A range of readily available 3,4-dihydro-lH-benzo[e][l,4]diazepine-2,5-diones underwent iV-acylation followed by a dehydrative ring-contraction rearrangement process to afford the corresponding oxazoloquinolinones (13OL1052). 

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