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Protective Groups – Silicon-Based Protection of the Hydroxyl Group Chem 115

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Protective Groups – Silicon-Based Protection of the Hydroxyl Group Chem 115 Myers Protective Groups – Silicon-Based Protection of the Hydroxyl Group Chem 115 General Reference: • In general, the stability of silyl ethers towards acidic media increases as indicated: TMS (1) < TES (64) < TBS (20,000) < TIPS (700,000) < TBDPS (5,000,000) Greene, T. W.; Wuts, P. G. M. Protective Groups In Organic Synthesis, 3rd4th ed. John Wiley & Sons: • In general, stability towards basic media increases in the following order: New Jersey,York, 1991 2007. TMS (1) < TES (10-100) < TBS ~ TBDPS (20,000) < TIPS (100,000) Important Silyl Ether Protective Groups: Greene, T. W.; Wuts, P. G. M. Protective Groups In Organic Synthesis, 3rd ed. John Wiley & Sons: New York, 1991. Half Life Half Life CH3 Et CH3 Silyl Ether (5% NaOH–95% MeOH) (1% HCl–MeOH, 25 °C) RO Si CH3 RO Si Et RO Si i-Pr n-C6H13OTMS 1 min 1 min CH3 Et CH3 n-C6H13OSi-i-Bu(CH3)2 2.5 min 1 min Trimethylsilyl (TMS) Triethylsilyl (TES) DimethylisopropylsilylIsopropyldimethylsilyl (IPDMS) n-C6H13OTBS Stable for 24 h 1 min n-C6H13OSiCH3Ph2 1 min 14 min Et CH3 Ph n-C6H13OTIPS Stable for 24 h 55 min RO Si i-Pr RO Si t-Bu RO Si t-Bu n-C6H13OTBDPS Stable for 24 h 225 min Et CH3 Ph Davies, J. S.; Higginbotham, L. C. L.; Tremeer, E. J.; Brown, C.; Treadgold, J. Diethylisopropylsilyl (DEIPS) t-Butyldimethylsilyl (TBS) t-Butyldiphenylsilyl (TBDPS) Chem. Soc., Perkin Trans . 1 1992, 3043. • A study comparing alkoxysilyl vs. trialkylsilyl groups has also been done: i-Pr R Half Life Half Life i-Pr R O Si i-Pr O + – t-Bu Silyl Ether Bu4N F (0.06 M, 6 equiv) HClO4 (0.01 M) RO Si i-Pr O Si t-Bu n-C12H25OTBS 140 h 1.4 h i-Pr R O Si i-Pr O i-Pr R n-C12H25OTBDPS 375 h > 200 h n-C H OSiPh (Oi-Pr) Triisopropylsilyl (TIPS) TetraisopropyldisiloxanylideneTetraisopropyldisilylene (TIPDS) (TIPDS)Di-t-butyldimethylsilyleneDi-t-butylsilylene (DTBS) (DTBS) 12 25 2 <0.03 h 0.7 h n-C12H25OSiPh2(Ot-Bu) 5.8 h 17.5 h n-C12H25OPh(OSiPh(t-Bu)(OCHt-Bu)(OCH3) 3) 22 h 200h General methods for the formation of silyl ethers: Gillard, J.W.; Fortin, R.; Morton, H. E.; Yoakim, C.; Quesnell, C. A.; Daignault, S.; Guindon, Y. J. Org. Chem. 1988, 53, 2602. R'3SiCl ROH ROSiR'3 • Silyl groups are typically deprotected with a source of fluoride ion. The Si–F bond stength is imidazole, DMF about 30 kcal/mol stronger than the Si–O bond. Fluoride sources: Corey, E. J.; Venkateswarlu, A. J. Am. Chem. Soc. 1972, 94, 6190. + – Tetrabutylammonium fluoride, Bu4N F (TBAF) Pyridine•(HF)x R'3SiOTf ROH ROSiR'3 Triethylamine trihydrofluoride, Et3N•3HF 2,6-lutidine,2,6 lutidine, CH2Cl2 Hydrofluoric acid Tris(dimethylamino)sulfonium difluorotrimethylsilicate (TASF) + – Corey, E. J.; Cho, H.; Rücker, C.; Hua, D. H. Tetrahedron Lett. 1981, 22, 3455. Ammonium fluoride, H4N F P. Hogan 1 • Monosilylation of symmetrical diols is possible, and useful. • Selective Selective deprotection deprotection of of silyl silyl ethers ethers is isalso also important, important, and and is isalso also subject subject to to NaH, TBSCl, THF HO OH TBSO OH empirical determination. n 75-97% n n = 2-6,10 TESOTESO HOHO H C McDougal, P.G.; Rico, J.G.; Oh, Y.; Condon, B. D. J. Org. Chem. 1986, 51, 3388. H3C 3 CH CH3 CH3OBOM CH CHOBOM3 CH3 3 OBOM 3 OBOM TBDPSCl, i-Pr2NEt, DMF, 