Reductive-cyclization general reaction schemes

A general method for the synthesis of N-unsubstituted aziridine-2-carboxylates involves a triphenylphosphine-mediated reductive cyclization of hydroxy azido esters [17-22]. A recent example involves the treatment of [1-hydroxy-a-azido ester 15 (Scheme 3.6) with PPh3 to give aziridine 16 in 90% yield [19]. a-Hydroxy- 3-azido esters undergo similar reactions to give aziridine-2-carboxylates [20-22],... 

The excellent reducing properties of potassium tetracarbonylhydrido-ferrate have been used to good effect in providing an efficient general piperidine synthesis from alkyl and aryl amines and glutaraldehyde210 (Scheme 138). The reaction probably proceeds via reduction of intermediate Schiff bases and cyclization via the iminium salt. 

Several recent examples of metal-promoted cyclizations of perchlorocarbonyl compounds are presented in Scheme 28, and a full paper by Weinreb is recommended as an excellent source of references to prior work in this area (including mechanistic studies on the role of the metal).127 The first two examples illustrate that the choice of substrates can dictate the types of products that are formed the initially formed y-chloro esters are stable to subsequent ionic reactions, but the ris-y-chloro acids form lactones. Interestingly, Weinreb has shown that the metal can equilibrate the cis- and /rans-y-chloro esters by reversible chlorine atom transfer. The third example128 illustrates a general feature of the atom transfer method yields at high concentration are comparable to (and sometimes better than) those provided by using tin hydride at low concentrations. Indeed, in the third example, the three chlorines on the ester provided three opportunities for cyclization during the tin hydride reduction, but 40% of the product still failed to cyclize. (Unfortunately, the tin hydride concentration was not specified.)... 

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