Double bonds diazoalkane 1,3-dipolar cycloadditions

The 1,3-dipolar cycloaddition reactions to unsaturated carbon-carbon bonds have been known for quite some time and have become an important part of strategies for organic synthesis of many compounds (Smith and March, 2007). The 1,3-dipolar compounds that participate in this reaction include many of those that can be drawn having charged resonance hybrid structures, such as azides, diazoalkanes, nitriles, azomethine ylides, and aziridines, among others. The heterocyclic ring structures formed as the result of this reaction typically are triazoline, triazole, or pyrrolidine derivatives. In all cases, the product is a 5-membered heterocycle that contains components of both reactants and occurs with a reduction in the total bond unsaturation. In addition, this type of cycloaddition reaction can be done using carbon-carbon double bonds or triple bonds (alkynes). 

The mechanism for the addition of diazoalkanes to a C=—O double bond is generally written along lines similar to those discussed so far (Scheme 1 X" = N2+). The initial adduct is also the progenitor of the various rearrangement pathways. However, the subject of mechanism is by no means settled, with 1,3-dipolar cycloadditions and carbonyl ylide formation" considered to be prominent alternatives. In general, successful epoxidation of carbonyl compounds improves with increasing electron-poor character of the C—O bond. When the diazoalkane is electron poor, yields of epoxide diminish. 

Reports on the advances of asymmetric 1,3-dipolar cycloadditions include the reaction of diazoalkanes to 7V-(2-alkenoyl)oxazolidin-2-ones catalyzed by Mg or Zn complexes of 73, showing cooperative chiral control by the achiral oxazolidinone auxiliary and the chiral ligand.An intramolecular cycloaddition of the same kind from substrates containing a chiral cyclic AiA -dimethylaminal unit adjacent to the dipolarophilic double bond (i.e., 74) proves very successful in the asymmetric sense, although the reaction of an analogous nitrone lacks stereoselectivity. 

Dipolar cycloaddition of the diazoalkane ((CH3)2CN2) to (72) in acetone gave the tetracyclic compound (73). The unsubstituted compound did not react. The nitro group at the 6-position activates the C(5)—C(6) and C(7)—C(8) double bonds for cycloaddition <89H(28)259>. It was originally reported that [l,2,4]triazolo[4,3-a]pyridine (29) reacted with DMAD in the presence of 5% Pd/C to give (74) <81IJC(B)10>. It was subsequently shown that this reaction produces three products independent of the use of the palladium catalyst. Initial Michael addition of DMAD to (29) via its... 

---