Acyl- and Cyanonitroso Compounds

The pioneering studies of Kirby et al. have established that acylnitroso compounds are useful heterodienophiles in [4 -f 2] cycloadditions. These species, which can best be generated by periodate oxidation of a hydroxamic acid, are unstable and may undergo rapid solvolysis or dimerization. However, in the presence of a diene such as 9,10-dimethylan-thracene an isolable adduct such as 14 is produced [Eq. (18)]. 

Compound 14 undergoes a retro-Diels-Alder reaction thermally at about 60 C to generate the acylnitroso intermediate which can be trapped by a different diene. Some adducts of acylnitroso compounds with cyclopenta-diene also tend to undergo facile retro-[4 -I- 2] reactions readily and can 

However, [4 + 2] cycloadditions are usually more rapid than the corresponding ene reaction. Table 3-III shows some representative examples of Diels-Alder cycloadditions of acylnitroso dienophiles.  

These results, which seem rather surprising, have not been satisfactorily rationalized. 

Baldwin and co-workers have used a nitroso Diels-Alder reaction as a key step in total synthesis of tabtoxin (17), a metabolite causing leafspot disease in tobacco. This group has also prepared tabtoxinine -lactam (18), the active principle generated by in vivo enzymatic hydrolysis of 17. 

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