Problem session (3) Daiki Kuwana. Please fill in the blank and explain reaction mechanisms and stereoselectivities.
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1 Problem session (3) Daiki Kuwana Please fill in the blank and explain reaction mechanisms and stereoselectivities (Ac) 2 (10 mol%), DPEphos (20 mol%) Et 3, toluene, 90 C 2. s 4 (14 mol%), M; ai 4 acetone/ 2 (3/1), rt 3. m-cpba, ac 3, C 2 Cl 2, rt 4. Li(TMS) 2, -78 to 0 C; Me, -78 C to rt; Stryker's reagent (0.4 eq.), rt, 28% (4 steps) 1-2 (tricyclic compound) 5. TMS, Et 3, C 2 Cl 2, rt 6. p-absa, DBU, C 3 C, 95% (2 steps) W g-lamp, Me, Et 3, 83% R a C 2 Me 1-3 (R = TMS) a. Stereochemistry of this position was not determined K(TMS) 2, 2-2, TF, 0 C to rt; 2. m-cpba, C 2 Cl 2, rt + 3. Cu (12.5 mol%) pyridine/me(10/ 1), reflux C 2 Me C 2 Me % (3 steps) 18% (3 steps) 3. (i) Cl 2, n-buli, TF, -78 C; 3-1, -78 C to rt; 3-2, LDA, -78 C; -C CLi (3.0 eq), -78 C; Me, ac 3 aq. rt, 82% n-c (ii) Me Cl 2, n-buli, TF, -78 C; 3-4, -78 C to rt; 3-2, LDA, -78 C; -C CLi (3.0 eq), -78 to -55 C; Me, ac 3 aq. rt, 77% Me Me Me n-c 6 13 P 2 P 2 DPEphos + - M [( 3 P) 3 Cu] 6 Stryker's reagent p-absa S 3 Tf I 2-2 Br n-c 6 13 Br 3-2
2 Problem Session (3) - Answer Topic: trans-fused 5,5-ring X Daiki Kuwana Y 0. Introduction 0-1. atural products including trans-fused 5,5-ring 2 Cl 2 2 Palau'amine (+)-epoxydictymene 0-2. Difficulty of formation of trans-fused 5,5-ring kinetics u or E funebrene ( 3,4 ) -funebrene ( 3,15 ) (-)-presilphiperfolan-8-ol (Problem 1) 0-1 concave R cis-fused thermodynamics (kcal/mol) trans-fused 5,5-ring is less stable than cis-fused both kinetically and thermodynamically Methods to construct trans-fused 5,5-ring Connection of two alkyl chains Richard,., B. et al. J. Am. Chem. Soc. 1970, 92, P, imid, I 2, 81% 2. t-buli, 74% 0-2 C C 0-3 Schreiber, S., L. et al. J. Am. Chem. Soc. 1994, 116, Ring contraction (Problem 1) Zirconacyclopentane (Problem 3) Generation as byproduct (Problem 2) 0-4 MeAlCl 2 Me 2 AlCl 28% 1. LiAl 4 2. ArSeC, Bu 3 P, :0-7 = 2: Shenvi, R., Y. et al. ature Chem. 2012, 4, 915.
3 1. Total synthesis of (-)-presilphiperfolan-8-ol 1-1. Brief introduction Isolation Bohlmann F. et al. ytochemistry 1981, 20, X-ray structure determination Wilson, S. et al. J. Am. Chem. Soc. 1996, 118, (-)-presilphiperfolan-8-ol (1-0) 1-2. Synthetic route Structural features 5 stereocenters trans-fused 5,5-ring 3 steps 4 steps 3 steps (+)-pulegone alkylation 1-1 eck cyclization 1-2 ring contraction R 3 steps C 2 Me removal of ester 1-3 (R = TMS) 1-3. Answer 1-0 (13 total steps) (Ac) 2 (10 mol%), DPEphos (20 mol%) Et 3, toluene, 90 C 2. s 4 (14 mol%), M; ai 4 acetone/ 2 (3/1), rt 3. m-cpba, ac 3, C 2 Cl 2, rt 4. Li(TMS) 2, -78 to 0 C; Me, -78 C to rt; Stryker's reagent (0.4 eq), rt, 28% (4 steps) TMS, Et 3, C 2 Cl 2, rt 6. p-absa, DBU, C 3 C, 95% (2 steps) W g-lamp, Me, Et 3 83% R a C 2 Me P 2 P 2 DPEphos p-absa S (R = TMS) a. Stereochemistry of this position was not mentioned. [( 3 P) 3 Cu] 6 Stryker's reagent Answer (Ac) 2 Et 3 DPEphos (= L 2 ) Ac 0 Ac L 2 L L - Ac A 1-B 2
4 Tf 0 L L 2 Tf L step 1 eck-type tandem cyclization 5-exo Discussion exo no hydride to eliminate 1-5 L 2 VI s s VI - s 1-8 VI s M 1-9 s 4 I a ai 4 - C Discussion 2 - a 2 I I 1-11 step a Me m-cpba [( 3 P) 3 Cu] 6 step 3 Li(TMS) 2 L Cu
