Azides, acid catalyzed ring

The functional group compatibility of the click cyclization approach also makes it amenable to a variety of other polymerization techniques. For example, cyclic polyesters have been prepared using the Lewis acid-catalyzed ring-opening polymerizations (ROPs) of lactone monomers, including e-caprolactone [47, 48] and 5-valerolactone [49] from an azide-functionaUzed alcohol. Esterification of the terminal alcohol with pentynoic anhydride provided the complementary functional groups to enable an efficient click cycHzation for molecular weights up to 15 kDa. 

McEwan and co-worker" examined the acid-catalyzed decomposition of unsymmetrical benzhydryl azides 18 and some related species (Fig. 13.11). The aryl migration step showed very little discrimination between aryl rings with electron-donating and those with electron-withdrawing substituents. This low selectivity was judged to be more consistent with formation of a discrete nitrenium ion intermediate (19). These workers reasoned that a concerted migration would exhibit higher selectivity toward donor-substituted arenes, because in that mechanism the electrons from the arene would participate in the reaction. 

A mechanism that explains some of the more important observations in the acid-catalyzed hydrolysis of epoxides 49a-d is outlined in Scheme 15. The cis/trans diol product ratios from the acid-catalyzed hydrolysis of 49a-c, which have either hydrogen- or electron-donating groups in the para position of the phenyl ring, are 74 26, 83 17 and 65 35, respectively. An intermediate carbocation 52a is trapped by azide ion in the acid-catalyzed hydrolysis of 49a and the rate constant for reaction of 52a with water in 10 90 dioxane-water solvent is estimated, by the azide clock technique, to be 1.7 x 108 s 1. Azide ion also traps an intermediate 52b in the acid-catalyzed hydrolysis of 49b, but somewhat less efficiently. The rate constant ks for reaction of 52b with solvent is estimated to be 2 x 109 s-1. The somewhat greater reactivity of 52b compared to that of 52a is consistent with the observation that... 

Aziridines2 )3-Iodoazides, available by the addition of iodine azide to olefins, undergo reduction of the azide function followed by base-catalyzed ring closure to aziridines. The most satisfactory reagent for this purpose is lithium aluminum hydride, which can accomplish both reaction steps since it is a Lewis acid as well as a reducing agent. Competing side reactions are elimination of the elements... 

Dihydropyridones 110 are prepared from l,4-dien-3-ones 105 by a combination of electrocyclization with Schmidt reaction. Thus, enone 105 undergoes Lewis acid-catalyzed Nazarov electrocyclization, the resulting Nazarov intermediate 106 is trapped with benzyl azide, and ring expansion of the resulting azide 107 delivers zwitterion 108. This intermediate rearranges to the dihydropyridone 110 via either proton transfer or 1,5-hydride shift. 

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