Advanced Organic Chemistry

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1 D. A. Evans, G. Lalic Question of the day: Chemistry 530A TBS Me 2 C Me toluene, 130 C 70% TBS C 2 Me H H Advanced rganic Chemistry Me Lecture 16 Cycloaddition Reactions Diels _ Alder Reaction Photochemical [2+2] Cycloadditions Ketene [2+2] Cycloadditions Cheletropic reactions [3+2] Cycloadditions Crimmins, M. T. "Synthetic Applications of Intramolecular Enone-lefin Photocycloadditions", Chem. Rev. 1988, 88, (pdf) Bach, T. "Stereoselective Intermolecular [2+2]-Photocycloaddition Reactions and Their Application in Synthesis", Synthesis 1998, 683. (pdf) Nicolaou, K. C. "The Diels _ Alder Reaction in Total Synthesis", Angew. Chem., Int. Ed. 2002, 41, (pdf)

2 diene dienophile Looks simple? How about this one? FR182877

3 Steric Effects HM-LUM Gap and the Reaction Rate Steric hindrance at the terminus of the diene slows the reaction. The effect of the methyl substituent is a combination of electronic and steric effects. substituents at the terminal position have an unfavorable effect on the equilibrium between s-cis and s-trans conformers. In most cases the size of substituents in positions 2 of a diene has little effect on the rate of the reaction. Provide an explanation for the observed difference in relative rates. Substituents on both the diene and the dienophile that decrease the gap will contribute to the rate acceleration. Electron donating substituents on the diene and electron withdrawing substituents on the dienophile.

4 Lewis acid coordination to dienophile lowers the LUM and accelerates the reaction. Bases for a vast majority of catalytic DA reactions. rbitals with similar coefficients overlap. X C X C Calculated HMs and LUMs for some alkenes and dienes. Regiochemistry of DA Reaction rtho-para rule. Meta products are not favored. NEt 2 NEt 2 C 2 Me C 2 Me 20 C + + "ortho" is only product NEt 2 C 2 Me Et + C 2 Me 160 C Et + C 2 Me "para" is only product Et C 2 Me Resonance structure approach. allyl cation superimposed on alkene LUM HM alkene LUM C C X C C X C C X allyl cation C C X LUM Me Me Positive charge is on the terminal carbon Draw relevant resonance structures of the starting materials that explain the observed regiochemistry.

5 Lewis acids improve regioselectivity of DA reactions by increasing the contribution of the more polarized resonance structures. Stronger group wins The endo Diels-Alder adduct is formed faster even though the exo product is more stable. There is thus some special stabilization in the transition state leading to the endo product which is lacking the exo transition state. Nitro group is particularly useful removable electron withdrawing group. Secondary orbital interactions between the diene and the dienophile favor the formation of the endo product.

6 Relative stereochemistry of the product: Endo rule DA is both stereospecific and stereoselective. Relative orientation of substituents in the diene and dienophile is preserved. There is a preference for a relative orientation of substituents from the two reacting molecules. Inverse electron demand DA Diels Alder reaction is also possible when the HM of the dienophile is close in energy to the LUM of the diene. Such Diels Alder reactions are called inverse electron demand. Still endo-selective. Regiocemistry is a result of the same considerations Hetero DA reaction E E + n n x x n n E x E x E E x x n n Lewis acids improve the endo-exo selectivity of DA reactions by increasing the importance of secondary orbital interactions.

7 Biomimetic Synthesis of FR Transannular Diels-Alder Reaction Biomimetic Synthesis of longithorone A Shair, M. D. J. Am. Chem. Soc. 2002, 124, 773. E E + n n E x E x E E x x n n

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9 New Type of Hydrogen Bonding Similar model for coordination of enals with Yamamoto s catalyst correctly predicts the stereochemistry of the product. Aldehydes are monodentate dienophiles? See Sherburn, M. S. Angew. Chem. Int. Ed. 2008, 47, 7013 for another model operational with certain substrates.

10 State of The Art Enantioselective Hetero-Diels-Alder Reactions Lewis acid activated Lewis acid. Concept first introduced by Yamamoto. Lewis activation in this case expands the scope of both dienes and dienophiles. Much less reactive dienes can be used and a variety of dienophiles including aldehydes esters, quinones, cyclic ketones. Jacobsen, E. N. Angew. Chem., Int. Ed. 1999, 38, Enantioselective Transannular Diels-Alder Corey, E. J. J. Am. Chem. Soc. 2007, 129, Catalyst Controlled Diastereoselective Hetero-Diels-Alder Reaction Jacobsen, E. N. Science 2007, 317, 1736.

11 [2+2] Cycloadditions Applications of Photochemical [2+2] Cycloadditions Thermal activation [ 2a + 2s] Paterno-Buchi reaction (carbonyl + alkene) Reaction proceeds through a triplet carbonyl excited state and a diradical intermediate. ften gives a mixture of head-head and head-tail isomers. Asteltoxin synthesis Schreiber, S. L. J. Am. Chem. Soc. 1983, 105, Photochemical activation [ 2s + 2s] Me Me + Enone alkene cycloadditions H hv 80% Me anti H Me >20:1 exo H Ph Reaction proceeds through a triplet carbonyl excited state and a diradical intermediate. Intramolecular reactions are predictable and often used. Fused ring is a five membered when ever possible. Ac Me hv 76% Ac H Me Ac H Me Substituents that stabilize the radical intermediate, promote the reaction. ppolzer, W. Helv. Chim. Acta 1980, 62, rev: Crimmins, M. T. Chem. Rev. 1988, 88, 1453.

12 Cycloadditions of Ketenes Deprotonation/elimination from acyl derivatives. Decomposition of -diazocarbony compounds, Wolff rearrangement. [2+2] cycloadditions preferred over [4+2[ cycloaddtions with dienes Keten Formation Reactions Reduction of chloroacyl derivatives (most often chlorides). Electrocyclic ring opening.

13 FM Description Examples: Key step in Corey's synthesis of Ginkgolide B Concerted vs. Stepwise Mechanism E alkenes stepwise process: 1. Slower reaction 2. Mixture if syn and anti products 3. KIE alkenes concerted reaction: 1. Stereospecific 2. Faster Me Me Bu 3 N tbu tbu 80% yield toluene, H C 2 Cl H Corey, E. J. J. Am. Chem. Soc. 1988, 110, 649. Catalytic Enantioselective [2+2] Cycloadditions Stepwise reaction with excellent diastereo and enantioselectivities.

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16 Stereospecificity: The Dipole

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18 Azomethine Ylides R 2 is usually electron-withdrawing group Wada, E. Bull. Chem. Soc. Jpn. 1990, 63, Livinghouse, T. J. rg. Chem. 1983, 48, 1554.

19 Huisgen 3+2 cycloaddition Carbonyl Ylides Huisgen, R. Angew. Chem. 1963, 75, 604. Sharpless Angew. Chem. Int. Ed. 2002, 41, 2596.

20 Padwa, A. Tetrahedron Lett. 1992, 33, Provide the product and a detailed mechanism.

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