Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2
➤ Gửi thông báo lỗi ⚠️ Báo cáo tài liệu vi phạmNội dung chi tiết: Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2
Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2
Two-Group C-C Disconnections I: Diels-Alder ReactionsBackground Needed for this Chapter Reference to Clayden, Organic Chemistry;Chapter 35: Pericyclic Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2c Reactions I: CycloadditionsThe Diels-Alder reaction,1 e.g. 1 + 2, is one of the most important reactions in organic synthesis because it makes two C-C bonds m one step and because it is regio- and stereoselective. It is a pericyclic reaction between a conjugated diene 1 and an alkene 2 or 4 (the d Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2ienophile) conjugated with, usually, an electron-withdrawing group z forming a cyclohexene 3 or 5.The disconnection is often best found by drawing theEbook Organic synthesis - The disconnection approach (2nd edition): Part 2
reverse reaction mechanism. You may draw the arrows cither clockwise or anticlockwise but one must start from the alkene. It makes sense to draw thisTwo-Group C-C Disconnections I: Diels-Alder ReactionsBackground Needed for this Chapter Reference to Clayden, Organic Chemistry;Chapter 35: Pericyclic Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2eed only to be heated together in a sealed tube2 (because they arc volatile) to give 6.This is a two-group disconnection because it can be carried out only when two features are present in the target molecule: the cyclohexene ring and the electron-withdrawing group outside the ring and on the opposi Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2te side to the alkene. The relationship between these features must be recognised. No matter how complicated a molecule may be. if those features areEbook Organic synthesis - The disconnection approach (2nd edition): Part 2
present we should first try' a Diels-Alder reaction. Other features, such as the two four-membered rings in 9, shouldn't distract you. In fact wc madeTwo-Group C-C Disconnections I: Diels-Alder ReactionsBackground Needed for this Chapter Reference to Clayden, Organic Chemistry;Chapter 35: Pericyclic Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2d benzene 12.Organic Synlhcíií ' The Diiconntction Approach. Second Edition Stuart Warren and Paul Wyatt © 2008 John Wiley & Sons. Ltd12217 Two Group C-C Disconnections I: Die If Alder Reactions1011heat —*■ 912StereospecificityThe reaction occurs in one step so there is no chance for either the dien Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2e or the dienophile to rotate and the stereochemistry of each must be faithfully reproduced in the product.4 The two Hs in 3 are cis because they wereEbook Organic synthesis - The disconnection approach (2nd edition): Part 2
cis in the starting anhydride 2. The two Hs in 14 are trans because they were trans in the diester 13.MeOjC Hsealed tubeH C02M. 15O.CThe synthetic atTwo-Group C-C Disconnections I: Diels-Alder ReactionsBackground Needed for this Chapter Reference to Clayden, Organic Chemistry;Chapter 35: Pericyclic Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2or the reaction.5In manufacture it is easier to use the cheap methyl ester and exchange the esterifying group after the Diels-Alder reaction.s-BuOH ----——*- 15; siglureH®Stereospecificity of the DieneThe stereochemistry of the diene is also faithfully reproduced in the product but this is not so eas Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2y to sec. Diene 19 with two E double bonds, adds to the acetylene 20 to give a product 21 with the two phenyl groups cis. This is because the two reagEbook Organic synthesis - The disconnection approach (2nd edition): Part 2
ents approach in parallel planes 22. There are two other ways to work this out, both based on a diagram 23 that looks downwards onto the planes. You mTwo-Group C-C Disconnections I: Diels-Alder ReactionsBackground Needed for this Chapter Reference to Clayden, Organic Chemistry;Chapter 35: Pericyclic Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2symmetrical, having a plane of symmetry marked with the dashed line in 23 and that the product must keep the same symmetry. A more detailed analysis of all these questions appears in Ian Heming’s books.617 Etido-Seiectiviiỵ123PhC0?McII -CO^Me20These two aspects of the Diels-Alder are stereospecific Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2in that the stereochemistry of the product is determined only by the stereochemistry of the reagents and not at all by how favourable one or other patEbook Organic synthesis - The disconnection approach (2nd edition): Part 2
hway may be. We deliberately used an acetylenic dienophile in the last example as it gives rise to no new stereochemistry. But if both diene and dienoTwo-Group C-C Disconnections I: Diels-Alder ReactionsBackground Needed for this Chapter Reference to Clayden, Organic Chemistry;Chapter 35: Pericyclic Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2nts can be maintained.Endo -SelectivityIn a classic Diels-Alder reaction, cyclopentadiene 24 combines with maleic anhydride 2 with complete stereospecificity to give either 25 or 26: so the two Hs on 2 remain cis to each other in both 25 and 26. These arc called endo- and exo- adducts. This refers t Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2o the relationship between the alkene on the diene side and the carbonyl groups from the dienophile. These arc much closer in the Í7u/o-adduct 25. TheEbook Organic synthesis - The disconnection approach (2nd edition): Part 2
result is easy to see when both reagents are cyclic.The experimental result is that the e/ufo-adduct 25 is kinetically favoured while the exo-adducl Two-Group C-C Disconnections I: Diels-Alder ReactionsBackground Needed for this Chapter Reference to Clayden, Organic Chemistry;Chapter 35: Pericyclic Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2ron-withdrawing groups in the dienophile is to attract the diene through space. This interaction does not lead to any bonding between these atoms and is a secondary orbital interaction. You may find this easier to see in the 3D diagrams 27 and 28 where dotted lines show the secondary orbital interac Ebook Organic synthesis - The disconnection approach (2nd edition): Part 2tions or in the flat diagram 29.27 gives 2528 gives 26Two-Group C-C Disconnections I: Diels-Alder ReactionsBackground Needed for this Chapter Reference to Clayden, Organic Chemistry;Chapter 35: PericyclicTwo-Group C-C Disconnections I: Diels-Alder ReactionsBackground Needed for this Chapter Reference to Clayden, Organic Chemistry;Chapter 35: PericyclicGọi ngay
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