Protecting Groups. Tactical Considerations
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1 Tactical Considerations Cheap & commercially available Easy & efficient introduction Should not create any stereogenic center Stable throughout reaction, work-up & purification Efficient removal By-products of the removal should be easily separated CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 1
2 1. Methyl ethers: Protecting Groups Ethers R H R Me Difficult to remove except for phenols CH 2 N 2 NaH, MeI, THF BBr 3, CH 2 Cl 2 PhSe - Ph 2 P - Me 3 SiI CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 2
3 Ethers 2. Methoxymethyl ethers (MM): R H R CH 2 Me Stable to base and mild acid MeCH 2 Cl, NaH, THF MeCH 2 Cl, CH 2 Cl 2, i-pr 2 EtN Me 2 BBr HCl/ THF, reflux CH 3 CCl, MeH CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 3
4 Ethers 3. Methoxyethoxymethyl ethers (MEM): R H R CH 2 CH 2 CH 2 Me Stable to base and mild acid MeCH 2 CH 2 CH 2 Cl, NaH, THF MeCH 2 CH 2 CH 2 Cl, CH 2 Cl 2, i-pr 2 EtN Lewis acids such as ZnBr 2, TiCl 4, Me 2 BBr 2 CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 4
5 Ethers 4. Benzyloxymethyl ethers (BM): R H R CH 2 CH 2 Ph Stable to base and mild acid PhCH 2 CH 2 Cl, CH 2 Cl 2, i-pr 2 EtN H 2, Pt 2 Na/ NH 3, EtH CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 5
6 Ethers 5. Tetrahydropyranyl ethers (THP): R H H +, PhH R Stable to base DHP, PPTS, CH 2 Cl 2 PPTS, EtH Amberlyst H-15, MeH Creates one more stereogenic center CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 6
7 6. Benzyl ethers (Bn): Protecting Groups Ethers R H R CH 2 Ph NaH, BnBr, THF/ DMF/ DME KH, BnCl, THF BnC(=NH)CCl 3, CF 3 S 3 H H 2 / Pt 2 Li/ NH 3 H 2 / Pd-C CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 7
8 Ethers 7. p-methoxybenzyl ethers (PMB): R H Me Cl Me R KH, p-methoxybenzyl chloride, THF PMBC(=NH)CCl 3, CF 3 S 3 H DDQ CAN H 2 / Pd-C Li/ NH 3 R H PMB DDQ MS R PMP DIBAL-H PMB R H CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 8
9 8. Silyl ethers: Protecting Groups Ethers R H R SiR 3 R 3 SiCl, Pyridine, DMAP R 3 SiCl, Imidazole, CH 2 Cl 2 / DMF/ CH 3 CN, DMAP R 3 SiTf, i-pr 2 EtN, CH 2 Cl 2 Acid F - (KF, CsF, HF, Py, n-bu 4 NF) CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 9
10 Ethers 8. Silyl ethers: 8.1. Trimethylsilyl ethers (TMS): Properties: Acid and water labile Useful for transient protection CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 10
11 Ethers 8. Silyl ethers: 8.2. Triethylsilyl ethers (TES): Properties: Considerably more stable than TMS Can be selectively removed in presence of more robust silyl ethers with F - or mild acids CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 11
12 Ethers 8. Silyl ethers: 8.3. Triisopropylsilyl ethers (TIPS): Properties: More stable to hydrolysis than TMS CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 12
13 Ethers 8. Silyl ethers: 8.4. t-butyldimethylsilyl ethers (TBS/ TBDMS): Properties: Stable to bases and mild acids Under controlled condition it is selective for primary alcohols t-butyldimethylsilyl triflate, base t-butyldimethylsilyl chloride, base H TES H 2 / AcH/ THF TBS TBS CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 13
14 Ethers 8. Silyl ethers: 8.5. t-butyldiphenylsilyl ethers (TBDPS): Properties: Stable to bases and mild acids Selective for primary alcohols Me 3 Si & i-pr 3 Si groups can be selectively removed in presence of TBS or TBDPS TBS group can be selectively removed in presence of TBDPS by acid hydrolysis AcH/ H 2 THF TBDPS TBS TBDPS H CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 14
