Answers To Chapter 4 In-Chapter Problems.

Transcription

1 Answers To Chapter In-Chapter Problems... The numbering of the atoms is quite difficult in this problem. The number of groups in the product suggests that at least two equivalents of the bromide are incorporated into the product. But which ring atoms are C and which one is C? And even if one of the ring carbons is arbitrarily chosen as C, there is still the question of whether C or C becomes attached to C. This problem is solved by noting that step turns the bromide into a Grignard reagent, which is nucleophilic at C, so it is likely to attack C, and electrophilic atom. Make: C C, C C, C C. Break: C, C, C Br, C Br. Br Mg C CEt + In the first step, the bromide is converted to a Grignard reagent. In the second step, two equivalents of the Grignard reagent react with the ester by addition elimination addition. (Remember, the ketone that is initially obtained from reaction of a Grignard reagent with an ester by addition elimination is more electrophilic than the starting ester, so addition of a second Grignard reagent to the ketone to give an alcohol is faster than the original addition to give the ketone.) In the last step, addition of acid to the tertiary, doubly allylic alcohol gives a pentadienyl cation that undergoes electrocyclic ring closure. Loss of + gives the observed product. Br Mg MgBr C CEt Et BrMg product.. As usual, the key to this problem is numbering correctly. The main question is whether the ester C in the product is C or C. Because a ring contraction from - to -membered is likely to proceed by a Favorskii rearrangement, where the last step is cleavage of a cyclopropanone, it makes sense to label the +

2 Chapter ester C as C. Make: C C, C C. Break: C, C C, C, Ts. ECE 0 Ts Ts Ts a old on! If the Ts bond is broken, and the electrons go to (as seems reasonable), what happens to the Ts? ome nucleophile must form a bond to it. The only nucleophile in the mixture is, so let s add Ts to our make list. a is a good base, and with all these Ts groups, an E elimination reaction to break a C Ts bond seems reasonable. Either the C or the C bond can be cleaved; we choose the C bond here, but cleavage of the other bond works, too. The product is an enol tosylate. A second elimination reaction is not possible, but at this point we can form the Ts bond and cleave the Ts bond by having attack Ts, displacing to make an enolate. Electrocyclic ring closing with concurrent cleavage of the C bond gives a cyclopropanone. Addition of to C and then C C bond cleavage with concurrent protonation of C by solvent gives the product. Ts Ts Ts Ts Ts Ts.. Make: C C. Break: none. 0 C C C C a C 0 C C Deprotonation of C gives an enolate ion, which in this compound is actually a,,-hexatriene. As such it can undergo an electrocyclic ring closing. Protonation gives the product.

3 C C C C Chapter C C C C C C C C product You may have been tempted to draw the C C bond-forming reaction as a conjugate addition. owever, once C is deprotonated, the carbonyl group is no longer electrophilic, because it is busy stabilizing the enolate. It is much more proper to think of the bond-forming reaction as an electrocyclic ring closure. This problem illustrates why it is so important to consider all the resonance structures of any species... Make: C C. Break: C C. You may be very tempted to draw the following mechanism for the reaction: ~ + owever, this mechanism is not correct. It is a [,]-sigmatropic rearrangement, and for reasons which are discussed in ection.., [,]-sigmatropic rearrangements are very rare under thermal conditions. A much better mechanism can be written. The C C bond is part of a cyclobutene, and cyclobutenes open very readily under thermal conditions. After the electrocyclic ring opening, a,,-hexatriene is obtained, and these compounds readily undergo electrocyclic ring closure under thermal conditions. Tautomerization then affords the product. C ~ + product.. The product has a cis ring fusion.

4 Chapter e.. The first electrocyclic ring closure involves eight electrons, so it is conrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the tetraene, which are both in, become trans. The second electrocyclic ring closure involves six electrons, so it is disrotatory under thermal conditions, and the two hydrogen atoms at the terminus of the triene, which are both out, become cis. This is the arrangement observed in the natural product. C R C R C R C R.. The M of the pentadienyl cation is ψ, which is antisymmetric, so a conrotatory ring closure occurs, consistent with the four electrons involved in this reaction. The M of the pentadienyl anion is ψ, which is symmetric, so a disrotatory ring closure occurs, consistent with the six electrons involved in this reaction. pentadienyl cation pentadienyl anion ψ Ms of the pentadienyl π system ψ ψ ψ ψ

5 Chapter pentadienyl cation M conrotatory pentadienyl anion M disrotatory.. Make: C, C, C0 C. Break: C. 0 C C = 0 C The new five-membered, heterocyclic ring clues you in to the fact that a,-dipolar cycloaddition has occurred here to form bonds C and C0 C. Disconnect these bonds, putting a + charge on C and a charge on, to see the immediate precursor to the product. C C C C 0 C When this disconnection is written in the forward direction along with some curved arrows, it is the last step in the reaction. ow all you have to do is make C and break C. This is easy to do: attacks C, proton transfer occurs, expels, and deprotonation gives the nitrone. C = C C ~ + C C C

6 Chapter C ~ + C C product C.. attacks one of the atoms involved in the bond, displacing. emiacetal collapse to the carbonyl compounds then occurs. R R R R R R.0. Make: C C, C. Break: C. R R R= sugar phosphate backbone of DA. hν R R Making the C C and C suggests a [ + ] photocycloaddition. Then the lone pair on expels from C to give the observed product (after proton transfer). hν R R R R

7 Chapter ~ + product R R.. The numbering is not straightforward in this reaction, but if you draw in the atoms you can see that the two C groups in the new benzene ring in the product probably come from two C groups in norbornadiene. Atoms unaccounted for in the written product include C and 0 (can be lost as C), C to C (can be lost as cyclopentadiene), and and (can be lost as ). Make: C C, C C, C C, C C. Break: C, C, C C, C C, C C, C C. 0 C C, cat. glycine 0 C C 0 C 0 Glycine acts as an acid base catalyst in this reaction. C and C are very acidic, and once deprotonated they are very nucleophilic, so they can attack C and C in an aldol reaction. Dehydration gives a key cyclopentadienone intermediate. (The mechanism of these steps is not written out below.) Cyclopentadienones are antiaromatic, so they are very prone to undergo Diels Alder reactions. uch a reaction occurs here with norbornadiene. A retro-diels Alder reaction followed by a [ + ] retrocycloaddition affords the product. C C C C

8 Chapter C C C C C product C.. Make: C C, C C, C C0, C C. Break: C, C. C C + 0 C 0 C 0 C The C C and C C0 bonds can be made by a Diels Alder reaction. Then loss of and cleavage of the C and C bonds can occur by a retro-diels Alder reaction. This step regenerates a diene, which can undergo another, intramolecular Diels Alder reaction with the C C π bond to give the product. C C C C C C product.. The [+] cycloaddition involves five pairs of electrons (an odd number), so it is thermally

9 Chapter allowed. The [+] cationic cycloaddition involves three pairs of electrons, so it is also thermally allowed... Make: C C. Break: C C. i-pr i-pr 0 0 C, h i-pr i-pr 0 Making and breaking bonds to C suggests a [,n] sigmatropic rearrangement, and a [,] sigmatropic rearrangement, one of the most common types, is possible here. nce the rearrangement is drawn, however, the mechanism is not complete, even though all bonds on the make & break list have been crossed off. C still has one extra and C has one too few. Both these problems can be taken care of by another [,] sigmatropic rearrangement. This step, by the way, reestablishes the aromatic ring. i-pr i-pr.. Make: C C. Break: C. i-pr i-pr product C Et Bn 0 DBU Bn C Et 0 Deprotonation of C by DBU gives an ylide (has positive and negative charges on adjacent atoms that cannot quench each other with a π bond), a compound which is particularly prone to undergo [,] sigmatropic rearrangements when an allyl group is attached to the cationic center, as is the case here. Esters are not normally acidic enough to be deprotonated by DBU, but in this ester the + stabilizes the enolate by an inductive effect.

