Science of Synthesis

Marsden, S. P., Science of Synthesis, (2005) 26, 1102.

α,α′-Dienones undergo Brønsted or Lewis acid mediated isomerization to cyclopenten­ones, a process commonly referred to as the Nazarov cyclization.[‌254‌,‌255‌] The reaction involves acid-promoted conrotatory electrocyclic ring-closure, followed by proton loss and keto–enol tautomerism. As a result of this mechanism, the reaction of unsymmetrical dienones can generate mixtures of regioisomeric products resulting from proton loss from different sites. This can impose a limitation on the utility of the process, although in general the formation of products containing the more substituted alkenes dominates, as in the conversion of dienone ‌330‌ into hexahydroindanone ‌331‌ (‌Scheme 57‌).[‌254‌] A further problem is that the strongly acidic conditions can encourage alkene migration following the cyclization. A successful approach to solving these regiochemical problems involves the use of an alternative electrofuge such as a trialkylsilyl[‌256‌] or trialkylstannyl[‌257‌] group in place of a proton. The preferential loss of the silicon or tin function defines the regiochemistry of the process, as illustrated by the reactions of enones ‌332‌ and ‌335‌. The nonsilylated variant ‌332‌ yields a mixture of the more substituted enone product ‌333‌ and an isomerization product ‌334‌, whereas the silylated enones ‌335‌ yield the less substituted enones ‌336‌ cleanly.[‌256‌]

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