Allene Tethered to a.l-Unsaturated Ester

An exceptionally interesting example of the electrohydrocyclization reaction involves the use of allenes which are tethered to a,/S-unsaturated esters (Table 3) [19]. The chemistry takes place in a manner wherein the new carbon-carbon bond forms between the central carbon of the allene and the /S-carbon of the unsaturated ester. Of particular value is the preservation of one of the double bonds of the original allene, thereby providing functionality for further elaboration. It is important to carry out these transformations in an undivided cell, as the use of a divided cell led to hydrogenation of the olefins instead of cyclization. 

In the absence of the acid, cyclization does not occur. JV,Af -Diben-zylidenethylenediamine (16a), for example, is converted to lV,lV -diben-zylethylenediamine (19) in 60% yield in its absence. 

While the yields are generally good for the cyclization of diimines derived from aldehydes, the process does not work satisfactorily using diketoimines such as 20. This is not too surprising, given the need to form two contiguous quaternary centers in 21. 

The method lends itself nicely to the construction of enantiomerically enriched tri- and tetrasubstituted piperazines such as 23, substances that prove useful as ligands in enantioselective synthesis, such as that illustrated in Eq. (11) [20], 

As noted previously, many of the cathodic cyclizations discussed in this article are variations on the electrohydrocyclization theme developed by Baizer and coworkers [8-14,16,17,21], The next section of this article, for example, deals with what has been referred to as the electroreductive cyclization (ERC) reaction, a process that leads to cycUzation between an electron-deficient alkene and an aldehyde or ketone. With this thought in mind, several of the section titles are formulated to highlight the functional groups to be joined the following is representative. 

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