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Spring 07 Exam 6 Key - CHEMISTRY 22S TEST#6 i(25 A t 2(22...

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Unformatted text preview: CHEMISTRY 22S TEST #6 JUNE 22, 2007 i. __ (25) A t 2. (22) NAME Wfivw e v y 3. (16) 4. (.12) TOTAL (75) G O 0 D L U C K I 1. (25 Points) ORGANIC REACTIONS. a. ( f 16) Compiete FOUR of the following organic reactions by providing structures for the reactant, product, or reagent as required. Be sure to show stereochemistry andfor regiochemistry in cases where such characteristics are important. If no reaction is expected under the reaction conditions, write “N. R.” and explain briefly why the reaction fails. .If you complete ail five equations, pfease indicate clearly whrchfour you with to have graded. Br\ KCH3 i .\ 7/" 9 / A" i (cream + t V we. a / EL —\ 1' HQ(OAC)2, H20 <7)i3H2C3H=CH2 ....__._....._.__* i—ii 2. NaBH4 f iiiii fl PdrC \ \ \ (y /-----CH;CH2 + D2(excess) -—* \— HQ Br {I i B r I“... fl .. .. i N I?" H C C "'H or; i 20 / \ I H3 C C H3 Gui: extingu- CchHQCH=CHCHQCHQCH3 flag), b. ( f 9) Write complete equations (including reactant, reagent, and product) for reactions satisfying the following criteria. Be sure to show the stereochemistry of the reactant andror product cleariy Your reaction sequences may involve more than one reagent or step. A particular reaction type (eg. hydroboration) may only be used once, even though it may satisfy more than one criterion. 0 (3) A syn addition to a rrans— double bond. NV C/Pz' COKE ,, /~\\| /@ ._ a “a \ o (3) An addition reaction with a tertiary carbocation intermediate. an; .>__._., w w, ,_ lira. o (3) An addition reaction that produces a meso product. DJ 2. (22 Points) REACTION MECHANISMS. a. ( .I' 5) Recent experimental results _ from Villanova University suggest that what BI IE” was thought to be a cyclic bromonium ion exists "CPS? . , §__é _ instead as a pair of rapidly equilibrating / \ f— / \" carbocations as shown to the right [Organic Lea. 2007, 9, 2317]. o (3) Write structures for the two equilibrating cations that might be formed in the reaction below. 0 (2) Write a structure for the bromonium ion that would be formed from the same alkene. at; ! l.-..\ l / l dig/CH3 Br2 ‘ I ® _ __,_ pg. it — * . r—" l 9 ‘H-.,___/" ~..CH3 : (a? . l or? b. ( __ f 5) Japanese workers have reported the following cyclization reaction [Tetrahedron Len; 2007, 48, 3259]. In the original paper the transformation was brought about by Lewis acid catalysis, but the same result could be achieved using a strong mineral acid. Write a detailed, stepwise reaction mechanism to account for the formation of the product. A complete reaction mechanism will include structures for all intermediates and curved mechanistic arrows to represent the “flow" of an electron pair. HINT: The bond formed in the cyclizatiori process has been indicated for you Ph \ + Pha. H H OW ___,. Al/ .0 ”—7 / . Ph Ph l 77? Hwy H+ A gyp\ HG/YEK\*+ "*9 alt/[gage FL ifl c. ( ____ / 4) BONUS QUESTION! Chemists in Italy and the Netherlands jointly discovered a mechanistically similar cyclization [Organic Left 2007,. 9, 2365]. Complete the “minimechanism” below with structures for the missing intermediates. LJ.) 4 d. ( _ i’ 4) A betaine is a reactive organic compound with positive and negative charges separated by two carbon atoms. An example is shown below (compound A). Canadian chemists have found that compound A undergoes intramolecular cyclization by an Syd—like mechanism to form an epoxide [Organnz Len. 2007., 9, 2397]. -- ——» H 'S' H/C ct} Cu—H'C \(‘F + (C 3)2 Ph SET3H .4*““ H30 ..'+- 3 ii p 1P\/ ‘ 4' compound A epoxide product 0 (3) Complete the equation with a structure ofthe cyclized product. 