Other Rearrangements and Cyclizations

Sheldon and co-workers859 have performed the cyclization of A-formyl enamide 249 in triflic acid or over Nafion-H to form the corresponding A-formyl octahydroisoq u i -nolines starting compounds in the synthesis of A-formylmorphinanes [Eq. (5.314)]. 

A novel one-pot tandem oxidation-cyclization-oxidation process was successfully applied in the transformation of unsaturated alcohols 250 [Eq. (5.315)].860 The intermediate aldehyde formed by oxidation with pyridinium chlorochromate (PCC) undergoes a carbonyl-ene cyclization followed by an additional oxidation to form 3-substituted piperidinones. 

The allyl-transfer reaction based on 2-oxonia Cope rearrangement allows highly stereocontrolled chirality transfer. Triflic acid has been shown to induce the rearrangement of the 251 allyl sterols into 22-homoallylic sterols with high stereoselectivity without side reactions86 [Eq. (5.316)]. The protocol, however, is not effective for syn substrates (for example, 251, R = H, R = COOEt). 

Isomerization of pivalaldehyde to yield isopropyl methyl ketone has been shown by Olah et al.451 to strongly depend on the acidity. Triflic acid, anhydrous HF, and BF3—2CF3CH2OH, but not trifluoroacetic acid, induce quantitative isomerization that is, acids with //0 11 are required for complete isomerization. These 

By studying stable ion chemistry of polycyclic aromatic systems, Laali et al.862 observed the ring closure of dicyanometacyclophanediene 254 with the involvement of diprotonated intermediate 255 [Eq. (5.318)]. When product 256 was treated again in superacids under different conditions, rearrangement took place to yield 1-cyanopyr-ene through mono- and diprotonated intermediates [Eq. (5.319)]. 

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