Answers To Chapter 4 Problems.

Transcription

1 Answers To 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, (b) The two C groups are both out groups, so they are cis in product. so they are cis in product. Bn C C C C C.

2 . (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 ime 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 Me i C C C (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 Me groups remain trans; is in, C Me is endo, so they are trans in product. Et Et C Me Ms (g) tereo: C Me groups remain trans. Ar group is probably out for steric reasons, C Me 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 Me C C Me

3 ..,,,-Cyclononatetraene can theoretically undergo three different electrocyclic ring closures. e electrocyclic ring closing conrotatory e electrocyclic ring closing disrotatory 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 Me on the C(sp ) equatorial. C C C C C C (b) Chair T, with the equatorial. ir (R) Bn Bn ir Bn () ir

4 . (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. 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

5 (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 ir ir trans double bond ir ir ir ir ir ir (h) The chair T is enforced in this macrocyclic compound. 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

6 . 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. 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.

7 . C C Δ Me i ime 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 ime Δ C Me i C C Me i C (b) Electrocyclic ring closing followed by base-catalyzed tautomerization (both starting material and product are bases) gives the product. Me Me Me Me B Me Me Me Me B (c) Diels Alder reaction followed by spontaneous elimination of Me i and aromatization gives the product. Loss of Me i occurs so readily because the Me i group is a π electron withdrawer like a carbonyl group.

8 . Ar ime C Me Ar ime C Me Me i Me i Ar C Me Ar C Me work-up Ar C Me Me i ime Me 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 Me ime Δ Br C Me Br C Me ime Br C Me ime C Me aq. workup C Me C Me Br ime 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

9 . starting material, and since C has a C group attached, we can identify it and C0 in the product as the 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. Me i C 0 C C Li ; warm to RT; ac 0 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 ime ). Upon quenching with water, however, C becomes an electrophilic carbonyl C, whereupon C attacks with concomitant desilylation of to give the product. Me i C Li C Me i C 0 C Me i C C Me i C C Me 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

10 .0 to give the observed product. C C Me i Me i Me i Me i C C Me i Me i Me i Me i C Me i Me i (g) We form C C and C C bonds, and we eliminate the elements of Me 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 Me i + group from C then gives the product. 0 CCF Zn C C Me i C 0 C

11 . Me i C C CF Zn Zn F C C C Me i C C Me i C C Me 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 C ' ' Bu

12 . Bu B C 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 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

13 . (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. C C th pyr C C 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

14 . 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 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 C C 0 C C C C C C ~ + C C C

15 . 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 ~ + C C C C (n) Make: C. Break: C C. ince is nucleophilic, we must turn C into an electrophilic center.

16 . ir 0 ir Ag acetone- 0 ir In the first step, Ag + promotes the departure of to give a cyclopropyl carbocation. This undergoes twoelectron 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 C C Me C C C Me C

17 . 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 Me 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 Me C Me C C C C Me C C C C C Me C C C C Me C Me Me C C C C C C C C C (p) Make: C C, C. Br Me Me LDA Me Me 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

18 . is not very stable, and it is oxidized by air to give the fully aromatic product. Br Me Me i-pr Br i-pr Me Me Me Me Me Me Me Me work-up (q) Break:, C. Make: C C, C. We lose the elements of. Me Zn Me + 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. Me Zn Me Zn

19 . Me + Zn Me + Zn Me Me Zn + Zn Me Zn Me Zn ~ + Me Me Me 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. C C a Et C C Et C C C Et C Et

20 C C Et.0 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. C + C C C C ~ + C C C C C C C C (t) Make: C C, C C, ' C. Break: C. Me ' ' 0 C Me cat. Rh (Ac) Et ' ' ' Et ' Me 0 C Me 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

21 . 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! Me C Me cat. Rh (Ac) Et ' C Me Et Et ' Me Me C Me ' Et C Me (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

22 . (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 Me Δ C C Me C C Me (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

23 . (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. + (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.

24 . Me + Δ a 0 A Me 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. Me Me a Me A Me Me Me (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. (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

25 . 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.

26 . 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

27 . Et C C Et C Et C Et work-up 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

28 . [+] 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. 0 D hν D D D D D D D D D