0 (1) Add mechanistic arrows to show how the cyclized product is formed. e. ( _____ _i’ 8) Secondary alkyl halides react by a variety of reaction mechanism according to the reagents and the reaction conditions. For each of the following reactions identify the mechanism by which the reaction is occurring. o (2) 2-Bromopentane reacts with the conjugate base of l-methyl-2-propanol (icrt-butyl alcohol) to form l—pentene as the major product. EL 0 (2) Upon treatment with potassium iodide in acetone ci.r~2~methyl-l- 4 Z bromocyclopentane is converted to trans—Z—methyl-l—iodocyclopentane. N 0 When warmed in ethanol (25‘,31€)—2-brom0-3- (7 ) methylpentane is slowly converted to a pair of diastereomeric ethers. N o (2) One of the products formed when iodocyclopentane h is heated in aqueous acetone has the molecular formula CgHg, _H_LH3 i (16 Points) CARBOCATIONS AND REAGENTS IN THE NEWS! a. ( . f 9) The following transformations have all appeared in recent articles in chemical journals. For each reaction suggest a reagent, or combination of reagents, that would accomplish the conversion. {ff \ lf )1 \\ '\ ,2"; \'- ff; —/“' -- o / to or; o 1 Q l ' \_.—-'r, ""‘x '_/f .z' \. ”xx.“ "K M,/‘ R l 1/ l OR on OH Tetrahedron Lett. 2007, 48, 2871 LJ" Br Br 0 O. > > O O 0’"- Tetrahedron Lett. 2007, 48, 3047 II ______ 1+}: [P= protecting group, unaffected by reage nt(s)] I stereoisomeric product b. ( _ t 2) BONUS QUESTION? The reaction directly above was part of the muscarine iodide synthesis [Tetrahedron Lett. 200?, 48, 2971, Test #4, Test #5]. Provide a structure for a stereoisomer of the product that could have been formed in the reaction. c. ( I 3’) The tricyclic carbocation B below has been identified as an intermediate in the enzymatic cyclization of oxidosqualene to the natural product arabidiol [Organic Lett. 2007, 9, 2183]. E'— z 0% 8N1 product - Lt .\ , f . CeHH HV'V RR“, carbocation B rearrangement _,—m_,_ rearranged carbocation o (2) Write the structure of the product that would be formed (after loss of a proton) when carbocation B reacts with water (the SN] product). 6 o (2) Write the structure ofthe product that would be formed if carbocation B loses a proton (the El product). 0 (3) Carhocation B to undergoes rearrangement by a hydride shift. Write the structure of the rearranged carbocation. 4. (12 Points) MULTIPLE CHOICE. As usual, pick out the correct answer and write its letter to the left of the question. FD Which of the following is NOT intermediate in the synthesis of an alcohol by hydroboration‘? A. Alkylborane B. Trialkylborane C. Boronate ester D. Sodium borohydride __ _ l; Account for the fact that electrophilic addition of HCl(g) to l-rnethylcyclohexene produces only l-chloro-1-methylcyclohexane. The reaction proceeds by way ofa bridged chloronium ion. The hydrogen adds so as to form the most stable carbocation. Chlorine is a large atom so it prefers to attach to the least congested carbon. The initially formed secondary carbocation rearranges to the more stable tertiary carbocation. boar D Which reagent would serve as an oxidizing agent towards a carbon—carbon double bond? A. H2, PdfC B. H30" C. BQHb' .D. Cll3C03H C Which statement comparing the El and 'E2 reactions is TRUE? A. The E1 reaction produces the more substituted double bond, the E2 reaction produces the less substituted double bond. B. The El reaction requires an eclipsed orientation of hydrogen and halogen; the E2 reaction requires an anti orientation of hydrogen and halogen. C. The E1 reaction occurs under neutral reaction conditions; the E2 reaction occurs under strongly basic reaction conditions. D. The E1 reaction is favored for tertiary alkyl halides, the E2 reaction is favored for primary alkyl halides. Q Which alkene reaction is reversible under acidic conditions? A. Hydration of the double bond B. Hydroboration C. Catalytic hydrogenation D. Addition of hydrogen halide j) The conversion to the right was an early 0 stage of the synthesis of butylcyloheptylprodigiosin [Organic Len. -—» 2007, 9, 1889, and Test #2]. What term describes the outcome of the reaction (with respect to the double bond)? A. cis addition B. syn addition C. trans addition D. anti addition ...
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