Cyclization of carbonates

One problem associated with An + 2n cyclizations of carbon nanotubes is the reversibility of the process, and for this reason Diels-Alder reactions have been a less used synthetic route. One of the most representative examples of Diels Alder functionalization was reported by Langa and co-workers, who performed MW-assisted addition of o-quinodimethane on pentanol-ester functionalized SWCNTs [34]. 

Cyclizations of carbon radicals forming 6-membered rings during polymerizations were reported in 1957 (equation 75) and Julia and his co-workers began systematic studies of these reactions in 1960. ... 

From our discussion of intermolecular additions, we would not expect these cyclizations of carbon radicals to occur in the elimination direction, and indeed cyclic radicals in five-membered or larger rings do not ordinarily open according to Equation 9.115. If the ring is small or highly strained, on the other hand,... 

CYCLIZATIONS OF CARBON-CENTERED RADICALS TO CARBON-CARBON MULTIPLE BONDS 789... 

These cyclizations both involve the reductive intramolecular addition of an electron deficient alkene function to an aldehyde carbonyl function, and both are effected in ca 90 % yields. The mechanism of this latter type of electrochemically induced cyclizations of carbon-carbon double bonds to carbonyl double bonds have been studied rather extensively, with especial attention to the fundamental mechanistic question of whether the cyclization step involves an anion radical, radical, or anionic mechanism [122]. The latter two mechanisms would involve the protonation of the initially formed anion radical intermediate to form a radical, which could then cyclize or, alternatively, be further reduced to an anion, which could then cyclize. Extensive and elegant electrochemical and chemical studies have led to the formulation of these reactions as involving anionic cyclization (Scheme 74). 

Yet another type of isomer formation has been discovered, the formation of a 1,2-dihydro-l,2,4-triazolo[l,5-a]pyrimidine-2-thione (XLVIII) from 2,3 - dihydro-1,2,4-triazolo[4,3-a]pj rimidine-3-thione (XLVII) during the cyclizations of carbon disulfide with 2-hydrazino-pyrimidine, 2-hydrazino-4-methylpyrimidine and 2-hydrazino-4.6-di-methylpyrimidine 316, 320). 

In general many of the features which govern the cyclization of carbon radicals are met with again in reactions involving heteroatomic radicals. But it is also true that the nature of Z may introduce subtle, or dramatic, changes in I I... 

Duan and Smith developed a diastereoselective electrophilic cyclization of carbonates derived from homoallylic alcohols 98 via reaction with iodine monobromide to afford a-iodocarbonates 99 (Figure 25.13). These can further be utilized as intermediates for the synthesis of epoxy alcohols, iodohydrins, diols, triols, cyclic carbonates, and so on ... 

---