15 Ethers 8. Silyl ethers: 8.5. t-butyldiphenylsilyl ethers (TBDPS): F -, n-bu 4 F HF/ H 2 / CH 3 CN HF.pyridine SiF 4. CH 2 Cl 2 CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 15
16 Ethers 9. o-nitrobenzyl ethers: R H NaH, THF R Cl N 2 N 2 o-nitrobenzyl chloride, NaH, THF Photolysis at 320 nm CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 16
17 Ethers 10. p-nitrobenzyl ethers: R H NaH, THF R 2 N Cl 2 N p-nitrobenzyl chloride, NaH, THF Selective removal with DDQ Hydrogenolysis Electrochemically CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 17
18 Ethers 11. Trityl ethers (Tr= CPh 3 ): R H R CPh 3 Selective for primary alcohols, stable to base Ph 3 C-Cl, pyridine, DMAP Ph 3 C + BF 4 - Mild acid CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 18
19 Esters R H R R' Activated acid, base, solvent Chem. Soc. Rev. 1983, 12, 129 Angew. Chem. Int. Ed. Engl. 1978, 17, 569 CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 19
20 1. Via acid chlorides: Protecting Groups Esters Cl R H R' H R' Cl R' N R' R N SCl 2 PCl 5 (CCl) 2 CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 20
21 2. Acetates (Ac): Protecting Groups Esters R H R Ac Properties: Stable to acid and mild base Not compatible with strong nucleophiles such as organometallic reagents Ac 2, pyridine Acetyl chloride, pyridine K 2 C 3, MeH, reflux KCN, EtH, reflux NH 3, MeH LiH, THF, H 2 Enzyme hydrolysis (Lipase) CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 21
22 Esters 3. Pivaloates (Piv): Cl R H R Selective for primary alcohols t-butylacetyl chloride t-butylacetic anhydride Mild base CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 22
23 Protection of 1,2 & 1,3-diols H H R 1 R 2 R 3 R 4 Acid R 3 R 4 R 1 R 2 1. Isopropylidenes (acetonides): H H Acid R 1 R 2 Acetone or Me or Me Me R 1 R 2 1,2-acetonide formation is usually favored over 1,3-acetonides Mild aqueous acid CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 23
24 Protection of 1,2 & 1,3-diols 2. Cycloalkylidene acetals: H H R 1 R 2 Acid R 1 R 2 or R 1 R 2 or Cyclopentylidenes are slightly easier to cleave than acetonides Cyclohexylidenes are slightly harder to cleave than acetonides CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 24
25 Protection of 1,2 & 1,3-diols 3. Benzylidene acetals: Ph H H Acid R 1 R 2 CH(Me) 2 CH R 1 R 2 or 1,3-Benzylidene formation is usually favored over 1,2- Benzylidene Benzylidenes are usually hydrogenolyed slower than benzyl ethers or olefins Acid hydrolysis or hydrogenolysis CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 25
26 Protection of 1,2 & 1,3-diols 4. p-methoxybenzylidene acetals: Me H H Acid R 1 R 2 CH(Me) 2 CH or R 1 R 2 Me Me Hydrolyzed about 10 times faster than regular benzylidenes Can be oxidatively removed with CAN CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 26
27 Protection of 1,2 & 1,3-diols 5. Carbonates: H H Im 2 C R 1 R 2 R 1 R 2 Stable to acid & more difficult to hydrolyze than esters Im 2 C or phosgene or triphosgene Removed with base CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 27
28 Protection of ketones and aldehydes MeH, H + R1 R 1 R 2 R 2 Me Me (CH 2 H) 2, H + R 1 R 2 1,3-dioxolanes H + R 1 R 2 1,3-dioxanes H H Ketones and aldehydes are generally protected as cyclic and acyclic ketals and acetals Stable to base CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 28
29 Protection of ketones and aldehydes Cleavage rate of substituted 1,3-dioxanes R 1 R 2 R 1 R 2 R 1 R 2 Ketal formation of α,β-unsaturated carbonyls are usually slower than for the saturated case (CH 2 H) 2 H + CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 29
30 Protection of ketones and aldehydes 1. Fluoride cleavable ketal: LiBF 4 Me 3 Si 2. Base cleavable ketal: H S 2 Ph H R 1 R 2 ptsa R 1 R 2 S 2 Ph DBU, DCM R 1 R 2 CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 30
31 3. 1,3-Dithiane derivative: Protecting Groups Protection of ketones and aldehydes HS SH S S R 1 R 2 R 1 R 2 or S S R 1 R 2 Aldehydes are selectively protected in presence of ketones HS(CH 2 ) n SH, BF 3.Et 2, DCM, 25 o C In α,β-unsaturated ketones, double bond does not migrate to the β,γ-positions S S CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 31
32 Protection of ketones and aldehydes 3. 1,3-Dithiane derivative: 1,3-dioxolanes and 1,3-dioxanes can be readily converted into 1,3-dithiolanes and 1,3-dithianes SH SH S S BF 3.Et 2 Hg(Cl) 4, MeH, CHCl 3, 25 o C NBS, acetone, 0 o C I 2, DMS m-cpba, Ac 2 CAN, aq. CH 3 CN DDQ, aq. CH 3 CN CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 32
33 1. Alkyl esters: Protecting Groups Protection of carboxylic acids Esters Fischer esterification (RCH + R H + H + ) Acid chloride + RH, pyridine t-butyl esters: Isobutylene & acid Methyl esters: Diazomethane LiH, THF, H 2 Enzyme hydrolysis t-butyl esters are cleaved with aq. acid Bu 2 Sn, PhH, reflux CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 33
34 Protection of carboxylic acids Esters 2. 9-Fluorenylmethyl esters (Fm): RCH DCC H R CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 34
35 1. t-butyl carbamate (Boc): Protecting Groups Protection of amines Carbamates R NH 2 R NHBoc (Boc) 2, NaH, H 2, 25 o C (Boc) 2, TEA, MeH/ DMF BocN 3, DMS 3M HCl, EtAc TFA, PhSH, DCM AcCl, MeH CAN, CH 3 CN CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 35
36 2. Allyl carbamate (Alloc): Protecting Groups Protection of amines Carbamates R NH 2 R NHAlloc CH 2 =CHCH 2 CCl, py (CH 2 =CHCH 2 C) 2, DCM Pd(Ph 3 P) 4, TBTH, AcH Pd(Ph 3 P) 4, Dimedone, THF CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 36
37 Protection of amines Carbamates 3. Benzyl carbamate (Cbz or Z): R NH 2 R NHCbz BnCCl, Na 2 C 3, H 2 (BnC) 2, dioxane, H 2 H 2 / Pd-C H 2 / Pd-C, NH 3 Pd-C, HCNH 4 BBr 3, DCM KH, MeH CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 37
38 Protection of amines Carbamates 4. 9-Fluorenylmethyl carbamate (Fmoc): R NH 2 R NHFmoc Fmoc-Cl, NaHC 3, aq. Dioxane Fmoc-C 6 F 5, NaHC 3, acetone Amine bases Piperidine, morpholine, diisopropylethyl amine TBAF, DMF CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 38
39 Protection of amines Carbamates 5. 2,2,2-Trichloroethyl carbamate (Troc): R NH 2 R NHTroc Cl 3 CCH 2 CCl, Py or aq. NaH Zn, THF, H 2, ph= 4.2 Zn-Pb couple, 4:1 THF/ 1M NH 4 Ac CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 39
40 Protection of amines Carbamates 6. 2-Trimethylsilylethyl carbamate (Teoc): TMSCH 2 CH 2 CCl (Teo-Cl) or Teoc-N3 Teoc-C 6 H 4-4-N 2, NaH Teoc-Su, TEA CH- 423 Course on rganic Synthesis; Course Instructor: Krishna P. Kaliappan 40
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