10 C Et Bn DBU Chapter 0 C Et C Et Bn Bn.. Make: C C0. Break:. R 0 C Et R 0 C Et The most unusual bond in this system is the bond. The nucleophilic substitution step must involve cleavage of this bond. o base is present, but is an excellent nucleophile, even in its neutral form, so the first step probably entails formation of an bond. ow we have to make the C C0 bond and make the bond. Deprotonation of C gives an ylide, which as discussed in problem. is likely to undergo a [,] sigmatropic rearrangement. Tautomerization to rearomatize then gives the product. R C Et R C Et base R C Et R C Et ~ + product.. The reaction in question is: To name the reaction, draw a dashed line where the new bond is made, draw a squiggly line across the bond that is broken, and count the number of atoms from the termini of the dashed bond to the termini of the squiggly bond.

11 Chapter This reaction would be a [,] sigmatropic rearrangement, an eight-electron reaction, and hence would require that one component be antarafacial. ot likely! A more reasonable mechanism begins with the same [,] sigmatropic rearrangement that gives -allylphenol. owever, instead of tautomerization to give the aromatic product, a second [,] sigmatropic rearrangement occurs. Then tautomerization gives the product. ~ + product.. Both the tevens rearrangement and the nonallylic Wittig rearrangement begin with deprotonation of the C atom next to the heteroatom followed by an anionic [,] sigmatropic rearrangement. Both involve four electrons, an even number of electron pairs, and hence if either is concerted then one of the two components of the reaction must be antarafacial. This condition is extremely difficult to fulfill, and hence it is much more likely that both reactions are nonconcerted. Both the tevens rearrangement and the nonallylic Wittig rearrangement are thought to proceed by homolysis of a C or C bond and recombination of the C radical with the neighboring C atom. C BuLi C C C C.. Make: C, C C. Break: C C, C. 0 C cat. + 0

12 Chapter The C bond is easily made first. eavage of the C bond gives an iminium ion that is also a,-(hetero)diene. The Cope rearrangement occurs to give a new iminium ion and an enol. Attack of the enol on the iminium ion (the Mannich reaction) affords the product. ~ + + product ow the stereochemistry. Assume the thermodynamically more stable iminium ion forms ( groups cis). The Cope rearrangement occurs from a chair conformation. This puts the,, and all pointing up both before and after the rearrangement. Assuming the Mannich reaction occurs without a change in conformation (a reasonable assumption, considering the proximity of the nucleophilic and electrophilic centers), the,, and should all be cis in the product..0. Deprotonation of one of the groups adjacent to gives an ylide which can undergo a retrohetero-ene reaction to give the observed products. R C Et R C R C C C C If (CD ) (deuterated DM) is used for the wern reaction, the E mechanism predicts that the sulfide product should be (CD ) ; the retro-hetero-ene mechanism predicts that it should be (CD )(CD ). Guess which product is actually found?

13 Chapter Answers To Chapter End-of-Chapter Problems.. (a) An eight-electron [+] cycloaddition. It proceeds photochemically. (b) A four-electron conrotatory electrocyclic ring opening. It proceeds thermally. (c) A six-electron ene reaction. (ote the transposition of the double bond.) It proceeds thermally. (d) A six-electron [,] sigmatropic rearrangement. It proceeds thermally. (e) A ten-electron [+] cycloaddition. It proceeds thermally. (f) A six-electron [,] sigmatropic rearrangement. It proceeds thermally. (g) A six-electron disrotatory electrocyclic ring opening. It proceeds thermally. (h) A four-electron disrotatory electrocyclic ring closing. It proceeds photochemically. (i) A six-electron disrotatory electrocyclic ring closing. It proceeds thermally. (j) A six-electron [+] (dipolar) cycloaddition. It proceeds thermally. (k) A four-electron [+] cycloaddition. It proceeds photochemically. (l) A six-electron conrotatory electrocyclic ring opening. It proceeds photochemically.. (a) Regio: R and C are,. tereo: C and C remain trans; R is out, C is endo, so they are cis in product. (b) The two C groups are both out groups, so they are cis in product. Bn C C C C C (c) Regio: C of diene is nucleophilic, so it makes a bond to electrophilic C of dienophile. tereo: Et is out, C Et group is endo, so they are cis in product. (d) Regio: C and i are,. tereo: the C C bridge is in at both ends of the diene, C is endo, so they are trans in product. Et C Et i C C C

