Classics in Tetrahedron Letters Jeremy Richter Baran Group Meeting: 9/24/03
The Plan Methodology Protecting Groups atural Products Syntheses
Methodology
xidation of Vicinal Diols R R' R'' 1. Cl 2, DMS, C 2 Cl 2, -78 o C 2. Et 3, RT R R' R'' Can selectively oxidize the secondary alcohol in the presence of the tertiary alcohol without cleavage. Corey. 1974, 287
cis-ydroxylation of alkenes Br C TsCl Py/C 2 Cl 2 Br 1. 5:1 TF:1 Cl 2. K 2 C 3, Me a C C Corey. 1982, 4217 Allows selectivity during bromohydrin formation
Indole Synthesis Br Li E E 1. MeLi 2. tbuli Li R R R Biselectrophiles used: a-chloroketones, a,b-epoxyketone (Li enolate), and enediones Also used 5-substituted anilines R is a general protecting group: best results with BC Yields: 50-90% Wender. 1981, 1475
An Interesting Reaction: B 1. ClDP 2. 2 2 /a acid or heat Cr 3 / 2 S 4 / 2 Mueller. 1976, 2925 Mueller. 1979, 1991
Alcohol xidations Collins Reagent 1 PCC 2 PDC 3 Collins. 1968, 3363 Corey. 1975, 2647 Corey. 1979, 399
Protecting Groups
,-Diallyl R 2 unig's Base Br R (Ph 3 P) 3 RhCl "Wilkinson's Catalyst" R 2 Acid/Base Stable and ucleophile Inert Ganem. 1981, 1483
MEM 1 and MTM 2 MEMCl unig's Base R ZnBr 2 or TiCl 4 R R 1. a 2. MTMI R S gcl 2 or Ag 3 1 Corey. 1976, 809 2 Corey. 1975, 3269
Protection for Alkynes Co 2 (C) 8 Fe 3+ (C) 3 Co Co(C) 3 Alkyne now protected from reagents which can affect alkenes or alkynes. icholas. 1971, 3475
ot ew, But Improved: Thioacetals R R' S PIFA S R R' Improved method of removal which is selective for thioacetals. Stork. 1989, 287
BB 1 R Protection: 1. Fisher Esterification 2. Mitsunobo Esterification 3. Jacobsen Asymmetric Epoxide pening with Carboxylic acids, catalyzed with Co(salen) 2 Deprotection: 1. ydrogenolysis/lactonization: 2, Pd/C then KtBu Protecting Capability: 1. Assumed similar to other esters 2. Can remove in presence of other esters 1 Ganem. 2000, 9523 2 Jacobsen. 1997, 773
atural Products Reported in Tetrahedron Letters
atural Products Br C 2 Me 3 C Johnstonol 3, 1972, 195 C 3 Cl Br Pseudopterane Metabolite, 36, 1991, 4661 Me 3 C C 3 Blazeispirol A 40, 1999, 329 C 3 C 3 C 3 Blepharocalyxin A 40, 1996, 7283 3 C C 3
atural Products Br Br Arabidopside A 44, 2003, 5553 Floreside 4 44, 2003, 1243 Brazilide A 43, 2002, 1731 Coleophomone A 41, 2000, 8705
Syntheses
(±)-Decaline Me Me Arata. 1973, 2355
