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Magazine Chapter 30: Orbitals and Organic Chemistry: Pericyclic Reactions
Q27E
Two sequential pericyclic reactions are involved in the following furan synthesis. Identify them, and propose a mechanism for the transformation.
Q.30-1
Look at Figure 30-1, and tell which molecular orbital is the HOMO and whichis the LUMO for both ground and excited states of ethylene and 1,3-butadiene.
Q.30-16
Q.30-17
The following thermal rearrangement involves two pericyclic reactions in sequence. Identify them, and propose a mechanism to account for the observed result.
Q.30-21E
Heating (1Z,3Z,5Z)-1,3,5-cyclononatriene to 100掳C causes cyclization and formation of a bicyclic product. Is the reaction conrotatory or disrotatory? What is the stereochemical relationship of the two hydrogens at the ring junctions, cis or trans?
Q.30-22E
(2E,4Z,6Z,8E)-2,4,6,8-Decatetraene has been cyclized to give 7,8-dimethyl-1,3,5-cyclooctatriene. Predict the manner of ring-closure- conrotatory or disrotatory for both thermal and photochemical reactions, and predict the stereochemistry of the product in each case.
Q.30-23E
Answer Problem 30-22 for the thermal and photochemical cyclizations of (2E,4Z,6Z,8Z)-2,4,6,8-decatetraene.
Q.30-3
trans-3,4-Dimethylcyclobutene can open by two conrotatory paths to give either (2E,4E)-2,4-hexadiene or (2Z,4Z)-2,4-hexadiene. Explain why both products are symmetry-allowed, and then account for the fact that only the 2E,4Eisomer is obtained in practice.
Q.30-4
What product would you expect to obtain from the photochemical cyclizationof (2E,4Z,6E)-2,4,6-octatriene? Of (2E,4Z,6Z)-2,4,6-octatriene?
Q.30-5
What stereochemistry would you expect for the product of the Diels鈥揂lder reaction between (2E,4E)-2,4-hexadiene and ethylene? What stereochemistry would you expect if (2E,4Z)-2,4-hexadiene were used instead?
