Acid-catalyzed electrocyclic formation

Acid-catalyzed electrocyclic formation of cyclopentenone from di-vinyl ketone. 

Ketone semicarbazones undergo acid catalyzed reversible cyclization to 3,3-disubstituted 1,2,4-triazolidinones (76JHC1257). The only example available in the literature where a fused triazoline was obtained involves the nitrosation of ethyl 2-isopropylidenehydrazonothiazole-4-acetate, resulting in the formation of a 3,3 -dimethyl-2//-s-triazolo[3,4-6Jthiazole (Scheme 14) (78JHC401).The reaction may be classified as a 1,5-electrocyclization. 

For the protic acid catalyzed reaction, the cyclization commences with protonation of the divinyl ketone 9 and formation of a pentadienyl cation 10. An analogous process is operational in the case of Lewis acid-catalyzed reactions. The pentadienyl cation 10 then undergoes a 47t electrocyclic closure to give a cyclopentenylic cation 11. This cyclization is a pericyclic reaction and is governed by the rules for conservation of orbital symmetry. Namely, this means the cyclization occurs stereospecifically in a conrotatory fashion, with predictable relative configurations of the substituents (i.e., the R groups in 11 are anti). Elimination of a proton, followed by tautomerization gives product 13. ... 

Trauner and colleagues [39] recently found a striking contrast in the thermal and catalyzed reactions of a triene. Thermal reaction of a trienolate readily underwent disrotatory electrocyclization to afford cyclohexadiene (delocalization band in Scheme 8) in accordance with the Woodward-Hoffmann rule. Surprisingly, treatment of the trienolate with Lewis acid did not result in the formation of the cyclohexadiene but rather gave bicyclo[3.1.0]hexene in a [4n +2nJ manner (pseudoexcitation band in Scheme 8). The catalyzed reaction is similar to the photochemical reaction in the delocalization band. 

Nazarov-type Reactions. It has been shown that the highly Lewis acidic Sc(OTf)3 and Yb(OTf)3 can catalyze the Nazarov cyclization. Recently, Dy(OTf)3 has been err5>loyed to catalyze the rearrangement of furfural and furylcarbinol reagents. These rearrangements are proposed to terminate in Nazarov-type 47T electrocyclizations. A Dy(OTf)3-catalyzed formation of trans-4,5-diamino-2-cyclopenten- 1-ones fromfurfural and nitrogen nucleophiles has been disclosed (eq 5). ... 

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