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Do the following electrocyclic reactions take place in a conrotatory or
disrotatory manner? Under what conditions, thermal or photochemical, would you carry out each reaction?

The following thermal isomerization occurs under relatively mild conditions. Identify the pericyclic reactions involved, and show how the rearrangement occurs.

Heating (1Z,3Z,5Z)-1,3,5-cyclononatriene to 100°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?

(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.

Answer Problem 30-22 for the thermal and photochemical cyclizations of (2E,4Z,6Z,8Z)-2,4,6,8-decatetraene.

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.

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?

What stereochemistry would you expect for the product of the Diels–Alder reaction between (2E,4E)-2,4-hexadiene and ethylene? What stereochemistry would you expect if (2E,4Z)-2,4-hexadiene were used instead?

1,3-Cyclopentadiene reacts with cycloheptatrienone to give the productshown. Tell what kind of reaction is involved, and explain the observed result.Is the reaction suprafacial or antarafacial?

Classify the following sigmatropic reaction by order [x,y], and tell whether itwill proceed with suprafacial or antarafacial stereochemistry: