Cycloadducts cyclic diketones

Similar adducts are produced from other cyclic diketones (see Table 2.4). N-pyridyl-(2)-triphenylphosphine imine 356 reacts with diphenylcarbodiimide via a [2+2] cycloaddition reaction. The initially formed cycloadduct 357 undergoes a cycloreversion reaction to give N-phenyl-N -pyridylcarbodiimide 358, which reacts with diphenylcarbodiimide to give a [2+4] cycloadduct 359 (yield 52%, mp 263-265 °C). ... 

Table 2.1 [2+4] Cycloadducts derived from Carbodiimides and Cyclic Diketones...

Tabh 3.19 [4- -2] Cycloadducts derived from carbodiimides and cyclic diketones... 

A total synthesis of functionalized 8,14-seco steroids with five- and six-membered D rings has been developed (467). The synthesis is based on the transformation of (S)-carvone into a steroidal AB ring moiety with a side chain at C(9), which allows the creation of a nitrile oxide at this position. The nitrile oxides are coupled with cyclic enones or enol derivatives of 1,3-diketones, and reductive cleavage of the obtained cycloadducts give the desired products. The formation of a twelve-membered ring compound has been reported in the cycloaddition of one of the nitrile oxides with cyclopentenone and as the result of an intramolecular ene reaction, followed by retro-aldol reaction. 

These experimental facts indicate that it is possible to synthesize differently bis-fimctionalized isoxazoles by cycloaddition of 6 with P-keto esters 34 direcdy. Among the numerous studies on nitrile oxides, only those examples in which a 1,3-dicarbonyl compound is employed as a dipolar-ophile are found in the literature (Scheme 9.14). Umesha et al. prepared 4-acetyl-3-arylisoxazoles by the cycloaddition of isolable aromatic nitrile oxides with acetylacetone 34b [59]. Suzuki and coworkers developed the cyclocondensation of hindered aromatic nitrile oxides with cyclic P-diketones, which was applied to the synthesis of natural products [60]. In these reactions, nitrile oxide was stable enough for isolation thus, a substituent at the 3-position of the cycloadducts was limited to an aromatic group. 

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