Radical ring-opening with electrophile

Figure 6. (A) Radical ring-opening polymerization of vinylcyclopropane with trimethylsiloxy groups and acidic hydrolysis of the poly(silyl enol ether)s. (B) Macromolecular reaction of the poly(silyl enol ether) with electrophile.

As depicted in Scheme 11, ylides 39 derived from 4-methyl-[l,2,3]triazolo[l,5- ]pyridine react with Michael acceptors, which, upon nucleophilic attack at C3 and ring opening, lead to nucleophilic displacement of nitrogen. The intermediate diradical led to a mixture of compounds, including alkenes and a cyclobutane derivative when methyl acrylate was used, and the indolizine 40 with methyl propiolate as the electrophile <1998T9785>. Heating 4-methyl triazolopyridine with benzenesulfonyl chloride in acetone also confirmed decomposition via a radical pathway. 

The first route relies on the ROP of cyclic ketene acetals [1-3]. The electron-rich double bond is prone to react with radicals and electrophiles. Therefore, this class of monomers undergoes cationic and radical polymerization. For example, radical initiators react with the double bond to provide a new tertiary radical (Fig. 2). Two distinct mechanisms of polymerization can then take place direct vinyl polymerization or indirect ring opening of the cycle accompanied by the formation of a new radical, which is the propagating species (Fig. 2). The ester function is formed... 

From epoxides. A very efficient access to tetrahydrofuran derivatives has been developed based on ring opening of oxiranes with selenolates. For instance, preparation of 2,4-disubstituted tetrahydrofurans from epoxides is shown in Eq. (3) [ 11 ]. Opening of the epoxide 19 with diphenyldiselenide/NaBH4 followed by prenylation gives the radical precursor 20 in excellent yield. Cyclization furnishes the tetrahydrofuran 21 in almost quantitative yield but with a modest stereocontrol. The oxirane opening approach described here competes with the electrophilic alkoxyselenenylation reported below (Sect. 2.1.2). 

It is possible to combine ring-opening reactions of the cyclobutoxy radical with a subsequent carbonylation/oxidation sequence (Scheme 4-48) [84]. The pattern of Cl-substitution affects the final products, cyclized or uncyclized, and this can be ascribed to the different reactivity of formyl and acyl functionalities to electrophilic attack by an acyl cation. 

The cyclopropane ring, due to its ring strain, can be considered as a functional group comparable to the double bond with the synthetic potential to generate functionalized three carbon chains via ring opening. Besides thermal-, photochemical-, oxidative-, reductive-, radical-, nucleophile- and Lewis acid or electrophile-mediated activation, the conversion of cyclopropanes mediated by transition metals plays an important role in synthetic uses of small-ring compounds. In most of these cases, prior to conversion, a complexation of the cyclopropane system by the transition metal is necessary. 

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