Spiro compounds ring opening

The formation of the tetrazine is interpreted in terms of an initial [3 -I- 2] cycloaddition leading to unstable spiro compounds. Ring opening of the dihydro-1,2,3-triazole ring generates the zwitterions... 

Nitro-substituted indolino spiroben2opyrans or indolino spironaphthopyrans are photochromic when dissolved in organic solvents or polymer matrices (27). Absorption of uv radiation results in the colorless spiro compound [1498-88-0], C22H2gN202, being transformed into the colored, ring-opened species. This colored species is often called a photomerocyanine because of its stmctural similarity to the merocyanine dyes (see Cyanine dyes). Removal of the ultraviolet light source results in thermal reversion to the spiro compound. 

Okada, M. Ring-Opening Polymerization of Bicyclic and Spiro Compounds. Vol. 102, pp. 1-46. 

An interesting strategy for the synthesis of pyrrolizidines and indolizidines has been developed by Brandi and co-workers. Cycloaddition between nitrones or nitrile oxides with methylenecyclopropanes generates strained tricyclic spiro compounds, which are prone toward further transformations, such as rearrangement, ring opening, and new ring closure (Scheme 10.17).116... 

Apparently, the driving force for the ring opening is the relief of the strain in the spiro system and the formation of the stable carbonate double bond. The double ring opening is probably a concerted process from the initial radical addition product to the open-chain radical. Even though the spiro compound XI is an allyl monomer, it does copolymerize with a wide variety of comonomers. 

Section IV,A,2 Competition between van Alphen-Huttel rearrangement and ring opening to a vinyldiazo compound is found to be substituent dependent for transient spiro-3//-pyrazoles formed from methyl propynoate and 4-diazopyrazolones.177... 

The reaction of phenols with the spiro /3-lactone (397), obtained from the reaction of diketene with ethyl diazoacetate, leads to coumarins (79JCS(Pl)525). Initial ring opening of the spiro compound to the diketo ester followed by regioselective intramolecular acylation would seem to be a possible mechanism (Scheme 129). 

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