Synthesis of Rings with Three or More Heteroatoms

Synthesis of Rings with Three or More Heteroatoms 

On thermolysis, bis(diazomethyl)phosphanes 647 split off a nitrogen molecule to produce 4//-l,2,4-diazaphospholes 648 via the intermediate formation of phosphinocarbenes 649 that undergo [l,5]-ring closure (88TL925). 

Flash vacuum pyrolysis of phosphorane 650 results in the extrusion of BujP to give 1,2,4-benzotriazine 651 (26%) and o-cyanoacetophenone (16%). The products are apparently derived from the rearrangement of the initially formed carbene 652, which involves, as in the first example, [1,2]-N migration (93CC1517). 

Cu(hfacac)2-catalyzed elimination of N2 from aroyl(a-diazoacyl)hydra-zines 653 yields 2//-l,3,4-oxadiazin-2-ones 654 (70%). The reaction occurs consecutively via intramolecular carbonyl ylide formation, 1,3-cyclization into oxirane 655, and ring opening by the carbon-oxygen bond followed by a C- 0 shift with ring expansion (88CB887). 

Thermolysis of 2-diazo-4,5-dicyano-2//-imidazole 656 in benzonitrile gives nitrilium ylide 657, which reacts with a second benzonitrile molecule to afford [4+2]-cycloadduct 658 (79JOC1717). Intermediate formation of nitrile ylide 659 was also supported in a reaction of N-fluoropyridinium 

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VIII. Synthesis of Rings with Three or More Heteroatoms. 195... 

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