14 Chapter (e) Dienophile adds to less hindered face of diene. C(sp ) of five-membered ring is in, is endo, so they are trans in product. (f) Regio: ucleophilic adds to electrophilic β C of unsaturated ester. tereo: alkyl and C groups remain trans; is in, C is endo, so they are trans in product. C Et Et Ms (g) tereo: C groups remain trans. Ar group is probably out for steric reasons, C is endo, so the two are cis in the product. (h) The [+] cycloaddition must be antarafacial with respect to one component. The two in groups of the -atom component become trans in the product. Ar C C C C C C.,,,-Cyclononatetraene can theoretically undergo three different electrocyclic ring closures. e electrocyclic ring closing conrotatory e electrocyclic ring closing disrotatory

15 Chapter e electrocyclic ring closing conrotatory When small rings are fused to other rings, the cis ring fusion is almost always much more stable than the trans ring fusion. The opposite is true only for saturated - or larger ring systems. (Make models to confirm this.) The order of stability of the three possible products shown above is: cis-- > trans-- > trans- -.. (a) Chair T, with the on the C(sp ) equatorial. C C C C C C (b) Chair T, with the equatorial. ir (R) Bn Bn ir Bn () ir (c) Chair T, with both substituents equatorial. C C C (d) Two different chairs are possible, but one ( equatorial) is lower in energy than the other. C C C C (e) A chair T is not possible, so it goes through a boat T.

16 Chapter chair T no way José C C C C boat T much better C C C C C C C C C C C C (f) Again, a boat T is necessary. 0 C (g) A chair T would produce a trans double bond in the seven-membered ring, so the boat T is operative, and the and ir groups on the two stereogenic atoms are cis to one another. ir ir trans double bond ir ir ir ir ir ir ir ir (h) The chair T is enforced in this macrocyclic compound.

17 Chapter C C C C C C C C C. (a) First step: hetero-diels Alder reaction (six-electron, [+] cycloaddition). econd step: aisen rearrangement (six-electron, [,] sigmatropic rearrangement). (b) The diene is electron-rich, so it requires an electron-poor dienophile for a normal electron demand Diels Alder reaction. The C=C bond of ketenes is pretty electron-rich, due to overlap with the lone pairs on : C=C=Ö C C +. nly the C= bond of the ketene is of sufficiently low energy to react with the diene at a reasonable rate. (c) First, it is important to remember that in ketenes, the p orbitals of the C= bond are coplanar with the substituents on the terminal C. R R L Because of the ketene s geometry, in the T of the hetero-diels Alder reaction, either R or R L must point directly at the diene. The lower energy approach towards R is chosen, and the product in which R points back toward the former diene portion of the compound is obtained. + R L C R R C R R L R L econd step: The new σ bond forms between the bottom face of the double bond on the left and the bottom face of the double bond on the right, giving the observed, less thermodynamically stable product.

18 Chapter R R L R R L R R L R R L. (a) umber the C s. C, C, C and C are clear in both starting material and product. The rest follows. C C i i We break the C C bond, and we form C C and C C. The formation of the latter two bonds and the fact that we re forming a cyclobutanone suggests a [+] cycloaddition between a ketene at C=C= and the C=C π bond. We can generate the requisite C=C π bond by electrocyclic ring opening of the cyclobutene ring in the.m. C i C i C C i C (b) Electrocyclic ring closing followed by base-catalyzed tautomerization (both starting material and product are bases) gives the product. B B

