Cyclization through azides

Some azides [such as (58.1)] are resistant to thermal cyclization through the nitrene. A strong acid may help the cyclization of such compounds but concentrated sulphuric acid may cause simultaneous sulphonation of a benzene ring. 

Removal of the MOM and Boc groups followed by regioselective bromination with Py-HBr in dichloromethane followed by cyclization through an iminium ion intermediate furnished the bicycle 10 as a single isomer. Deprotection of the benzyl ether and reduction of the azide group afforded the amino diol 11, which was treated with BrCH2C02Ph and propylene oxide and subsequent Pb(OAc)4 oxidation of the amine to provide 12 (80%). Intermolecular addition of the phenolic compound 13 to 12 in the presence of TEA occurred at the sterically less hindered convex face to provide the key intermediate 14 (Scheme 2). ... 

Nevertheless, satisfactory results were often obtained in cyclizations through the azide. The method rests on the large difference in the rate of conversion of the hydrazide to the azide and that of deamination of the (protonated) N-ter-minal residue by nitrous acid. Hence, with the calculated amount of sodium nitrite (or alkyl nitrite) rapid conversion of the hydrazide to the azide can take place without significant deamination at the N-terminus. Cyclization occurs on addition of base that deprotonates the amino group and thus permits its acylation. This addition, however, should be preceded by dilution to decrease the extent of intermolecular reactions... 

The rapid development of C-H functionalization has helped in accomplishing the synthesis of a variety of complex heterocycles from simple precursors. Recendy, Ramana and co-worker demonstrated a simple one-pot procedure for the Cu(I)-catalyzed SNAr reaction of o-bromochalcones 1 with sodium azide followed by intramolecular cyclization through nitrene C-H insertion to provide 2-aroylindole derivatives 2 [56] (Scheme 7.1) on the basis of their previous work [57]. Furthermore, this methodology is also applicable with the 2 -bromocinnamates giving the indole-2-carboxylates. 

The course of the photochemically mediated isomerization of vinylazirines is dependent on the stereochemistry of the vinyl group, as is illustrated in Scheme 94a (75JA4682). Under thermal conditions the isomerization proceeds through formation of the butadienylnitrene and subsequent pyrrole formation. Analogous conversions of azirines to indoles have also been effected (Scheme 94b). It is possible that some of the vinyl azide cyclizations discussed in Section 3.03.2.1 proceed via the azirine indeed, such an intermediate has been observed... 

Sodium azide also reacts with olefins of the type shown in Scheme 77 to give triazoles through the intermediacy of triazolines.295,296,298,299 The mechanism involving nucleophilic displacement of the substituent X by azide ion, followed by cyclization of the vinyl azide,295 does not seem to apply because neutral vinyl azides fail to cyclize.300... 

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