5 TMS TMS Et 3 TMS step R R S 2 Ar p-absa neopentyl 1-19 (R = TMS) step R 1-21 S 2 Ar R 1-22 S 2 Ar DBU 2 -S 2 Ar R 1-23 R R h R 1-24 step Me R R Wolff rearrangement C 2 Me
6 1-4. Discussion Stereoselectivity of eck-type cyclization (step 1) Tf 1-4 L2 L 2 L2 Tf 1-4a 1-4b 1-5 L2 1-5' L Stereoselectivity of epoxydation (step 3) Cl a a equatrial 1-12 b R b axial
7 2. Attempted synthesis of isocapnellenone using trimethylenemethane isocapnellenone (2-0) 2-1. Synthetic plan capnellene coriollin linear triquinane skeleton for total synthesis of coriollin : _PS_Yuki_AKAGAWA [3+2] cycloaddition I Answer KMDS, 2-2, TF, 0 C to rt 2. m-cpba, C 2 Cl 2, rt + 3. Cu (12.5 mol%) pyridine/me(10/ 1), reflux C 2 Me C 2 Me % (3 steps) 18% (3 steps) Tf I K(TMS) 2 I Tf I
8 6-endo exo step m-cpba 2-15 m-cpba cis-fused > trans-fused step 2 Me + Me + - Me 2 C= - C / / /2-4 C 2 Me step 3 Cu accelerates solvolysis. Methoxide anion is generated? Cu + n Me + 4 py. Cu(py) 4 (Me) n + n/2 2 7
9 2-3. Discussion: selectivity of [3+2] addition Regioselectivity a b ' 2-6'' from a : 5-exo or 6-endo vs from b : 7-exo or 8-endo Cyclization from a is preferred. neopentyl 3 radical Stereoselectivity endo 5-exo 6-endo 5-exo 2-12' 2-14-cis endo cyclization 1,3-allylic strain '
10 far exo cyclization ' 2-14'' exo vs 6-endo d c a Distance: a-c > a-d 2-6-2' d c a Distance: a-c < a-d There is steric hinderance between Me group and cyclopentene when Ca and Cc come close
11 3. formation of trans-fused 5,5-ring via zirconacyclopentane 3-1. Answer 3. (i) Cl 2, n-buli, TF, -78 C; 3-1, -78 C to rt; 3-2, LDA, -78 C; -C CLi (3.0 eq), -78 C; Me, ac 3 aq. rt, 82% n-c (ii) Me Cl 2, n-buli, TF, -78 C; 3-4, -78 C to rt; 3-2, LDA, -78 C; -C CLi (3.0 eq), -78 to -55 C; Me, ac 3 aq. rt, 77% Me Me Me n-c 6 13 n-c Br Br (i) Cl n-buli - 2 LiCl - 3-B 3-A 3-B (egishi reagent) LDA Br Li n-c 6 13 Br 3-C Li Br Br n-c LiBr n-c 6 13 Br n-c 6 13 Br n-c 6 13 Li formation of ate complex n-c 6 13 Zr Cp n-c 6 13
12 -addition from convex face 3-15 n-c 6 13 quench 3-3 n-c 6 13 (ii) Me 3-B Me Me Me Me Me Me Me Br Li n-c 6 13 Br 3-C Me Me Br Br n-c 6 13 Me Me Li n-c 6 13 Br Me Me Li Me 3-21 n-c 6 Br 13 - LiBr Me n-c 6 13 formation of ate complex 3-22 Me Me Me Me n-c 6 13 Me n-c 6 13 Me n-c quench Me Me 3-5 n-c
13 3-2. Discussion Selectivities of -addition t-buli Me/ 2-78 C t-bu ( -addition) n-buli -78 C t-bu 3-28 (88%) n-bu 3-29 ( -addition) 20 C 1,2-shift n-bu Me/ 2 n-bu (73%) -selective addition occurs when bulky nucleophile is used Selectivity of formation of zirconacyclopentane Proposal 1 Me Me B Me Me -Zr d orbital of Zr back donation Location of Zr is fixed by two C=C bonds. akamura, A. et al. Polyhedron 1991, 10, Zr Cp 2 too close Me Me Me Me 3-18'
14 Proposal 2 syn-pentane interaction Me Me Me Me Me Me 3-18' Me Me Appendix: metal vinylidenes vs metal alkynes Metal vinylidenes seem to more stable than metal alkynes. Wakatsuki, Y. J. rganomet. Chem. 2004, 689, There is back-donation from d-orbital of metal to empty p-orbital of carbene. Beckhaus, R. Angew. Chem. Int. Ed. 1997, 36,
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