19 Chapter (c) Diels Alder reaction followed by spontaneous elimination of i and aromatization gives the product. Loss of i occurs so readily because the i group is a π electron withdrawer like a carbonyl group. Ar i C Ar i C i i Ar C Ar C work-up Ar C i i i (d) The key atoms for numbering the C s are C (with the -bromoallyl group attached), C (ester group attached), and C ( attached). We form bond C C and break bond C C. ince C C is the central bond of a,-diene system terminating in C and C, i.e. C=C C C C=C, this must be a Cope rearrangement. Br C i Br C Br C i Br C i C aq. workup C C Br i Br Br (e) umbering the carbons is made easier by C, C, and C. These atoms make it easy to label C through C. ince C is a carbanion, we can expect that it will add to C, the only electrophilic C in the starting material, and since C has a C group attached, we can identify it and C0 in the product as the

20 Chapter 0 easternmost C s, with C attached to C. For C to C, we preserve the most bonds if we retain the C C C C sequence. o overall, we form C C, C C, and C0 C, and we break C C. i C C C 0 Li ; 0 warm to RT; ac C The first step is addition of C to C. We still need to form C0 C and break C C. ince we have a,-diene (C=C0 C C C=C), we can do an oxy-cope rearrangement. This gives a - system in which we only have to form the C C bond. C is neither nucleophilic nor electrophilic, while C is nucleophilic (conjugation from i ). Upon quenching with water, however, C becomes an electrophilic carbonyl C, whereupon C attacks with concomitant desilylation of to give the product. i C Li C i C 0 C i C C i C C i C C C C C C (f) It s clear that we form C C and C C bonds, and we break C C. The strained C C bond can be opened by an electrocyclic ring opening to give an o-xylylene, which undergoes an [+] cycloaddition to give the observed product.

21 Chapter C C i i i i C C i i i i C i i (g) We form C C and C C bonds, and we eliminate the elements of i CCF. The Zn is a Lewis acid, so it coordinates to the carbonyl and causes the cleavage of the carboxylate C bond to give the nice stable allylic cation C C0 C. This cation can undergo a six-electron, [+] cycloaddition with the C=C C=C diene to give a new carbocation at C. Loss of the i + group from C then gives the product. 0 CCF Zn C C i C 0 C i C C CF Zn Zn F C C C i

22 Chapter C C i C C i C C (h) The first product is formed by a hetero-ene reaction, with transfer of the attached to to the terminal C of styrene. C C The second product must incorporate two equivalents of the enol ether. We form C C, C C', and C' bonds, and we transfer a from to C. A hetero-ene reaction forms the C C bond and transfers the. As for the other two bonds, since and C are at the ends of a four-atom unit, we might expect a Diels Alder reaction. We can get to the requisite diene by eliminating the elements of Bu by an Ecb mechanism. The hetero-diels Alder reaction gives the product with endo stereoselectivity and the expected regioselectivity. Bu Bu B C C ' ' Bu C C Bu Bu C C Bu Bu (i) We form C C and C C bonds, and we break C and C bonds. ince C and C are the ends of a four-carbon unit, we can expect a Diels Alder reaction. The cyclohexene in the product should also tip you off. We can obtain the requisite diene by doing a [+] retro-cycloaddition, eliminating to give the C=C C=C diene. tereospecific and endo-selective Diels Alder reaction then gives the

23 Chapter product. Bn Bn Bn Bn Bn (j) When an acyl chloride is treated with Et, β-elimination takes place to give a ketene. When a sulfonyl chloride is treated with Et, β-elimination takes place in the same way. The intermediate undergoes [+] cycloadditions just like ketenes do to give the saturated four-membered ring. Et analogous to ketene (k) The second product provides the key. It is a six-membered ring with a single double bond, probably the product of a hetero-diels Alder reaction. The requisite diene can be made from the starting material by a vinylogous β-elimination, with th as the leaving group. The same diene intermediate can undergo a hetero-ene reaction to give the other observed product. An alternative mechanism for formation of the first product, i.e. direct attack of the alkene (nucleophile) on (electrophile, th as leaving group) to give a carbocation, followed by loss of +, is also possible, but is less likely, especially since we know the C= compound is formed under the conditions. If this mechanism were operative it s also likely that + would be lost from the other C of the carbocation to give the more substituted and more stable isomeric alkene.

24 C C th C C Chapter pyr C C C C C C C C C C pyr C C C C pyr C C C C C C (l) Whenever you see a five-membered heterocycle, think,-dipolar cycloaddition. The heterocyclic rings shown can be made from an intramolecular cycloaddition of a nitrone and the alkene. The nitrone must be made from the hydroxylamine and formaldehyde. C + ~ C C C AD (m) Make: C 0, C, C, C. Break: C C, C 0. C 0 0 C C C C C zone lives to do,-dipolar cycloadditions. After the cycloaddition to give the C and C

25 Chapter bonds, retro,-dipolar cycloaddition occurs to break the C 0 and C C bonds. Then can attack C and 0 can attack C to give the observed intermediate (after proton transfer). 0 0 C C C C C C C 0 C C C C C C ~ + C C C C C econd step. The elements of C are eliminated. The most likely by-products are and C. Make: one. Break: C C, C, 0. The base can deprotonate the on C, and the lone pair on can then push down to form a π bond with C, causing the C C bond to break. The electrons keep getting pushed around until they end up on again and the bond is broken, providing the driving force for the step. A keto-aldehyde and formate anion are obtained. ow C (deprotonated) is nucleophilic and C is electrophilic, so an aldol reaction followed by dehydration gives the observed product. 0 C C C aq. a C C 0 C C C C C C C C C C C C ~ +

26 Chapter C C C C (n) Make: C. Break: C C. ince is nucleophilic, we must turn C into an electrophilic center. ir 0 ir Ag acetone- 0 ir In the first step, Ag + promotes the departure of to give a cyclopropyl carbocation. This undergoes two-electron disrotatory electrocyclic ring opening to give the chloroallylic cation, in which the empty orbital is localized on C and C. Then can add to C; desilylation then gives the product. Ag + ir ir ir Ag ir ir ir ir ir ir (o) The product is a,-diene, specifically a γ,δ-unsaturated carbonyl, suggesting a aisen rearrangement. Work backwards one step from the product. C C C C ( C) C C ( C) C C C

27 Chapter C C C C C C The immediate precursor retains the C bond and would have a C bond and a C=C π bond. This calls for an substitution at C to replace the C bond with a C bond and an E elimination to make the C=C π bond. The overall reaction is an orthoamide aisen rearrangement. C C C C C C C C C C C C C C C C C C C C C C C C C C C C C (p) Make: C C, C. Br LDA ince and C are at the ends of a four-atom chain, we might expect a Diels Alder reaction. The dienophile in such a reaction would be benzyne; the key is the benzene ring fused to the new six-membered ring and the fact that the on C is gone in the product. (You could alternatively draw the π bond of the aromatic ring participating in the Diels Alder reaction, but this is unlikely, because the π bonds of aromatic rings are very bad dienophiles.) The first equivalent of LDA deprotonates to make the,-diene across =C C=C; the second equivalent induces an E elimination across C C to give an aryne. Cycloaddition gives the enolate, which is protonated on C to give the observed product. In fact, this compound is not very stable, and it is oxidized by air to give the fully aromatic product.

28 Chapter Br i-pr Br i-pr work-up (q) Break:, C. Make: C C, C. We lose the elements of. Zn + ince we are forming a σ bond at the end of a six-atom chain and breaking the σ bond in the middle, we might expect a Cope rearrangement. To do this, we must make a C=C π bond. We can do this by transposing the =C π bond. This transposition converts an imine to an enamine, which is exactly analogous to converting a ketone to an enol. The enamine then undergoes Cope rearrangement to give the C C bond. (ote how this Cope rearrangement is analogous to the aisen rearrangement of -allylphenols.) After reestablishing aromaticity by tautomerization, nucleophilic attacks electrophilic C to form the C bond. Finally, E elimination of gives the indole. Zn Zn + Zn + Zn

29 Chapter Zn + Zn Zn Zn ~ + Zn (r) Make: C C, C C. Break: C C. ince only one equivalent of malonate is incorporated into the molecule, the other equivalent must act as a base. The migration of C from C to C is a,-alkyl shift. Under these basic conditions, it is likely to proceed by a Favorskii mechanism. Deprotonation of C by malonate gives the enolate. Two-electron electrocyclic ring closing with expulsion of gives the cyclopropanone. Attack of malonate on C gives a tetrahedral intermediate; fragmentation of this with expulsion of gives the observed product. ther reasonable mechanisms can be drawn, some of which do not involve an electrocyclic ring closing. a C C C Et C C Et C C Et C Et C C Et C C Et C C C C C Et C Et C Et C Et C C C C C Et C Et (s) The five-membered heterocycle should alert you to a,-dipolar cycloaddition.

30 Chapter 0 C C + C C C C C C C C C ~ + C C (t) Make: C C, C C, ' C. Break: C. ' ' 0 C cat. Rh (Ac) Et ' ' ' Et ' 0 C C and C are at opposite ends of a four-carbon unit, but since one of these atoms (C) is saturated and quaternary, a Diels Alder reaction is unlikely (can t make diene). The combination of a diazo compound with Rh(II) generates a carbenoid at C. The nucleophile ' can add to the empty orbital at C, generating the ' C bond and a carbonyl ylide at C ' C. Carbonyl ylides are,-dipoles (negative charge on C, formal positive charge on ', electron deficiency at C), so a,-dipolar cycloaddition can now occur to join C to C and C to C, giving the product. ote how a relatively simple tricyclic starting material is transformed into a complex hexacyclic product in just one step! C cat. Rh (Ac) Et ' C Et Et ' C ' Et C

31 Chapter (u) The cyclobutanone should tip you off to a ketene alkene cycloaddition. Ketenes are generally made by Et -catalyzed elimination of from acyl chlorides. xalyl chloride CC serves to convert the acid into an acid chloride. C C C R R R R C R C C C Et C C C (v) Another five-membered heterocycle, another,-dipolar cycloaddition. The first step is formation of the requisite,-dipole, a nitrile ylide, by a two-electron electrocyclic ring opening. Then dipolar cycloaddition occurs. C hν C C (w) Formally this reaction is a [+] cycloaddition. In practice, concerted [+] cycloadditions occur under thermal conditions only when one of the components is a ketene or has a π bond to a heavy element like P or a metal. either of the alkenes in this reaction fits the bill. owever, one of these alkenes is very electron rich and the other is very electron poor, so a nonconcerted, two-step polar mechanism is likely. C C C C C C

32 Chapter (x) The extra six C s must come from benzene. A photochemically allowed [+] cycloaddition between the alkyne and benzene gives an intermediate that can undergo disrotatory electrocyclic ring opening to give the observed product (after bond alternation). (Either two or three arrows can be drawn for the electrocyclic ring opening, but the T for the reaction involves all eight π electrons, so to be disrotatory the reaction must be promoted photochemically.) Benzene does not usually undergo cycloaddition reactions, but here it evidently does. C C C C hν C C C C C C C C (y) The second product is clearly obtained by a hetero-diels Alder reaction between acrolein and isobutylene. The first product is less obvious. Two new C C bonds are formed, and atoms are transferred from C and C to C and. This suggests two ene reactions. + C C 00 C or + or C C C C C C C C (z) Elimination of allyl alcohol occurs by an E mechanism. Then a aisen rearrangement gives the product.

33 Chapter + (aa) The C to C unit in the product is numbered by virtue of the two s on C. Make: C C, C C, C. Break: C 0, 0. + a 0 A C Formation of C C and C C suggests a Diels Alder reaction, this one of the inverse electron demand flavor. The regioselectivity follows the ortho-para rule and the stereoselectivity is endo. The C bond can now be formed and the C 0 bond cleaved by a [,] sigmatropic rearrangement to give compound A. All that is left is to cleave the 0 bond. a attacks, with R acting as the leaving group, and protonation gives the final product. a A (bb) Retro Diels Alder reaction gives off and an ortho-xylylene. With no other substrates available, this extremely reactive substance dimerizes in another Diels Alder reaction to give the product.

34 Chapter (cc) The product is formally the result of a [,] sigmatropic rearrangement. TP! [,] sigmatroopic rearrangements are very rare, and they should be viewed with suspicion. They are thermally allowed only when one of the components is antarafacial. ometimes an apparent [,] shift is actually the result of two sequential reactions (polar or pericyclic). In this case, the presence of K suggests an oxyanion-accelerated concerted process. The one-atom component can be antarafacial if the back lobe of the sp orbital used to make the old bond to C is then used to make the new bond to C. After workup, aromaticity is reestablished by protonation-deprotonation. K + work-up + (dd) Make: C C, C, C C0,. Break: C,. C C C 0 Et 0 C C C The product looks very much like the result of a Diels Alder reaction that forms the C C and C C0 bonds. Work backwards one step from the product.

35 Chapter C C C C C C The intermediate might be made by a [,] sigmatropic rearrangement of an R compound. C C C C Et C C C C C C C C C C C (ee) Make: C C, C C. Break: C C, C. a Et C C Et C Et C Et The two new bonds can be obtained by a Diels Alder reaction. First, deprotonation gives an enolate that has an ortho-xylylene resonance structure. Diels Alder reaction followed by retro-diels Alder reaction gives the product. Et C C Et

36 Et C C Et Chapter C Et work-up C Et C Et C Et (ff) As in the previous problem, Diels Alder reaction followed by retro-diels Alder reaction establishes the desired C C bonds. Then E elimination of C gives the desired product. (An Ecb mechanism for elimination is also reasonable, but less likely in the absence of strong base.) C C C C C C C C C C C C C C C C C C C (gg) The D atoms give major clues to the numbering. Break: C C, C C0, C C. Make: C C, C0 C. D 0 hν D 0 D D If we break the C C and C C0 bonds by a retro-diels Alder reaction first, we get two molecules of benzene. But irradiation of benzene doesn t give the observed product, so this can t be right. Instead, let s form the C C and C0 C bonds first by a (photochemically allowed) [+] cycloaddition. This gives the strained polycyclic compound shown. ow the C C and C0 C bonds can be broken by a [+] retro-cycloaddition (thermal, supra with respect to both components) to give the tricyclic compound. This compound can then undergo disrotatory six-electron electrocyclic ring opening (thermal) to give the observed product. ote that only the first reaction in this series requires light.

37 Chapter hν D D D D 0 D D D D D D (hh) umbering the product is difficult. Because C in the starting material has no atoms, let s make it one of the C s in the product that has no atoms. Make: C C, C C, C C. Break: C C. ±C± Carbenes like to do [ + ] cycloadditions to alkenes. uch a cycloaddition between C and the C=C π bond gives a product which can undergo an -electron electrocyclic ring opening to cleave the C C bond, then a -electron electrocyclic ring closing to form the C C bond. All that is left to do is a [,] sigmatropic rearrangement to move the C to C. ±C± ± ± (ii) Following the instructions, we number all the atoms. The byproducts are. Make: C,

38 C,. Break: C,, 0. Chapter Et C 0 Et C The thermal reaction that azides undergo is the Wolff rearrangement (Chapter ). In the present case, the Wolff rearrangement allows us to make the C bond and cleave the 0 bond. A resonance structure can be drawn in which has a negative charge. This lone pair is used to attack, displacing. ext, the C bond is cleaved by a -electron electrocyclic ring opening to give a nitrilimine, which then undergoes a -electron electrocyclic ring closure to give the product. Et C Et C Et C Et C Et C Et C

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