Ring cleavage additive

Nucleophilic replacement of hydrogen on an isoxazole is unknown and replacement of substituents is discussed in Section 4.16.3.3. In this series it is difficult to identify reactions involving addition to the ring as, in many instances, they are rapidly followed by elimination or ring cleavage sequences. 

The interaction of diazomethane with 1-azirines was the first example of a 1,3-dipolar cycloaddition with this ring system (64JOC3049, 68JOC4316). 1,3-Dipolar addition produces the triazoline adduct (87). This material can exist in equilibrium with its valence tautomer (88), and allylic azides (89) and (90) can be produced from these triazolines by ring cleavage. 

Additions to acyclic enamines (337,343), to 1,1-dipiperidinoethylene (344), and to the analogous ethoxydimethylaminoethylene (344) gave products derived from ring cleavage of an initially formed cyclobutene adduct. 

Ketones are oxidatively cleaved by Cr(VI) or Mn(VII) reagents. The reaction is sometimes of utility in the synthesis of difunctional molecules by ring cleavage. The mechanism for both reagents is believed to involve an enol intermediate.206 A study involving both kinetic data and quantitative product studies has permitted a fairly complete description of the Cr(VI) oxidation of benzyl phenyl ketone.207 The products include both oxidative-cleavage products and benzil, 7, which results from oxidation a to the carbonyl. In addition, the dimeric product 8, which is suggestive of radical intermediates, is formed under some conditions. 

The regioselectivity in ketyl-radical-promoted ring cleavage of configurationally restricted tricyclo[3.3.0.0 ]octanone derivatives was investigated by Maiti and Lahiri, revealing new and additional information for this class of compounds in contrast to simple and unrestrained cyclopropyl ketones [9],... 

The properties of 5-azido and 5-diazomethyl-l,4-diphenyltriazole are unusual in that both compounds lose nitrogen under very mild conditions. The products derived from the azide are mainly those of ring cleavage (Section IV, G), but the diazoalkane gives a carbene which undergoes addition and insertion reactions with several solvents. These reactions are illustrated in Scheme 49. 

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