Name 215 F07-Exam No. 3 Page 2 I. (29 points) Streptomycetes are soil-dwelling, filamentous, Gram-positive saprophytic bacteria. They are responsible for over 50% of the known microbial metabolites, including many antibiotics used in human and veterinary medicine. In an article published earlier this year, a group of scientists from Czech Republic reported the isolation of a number of extracellular carbohydrate metabolites from Streptomyces coelicolor A3(2) [J. Nat. Prod. 2007, 70, 768]. The structures of some of these carbohydrates are shown below. 2 C 2 C C 2 1 C 2 2 3 C 2 Answer the following questions about the structures of these carbohydrates 1 3 and their hydrolysis products. (1) For each of these three carbohydrates, answer if it is a reducing sugar. Circle yes if it is a reducing sugar and, no if not a reducing sugar. For 1: For 2: For 3: yes; no 2 yes; no yes; no 2 2 (2) ow many of the sugar units of trisaccharide 1 are D sugars? Circle one that applies. 0 1 2 3 3 () Is the glycosidic bond in glycoside 2! or "? Circle one that applies.! " 3 (6) Draw in the box below a Fischer projection formula for the carbohydrate product, obtainable upon acid hydrolysis of glycoside 2, in an open-chain form. (3) Is the sugar unit of glycoside 2 D sugar? Circle one that applies. yes; no 3 (5) Is the stereochemistry of the anomeric in furanose 3! or "? Circle one that applies.! " 3 (7) Draw in the box below a Fischer projection formula for furanose 3 in an open-chain form. 6 5
Name 215 F07-Exam No. 3 Page 3 II. (17 points) Cyclopentanoid 5 shown below has been proposed to be biosynthesized from the C- oxidized D-glucose via the ene-diol intermediate under acid-catalyzed conditions (Tetrahedron Lett. 2002, 3, 51). Provide in the small box below a structure for the ene-diol and propose in the box below a step-by-step, curved-arrow reaction mechanism for this transformation from to 5. Use + B- and B: for the acid catalyst and its conjugate base, respectively. You do not need to explain the stereochemistry at the newly created chiral centers in 5. acid catalyst 5 B "ene-diol" intermediate 3 5 III. (8 points) Provide in the boxes below the structures of the reaction products. anomeric mixture 3 C 3 C 3 C C 3 C 3 C 1 20 9 p-ts or BF 3 (C 2 C 3 ) 2 (catalytic) PhS 1. NaC 3 (catalytic) 3 C 2. Na (excess) PhC 2 Br!-anomer; C 18 22 7 S (excess) TF + 3 CC(=)
Name 215 F07-Exam No. 3 Page IV (8 points) Treatment of keto-diol 6 with p-ts results in the highly diastereoselective formation of spiroketal 7 (Angew. Chem. Int. Ed. 2007, 6, 766). Draw the stereostructure of the major product 7 by completing the remaining structure in the box. 3 C 3 C p-ts (catalytic) benzene 3 C 3 C 6 7 + 2 3 C 3 C 7 5 The term used to rationalize the fact that the above stereoisomer is the more stable diastereomer is: 3 V. (16 points) Methyl α-d-glucofuranoside can readily be converted into methyl α-d-glucopyranoside under acidic conditions. Draw in the box below a step-by-step, curved-arrow mechanism for this reaction. Use + B and B: for the acid catalyst and its conjugate base, respectively. C 2 B C 3 methyl!-d-glucofuranoside C 2 C 3 methyl!-d-glucopyranoside 16
Name 215 F07-Exam No. 3 Page 5 VI (1 points) Draw in the box below a step-by-step, curved-arrow mechanism for the reaction shown below including the aqueous acid workup step (Synthesis 2007, 3185). ( - C 2 C 3 ) and its conjugate acid, respectively. Use B - and B- for the base 3 CC 2 N C 2 C 3 1.NaC 2 C 3 (1 mol equiv) C 2 C 3 2. 3 + workup (protonation) 3 CC 2 N + C 2 C 3 3 CC 2 B N C 2 C 3 3 CC 2 N 3 + workup 1 VII. (12 points) Complete the following reaction schemes by providing in the boxes the structures of the corresponding products for each of the following reaction schemes. (1) [Synthesis 2007, 3185] 3 CC 2 N 3 +! 1. LiAl (excess) TF 2. aqueous workup C 7 11 N 2+ C2 C 3 + C 2 C 7 15 N (2) [Tetrahedron Lett. 2007, 8, 761] Li N C Si(C 2 C 3 ) 3 3 3 + workup + TF C 3 + 2 N + (C 3 )C 3
Name 215 F07-Exam No. 3 Page 6 VIII. (16 points) Complete the following reaction schemes by providing in the boxes the structures of the corresponding products. (1) 3 C C 3 C 3 1. NaC 3 (1 mol equiv) benzene 2. Cl/ 3 + (protonation) + C 3 3 +! + C 3 + C 2 (2) [see: rg. Lett. 2007, 9, 1879] lithium enolate Cu Li TF -78 C + Cu(C 2 C 2 C 2 C 3 ) + enantiomer BrC 2 Ph + LiBr + enantiomer