Lista de Exercícios

Summary of cycloaddition reactions A cycloaddition is a onestep ringforming reaction between two conjugated π systems in which two new σ bonds are formed joining the two reagents at each end The mechanism has one step with no intermediates and all the arrows start on π bonds and go round in a ring The cycloadditions are supra facialthey occur on one face only of each π systemand for a thermally allowed reaction there should be 4n 2 electrons in the mechanism but 4n in a photochemical cycloaddition These rules are dictated by orbital symmetry Cycloaddition equilibria generally lie over on the righthand side in a thermal reaction because CC σ bonds are stronger than CC π bonds In a photochemical cycloaddition the product loses its π bonds and therefore its means of absorbing energy It is the kinetic product of the reaction even if it has a strained fourmembered ring The stereochemistry of each component is faithfully reproduced in the productthe reactions are stereospecificand the relationship between their stereochemistries may be governed by orbital overlap to give an endo product 940 35 Pericyclic reactions 1 cycloadditions Z Z TWO new σ bonds 4 2 cycloaddition all arrows start on π bonds 1 Give mechanisms for these reactions explaining the stereo chemistry 2 Predict the structure of the product of this DielsAlder reaction 3 Comment on the difference in rate between these two reactions It is estimated that the second goes about 106 times faster than the first 4 Justify the stereoselectivity in this intramolecular DielsAlder reaction 5 Explain the formation of single adducts in these reactions Problems COCl H O O O H H H O Et3N Et3N Me3SiO OMe CO2Me HO O O OMe O O OMe BnO HO OBn OR O Cl H O OR H O O Cl H H H 160 C O O CO2H O H H O HO2C O O O O 155 C 65 C halflife 15 hours halflife 1 hour 6 Revision elements Suggest two syntheses of this spirocyclic ketone from the starting materials shown Neither starting material is available 7 This reaction appeared in Chapter 33 Account for the selectivity 8 Draw mechanisms for these reactions and explain the stereo chemistry 9 Revision One of the nitrones used as an example in the chapter was prepared by this route Explain what is happening and give details of the reactions 10 Explain why this DielsAlder reaction gives total regio selectivity and stereospecificity but no stereoselectivity What is the mechanism of the second step What alternative route might you have considered if you wanted to make this final product and why would you reject it 11 Give mechanisms for these reactions and explain the regio and stereochemical control or the lack of it 12 Suggest a mechanism for this reaction and explain the stereo and regiochemistry How would you prepare the unsaturated ketone starting material 13 Photochemical cycloaddition of these two compounds is claimed to give the single diastereoisomer shown The chemists who did this work claim that the stereochemistry of the adduct is simply proved by its conversion into a lactone on reduction Comment on the validity of this deduction and explain the stereochemistry of the cycloaddition 14 Thioketones with a CS bond are not usually stable as we shall see in Chapter 46 However this thioketone is quite stable and undergoes reaction with maleic anhydride to give the product shown Comment on the stability of the starting material the mechanism of the reaction and the stereochemistry of the product Problems 941 O O CHO S MeO2C EtO S CO2Me H EtO MeO2C MeO2C Ph N O Ph OH NH2 1 2 LiAlH4 NO2 CHO NO2 OMe OMe NOH OMe OMe N O 1 MeO 2 MeOH HCl Zn NH4Cl H OMe NO2 CO2Me OMe O2N CO2Me OMe CO2Me base DBU was used N O Ph N O Ph N O Ph H H H 1 Zn HOAc 2 MnO2 oxidizes allylic alcohols to ketones mixture of stereoisomers Me H O MeO2C O MeO2C O O heat CO2Me CO2Me O MeO2C MeO2C H O MeO2C H O O h ν NaBH4 S O O O S O O O H H H 15 This unsaturated alcohol is perfectly stable until it is oxidized with CrVI it then immediately cyclizes to the product shown Explain 16 Suggest mechanisms for these reactions and comment on the stereochemistry of the first product 942 35 Pericyclic reactions 1 cycloadditions OH O H H CrVI O OH O O O O CO2H O O O CO2H O MeO MeOH