Torquoselectivity in the cationic cyclopentannelation of (2Z)-hexa-2,4,5-trienal acetals

Inter- and intramolecular trapping experiments and density functional theory ab initio calculations for model systems are consistent with the acid-catalyzed rearrangement of 2-[(1Z)-hexa-1,3,4-trienyl]dioxolanes 1 to tetrahydroalkylidenecyclopenta-1,4-dioxins 4; this involves the electrocyclic ring closure of substituted hydroxypentadienyl carbocations. The reaction, which may be considered a variant of the Nazarov cyclization, occurs much more readily than the standard Nazarov cyclization, proceeding rapidly even at -30 degrees C. B3LYP/6-31G**//HF/6-31G** calculations for models 36, 38 and 40 predict that the two alternative conrotations at the cyclization termini are associated with activation energies differing by 0.55, 0.56 and 1.60 kcalmol(-1), respectively, in favour of the R-outwards rotation. This last value corresponds to an E-41/Z-41 product ratio of >99:1 at -60 degrees C, in consonance with the experimental observation that divinylallene 1a rearranges exclusively to E-4a at temperatures below -30 degrees C. At higher temperatures the torquoselectivity of the reaction 1a-->4a is masked by subsequent isomerization to the Z isomer, the greater stability of which is attributable to steric interaction between the substituent at the exocyclic double bond and the bulky neighbouring tBu group in the E isomer.