(±)-Decaline + a 2 Br Me Br R Me R (Me) 2 S 2, a R= R=Me, 76% (2 steps) Br IrCl 4, Cl,Me 3 P, ipr, 84% "enbest catalyst" Me Me 9:1 axial:equitorial Proc. Chem. Soc. 1964, 361 rg. Synth. 1970, 13 Arata. 1973, 2355 Ac 2, Py R= R=Ac
(±)-Decaline Ac Me C 2 R R' Br Cu, Py 34% Me "Ullman Condensation" Me Me Me a, 2 R=Me, R'=Ac R=R'= ptsa, Ph D, 55% (2 steps) (±)-Decaline Me Arata. 1973, 2355 Me
(±)-Bakkenolide A Evans. 1973, 4691
(±)-Bakkenolide A 1. s 4, ai 4, t-bu/ 2, 93% 2. KtBu, tbu/et 2, 65% 3. 2, Pd/C, Et 1. Li 70% 2. PBr 3 3. Tos 75% Sa SMe S MeS a, TF, heat 60% S Evans. 1973, 4691 Tos SMe
(±)-Bakkenolide A Sigmatropic RAR Tos S Tos S SMe SMe S MeS S Evans. 1973, 4691 SMe
(±)-Bakkenolide A S MeS 1. g, gcl2, 75% 2. Se2 (±)Bakkenolide A Evans. 1973, 4691
Echinulin C 3 Kishi. 1971, 4665
Echinulin 2 2 ZnCl 2 Kishi. 1971, 4661
Echinulin 2 Et Br Et 1. LA 2. Ac 2 -DMS 3. Ph 3 P=C 2 Et2, C "Mannich" Kishi. 1971, 4665
Echinulin Me 2 C C 3 Me 2 C C 3 1. 0.1 a/dioxane 2. eat 3. Resolution C 3 Kishi. 1971, 4665
dl-cepharamine C 3 3 C Kitano. 1969, 1611
dl-cepharamine C 3 C 3 3 C 3 C I C C MVK, a C 3 3 C C 3 3 C aet C Kitano. 1969, 1611
dl-cepharamine C 3 C 3 3 C 3 C 1. Ethylene glycol 2. a, MeI 2 2-2 K C 3 C 3 Ac R R R 1. ydrolysis 2. Br 2, Ac Kitano. 1969, 1611 Freshly Fused aac in Ac R=Br R= R= R=Ac
dl-cepharamine C 3 C 3 Ac Ac methanol BF 3 3 C C 3 LA 3 C dl-cepharamine Kitano. 1969, 1611
(-)-Bilobalide tbu Corey. 1988, 3423. Racemic synthesis originally presented in J. Am. Chem. Soc. 1987, 7534
(-)-Bilobalide C 2 M DIBALCl 1 C 2 M M 2 C 88%, 99% de C 2 M M= 1. LDA 2. t-bu C 2 Ph 91% Co 2 M tbu C 2 M KMDS tbu 80% C 2 M C 2 M Corey. 1988, 3423. 1 Developed by Yamamoto. 1986, 4507
Co 2 M tbu (-)-Bilobalide B 3 -TF Co 2 M tbu Ph Ph C 2 M B 45% 10:1 desired isomer C 2 M Me tbu Me 1. 3, ac 3 DCM/Me 2. DMS 3. pts, Me 75% 1. LA, TF, 55 o C, 84% 2. i) xalyl Chloride, DMS ii) Et 3, -78 o C to -40 o C Me M 2 C tbu 3. 1 Cl/TF, 0 o C 4. PCC, 4A mol siv, DCM 62% M 2 C Me Corey. 1988, 3423. Me
(-)-Bilobalide R tbu tbu unig's Base 75% (3 steps) Me Me K MeS 2 Cl R=Me R= R=Cl 10eq 2 C 2 ac3, DCM, 95% Ac Ac tbu tbu 1. 0.5 Cl/ 2 /TF, 60 o C 2. Ac 2, DMAP, DCM, RT 89% Corey. 1988, 3423. Me Me
(-)-Bilobalide Ac Ac tbu 1. 0.5 Cl/Ac/TF, 80 o C Ac tbu 2. PCC, DCM, 96% R mcpba, BF 3 -Et 2 92% R' R=, R'=Me R=R'= trimethylsilane Tol, 300 o C, 90% R tbu R' s 4, Py, Et 2 Ac tbu Corey. 1988, 3423. Me 2 CCCl, unig's, 100% Bu 3 Sn, AIB, 55% 3 Cl, reflux, 36 hrs, 70% R=Ac, R'= R=Ac, R'=CC 2 C 3 R=Ac, R'= R=R'= (-)-Bilobalide
Conclusions