Intramolecular radical-induced 4+41 cycloaddition

When the cation radical of this alkyne is generated by y radiolysis in a solid matrix at 77 K and then warmed to 150 K, the ESR spectrum of the 1,2,3,4-tetramethyl-1,3-butadiene cation radical is observed. An analogous intramolecular reaction was also observed even in a rigid matrix at 77 K. The feasibility of the cycloaddition step itself is therefore indicated, but little work has yet been done in respect of the aminium salt or PET induced cycloadditions of alkynes in solution at ambient or near-ambient temperatures. Whether a chain or catalytic alkyne cyclodimerization can be effected is yet unclear, as is the potential fate of the cyclobutadiene products. 

These reactions can lead to carbon-carbon or carbon-heteroatom bonds, and their course can be intermolecular or intramolecular. Furthermore, cycloadditions can be initiated by anodic generation of the dienophile or by inducing a chain reaction with a radical cation as dienophile. 

The authors reason that direct irradiation of (54) must induce bond fission to yield the radical pair (57) as well as yielding the cycloadduct. Rebonding within the radical pair (57) can yield (53) which would then undergo the photocycloaddition to yield the cycloadduct (55). Intramolecular cydoaddition is observed on irradiation of the enones (58). The reaction is both wavelength and temperature dependent. Irradiation through Pyrex brings about the formation of a cisitrans mixture of the alkene moiety as the main reaction. Low yields of the two adducts (59a) and (59b) are also formed under these conditions. When a quartz vessel is used the cycloaddition assumes major importance. From the results obtained it is clear that there is a preference for the formation of isomer (59a). Both the isomeric products arise from the biradical intermediate (60) which is formed by addition at the P carbon of the enone moiety and affords the more stable biradical. 

Mono- and disubstituted N-alkyl and N-arylaziridines are reported to undergo a photoinduced electron transfer [3+2] cycloaddition to dipolarophiles to produce five-membered heterocycles, and it is suggested that in this process the radical cation intermediate behaves differently from the corresponding classical azomethine ylide. Intramolecular [4+4] photocycloaddition of the dipyridyl-propane (73) followed by Li/NHa reduction is a useful route to the eleven-membered ring system (74), and on irradiation of benzene solutions of 2,3-dicyano-5,6-dimethylpyrazine in the presence of allylic silanes a [2+2] cyclisation is induced followed by rearrangement to give 2,8-diazatricyclo[3.2.1.0 ]oct-2-ene (75 R = H, Me). ... 

Regardless of the precise structure of the chosen half southern synthon, the two main problems to be solved are the establishment of the carbon skeleton and the introduction of the necessary chirahty into the molecule. The published approaches have introduced chirality either by resolution, by starting with a chiral precursor, or via use of asymmetric synthesis techniques. The carbon skeleton has been established by use of a wide variety of techniques including the Diels-Alder and other cycloaddition reactions, heteroatom induced cyclizations, intramolecular Michael or Aldol cyclizations, intramolecular ether formation, and radical cyclization. 

Photolysis of 3-buten-l-ol nitrite affords no cyclized products (Cy5/Cy4) neither does 5-hexen-l-ol nitrite (Cy6/Cy7). The same result is obtained on peroxydisulphate oxidation of 5-hexen-l-ol. In the Cy6/Cy7 case an important competitive pathway is probably 1,5-intramolecular ally lie hydrogen abstraction and, indeed, esr spin trapping by nitrosodurene " provides evidence of this. Cyclization in the Cy6/Cy7 case was considered to explain the reaction products of tetrahalogeno-o-benzoquinones with 2,3-dimethylbut-2-ene but was discarded in favor of a direct cycloaddition process on the basis of spin trapping and deuteration experiments. As discussed before, cyclization in the Cy3/Cy4 case must be difficult to observe because of the high j5-scission rate of oxyranylalkyl radicals. Nevertheless, this pathway has been used recently to explain the formation of diepoxides in the thermal-, photochemical-, or ferrous-salt-induced decomposition of unsaturated cyclic peroxides. In view of the multistep scheme involved this conclusion must await further confirmation. 

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