CEM 852 Final Exam. May 6, 2010

Transcription

1 CEM 852 Final Exam May 6, 2010 This exam consists of 7 pages. Please make certain that your exam has all of the necessary pages. Total points possible for this exam are 150. n answering your questions, please write legibly and draw all structures clearly. Write all your answers in the exam booklets. Good luck.. For the following compounds provide their pkaʼs within 2 pka units. (5 pts) 1. DMS C 3 C 2 Et C 3 CC 2 C 2 Et 4. C 3 C=C 3 5. C 3 C 2 2. Explain, describe, or illustrate each of the following chemical processes. (15 pts) 1. dynamic kinetic resolution a type crotylation a catalytic asymmetric desymmetrization 4. a reaction that proceeds with double diastereodifferentiation 5. organocatalysis. Provide the product or products of the reactions outlined below. Show all intermediate compounds and be sure to indicate the product's relative or absolute stereochemistry. For reactions where multiple products are possible, be sure to indicate the major and minor species. (30 pts) LDA, TF, -78 C; then LDA, TF, -78 C; then allyl bromide 2 S 4 Bu A 1

2 1. TMS, Et 3, DMF 2 C C 2, heat Ti 4, C, C % aq. B (hint: the last step eliminates water) Piv TBS 1. DBAL DCC, DMS, 3 P 4 TBS C Et 2, i-pr, B(C 6 F 5 ) 3 (hint: this is T a [4+2] cyclization) 4. C 5 11 C 2 Et 1. S, KtBu (cat), Et, rt, 5 min oxone, C 2 2, 5 min () 3 P,,! D (hint: the first step opens the cyclopropyl ring) 5. C 2 Et 1. Baker's yeast reduction DP, PPTS LiAl 4 4. Ts, pyr. 5. a, acetone E t-bul, TF; then TMS S S then MPA; then Bn F g(ac)

3 benzoic acid 1.DCC, () Et 3, C 2 2, rt MgBr; then aq. B 3, TF, rt G 5 mol % B PCC, C 2 2 Pb(Ac) 4, C 6 12, CaC 3,! 3 P=C 2 4. Se 2, t-bu 9. Bu 1. ab 4 Ms, pyr. super hydride 4. 9-BB, TF, 0 C; then K, TP tbu 1. L-Selectride; then Tf 2 3 SnSn 3, cat. Pd(P 3 ) 4, Li C, cat. Pd 2 (dba 3 ), cat. 3 As,, MP J V. Provide conditions that will effect the transformations outlined below. Some of these conversions will require more than one reaction, so be sure to show all intermediate compounds. (30 pts) 1. Ac PMB PMB 3

4 Bn TBS 4. C C TBS 5. C 2 Bn Bz C 2 Bn 6. C 3 C 2 C 2 7. Ac TBS Bn Ac TBS 8. C C Et 2 Et 2 V. Provide the starting substrate for the reactions outlined below. (18 pts) 4

5 1. K Boc Sm 2, TF Boc L 1. n-bu 2 BTf, Et 3, C 2 2, 0 C TES -78 C; then 2 2 TES 4. Al 3, () M 1. LDA; then (ipr) 2 n-buli, ( )-sparteine; then acetone excess PMB, n-bu 4 (cat.) a, DMF, 22 C, 8 h Bu 3 Sn, TF, ( 3 P) 2 Pd 2 (C) 2 Pd 2, DMF, 22 C PMB PMB C 2 SEM 2 C SEM P=CC 2, 150 C 2, Pd/C 4. LiAl 4, TF C 2 6. C 2 Et P 1. LDA; then BrC 2 C 2 C 2 C 2 Et a, TF, eat 5

6 V. Provide complete arrow (electron) pushing mechanisms for the transformations shown. (12 pts) 1. LDA, Ac 2 mcpba 1. a, CS 2, C 3 1. mcpba Ts xylene,! mcpba csa V. Provide complete arrow (electron) pushing mechanisms for the transformations shown. (6 pts) i-pr C ( 3 P) 4 Pd C 2 Cs 2 C 3 2 V. Provide a complete arrow (electron) pushing mechanism for the transformation shown. (6 pts) C Et, + C 2 X. For the reaction shown below explain the stereochemical outcome. (6 pts) Et Ts LDA Et + Et 98 : 2 X. n class we saw examples of MacMillanʼs organocatalytic reactions with imidazolidinone catalysts (e.g. 1). Among his clever uses for this catalyst is his development of a merged iminium-enamine cascade. An example of this process is shown below. 6

7 C + 20 mol % 1 EtAc, TFA C n this reaction scheme, the enal reacts with 1 to generate an activated iminium species that can enantioselectively intercept a wide variety of nucleophiles (e.g. 2-methylfuran). The conjugate addition adduct is an enamine, which in turn can react with a wide array of electrophiles (e.g. the chlorinated quinone). Subsequent hydrolysis affords the product and regenerates the catalyst. llustrate this catalytic cycle and show the mechanistic steps involving the nucleophile addition and electrophile capture. (ote: You do not have to explain the stereochemistry.) (10 pts) X. Provide a stereoselective (relative stereochemistry only) synthesis of E of the compounds shown below, beginning with the indicated starting material (12 pts). Q S 2 R 2 S C Bonus Question: Which compound was found in the paint of all red Ferrariʼs built from ? a. b. c. S 2 2 X 2 Et 2 d. e. C 2 3 C C 3 C 3 7