Soluções Sugeridas Capítulo 35

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Chapter 35 Suggested solutions for Chapter 35 301 Suggested solution The dienes are the same, the ring sizes are the same, and the only difference is the presence of the benzene ring in the faster reacting compound. We should draw a mechanism for one of the reactions just to see what is happening. 5 7 6 4 [4 2] 8 3 9 10 2 1 0 We are making two new rings. The six-membered ring containing a cis-alkene presents no This reaction is part of a problem. The eight-membered ring with a ketone in it might present a problem, and the ten- synthesis of the taxane skeleton (K. J. Shea and P. D. Davis, Angew. membered ring with the trans-alkene is definitely a problem. It is much easier to make medium Chem., Int. Ed. Engl., 1983, 22, (8- to 14-membered) rings when there is a cis-alkene elsewhere in the ring and the benzene ring 419). helps there. It increases the population of conformations with the ends of their chains close together. It probably also lowers the energy of the LUMO. Problem 4 Justify the stereoselectivity in this intramolecular Diels-Alder reaction. 0 H 0 0 Purpose of the problem Investigating the stereoselectivity of the intramolecular Diels-Alder reaction. Suggested solution Intramolecular Diels-Alder reactions can give endo or exo products. We should first discover J. D. White and B. G. Sheldon, which this is. Drawing the transition state for the endo product, we find that the endo product is J. Org. Chem., 1981, 46, 2273. indeed formed. So electronic factors dominate, perhaps because the dienophile has such a low- energy LUMO and has two carbonyl groups for secondary orbital overlap with the back of the diene. all cis HO2C o H H H H 0 [4 2] 0 0 0 H