Reductive cyclization of ketones

Allenyl-substituted cycloalkanones have also been studied the one-electron reduction of these resulted in 5-exo-trig or 5-endo-trig cyclizations (depending on the relative position of the allene group) [95]. The reductive cyclization of ketones in the presence of acrylate side-chains have been investigated the reductant was magnesium metal in methanol [96]. 

A similar mechanism has been suggested for the reductive cyclization of ketones linked to Q ,)6-unsaturated esters by Mg in MeOH at —23°C, which gives high chemical yields (80-100%) of the cyclic alcohols [210]. 

In a unique twist on the use of oxocarbenium reduction to generate tetrahydrofurans, Romo and Mitchell reported the generation of 2,5-cis- and 2,5-trani-tetra-hydrofurans from the reductive cyclization of ketones having a pendant p-lactone (Scheme 5) [8]. This reaction builds upon previously reported results from Mead et al. and leads to the stereospecific synthesis of highly substituted tetrahydrofurans [9]. For example, when a fi-p-lactone 13 was subjected to TESOTf and excess Et3SiH, they isolated 2,5-trans-tetrahydrofuran 14 in 82 % yield as a 14 1 mixture of dia-stereomers, while the corresponding yn-p-lactone 16 gave 2,5-cfr-tetrahydrofuran 17 in 78 % yield as a >19 1 mixture of diastereomers. The stereochemical outcome of these reactions is consistent with the Woerpel et al. model as depicted for the reduction of 15 and 18. 

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