23 °C TBSO HO OH TBSO pyr•HF, CH3CN CHCH TBDPSO OH CHCH33 33 n 75-86% n 0 °C, 11 h, quant. O O H H n = 2,3,5,7,9 O O O AcO OO AcO O O Hu, L.; Liu, B.; Yu, C. Tetrahedron Lett. 2000, 41, 4281. O CH3 CH3 AcO OHOH BuLi, THF; TBSCl OTBS OTBS O Ph O H3C O HO HO HOHO CH CH 3 99%88% 3 3 OH CH3 CH3 N O H CH3 Roush, W. R.; Gillis, H. R.; Essenfeld, A. P. J. Org. Chem. 19831984, 49, 4674. OH 3 • Selective protection of alcohols is of great importance in synthesis. Conditions often must be HO H determined empirically. BzOBzOAcO OTIPS Taxol O O H OTIPS Holton, R. A., et al. J. Am. Chem. Soc., 1994, 116, 1599. H N O H CH O H O 3 H TESCl, 2,6-lutidine O H N H3C H N O H OH CH3 CH2Cl2, –78 °C O H CH3 H 97% H O H Cl CHCO H H3C O H 2 2 CH2 OH ONO N 2 AcO OAc AcO OAc HO CH 90% OAc CH3 H OAc TBSO OTES O TBSO O CH2 H OH • TESCl/imidazole and ONO N H O O H TESCl/imidazole and HO O TESOTf,• 2,6-lutidine both O CH3 OTES O CH3 gaveTESOTf, the bis-silylated 2,6-lutidine product. both O HO HO OTES OTBS OH OCH3 gave the bis-silylated product. H OH CH3 OCH O H OCH3 O H N O H O OH CH OAc O 3 H HO C H C O O H 2 O 3 HO C O O 2 CH CO2H 3 CH3 H HO CH ONO N 2 Zaragozic acid OH CH OH H 3 O Br Carreira, E. M.; Du Bois, J. J. Am. Chem. Soc. 1995, 117, 8106. OCH3 OCH3 Phorboxazole B • Selective deprotections in organic synthesis have been reviewed: Nelson, T. D.; Crouch, R. D. Synthesis 1996, 1065. Evans, D. A.; Fitch, D. M. Angew. Chem., Int. Ed. Engl. 2000, 39, 2536. P. Hogan 2 Myers Protective Groups – Protection of Hydroxyl Groups, Esters, and Carbonates Chem 115 Esters and Carbonates General methods used to form esters and carbonates: O pyr, DMAP O O O O O ROH RO RO RO RO Cl R' RO R' CH3 Cl Cl Cl Cl Cl Cl Acetate (Ac) Chloroacetate Dichloroacetate Trichloroacetate O O pyr, DMAP O ROH R' O R' RO R' O O O O RO RO RO RO O pyr O F CH3 ROH Cl OR' RO OR' F F H3C CH3 OCH3 R O Trifluoroacetate (TFA) Pivaloate (Piv) Benzoate (Bz) p-Methoxybenzoate N N X– O DMAP = 4-Dimethylaminopyridine: O RO RO O O O N N RO RO H3C CH3 H3C CH3 OCH3 O • Proposed intermediate Br p-Bromobenzoate Methyl Carbonate 9-(Fluorenylmethyl) Carbonate Allyl Carbonate (Fmoc) (Alloc) DMAP is used to accelerate reactions between nucleophiles and activated esters. Neises, B.; Steglich, W. Angew. Chem., Int. Ed. Engl. 1978, 17, 522. O O O • In general, the susceptibility of esters to base-catalyzed hydrolysis increases with the RO RO acidity of the product acid. RO O O O O RO CH Cl 3 Si(CH ) O CH Cl Cl 3 3 3 CH3 O O O H3C < OCH < < 2,2,2-Trichloroethyl Carbonate 2-(Trimethylsilyl)ethyl Carbonate Benzyl Carbonate t-Butyl Carbonate OCH 3 OCH3 H C 3 3 CH (Troc) (Teoc) (Cbz) (Boc) 3 CH3O S O O RO O O N CH3 < < Cl < F Cl OCH3 OCH3 H3C H3C OCH3 OCH3 Cl F Cl F Dimethylthiocarbamate (DMTC) P. Hogan/Seth B. Herzon 3 Acetate Esters: • Several methods for forming and cleaving acetate esters have been developed. Lipases can often • Good selectivity can often be achieved in the selective deprotection of different esters. be used for the enantioselective hydrolysis of acetate esters. The enantioselective hydrolysis of meso diesters is an important synthetic transformation and racemic esters have been kinetically resolved using lipases. OAc O OH O H3C H C O O 3 O Acetyl cholinesterase PCC H C 3 O H3C O 94%, 99% ee OAc OAc O OAc HO Cl HO O OH O O n-PrNH2 Deardorff, D. R.; Matthews, A. J.; McMeekin, D. S.; Craney, C. L. Tetrahedron Lett. 1986, O O O O 27, 1255. 71% CH O OH 3 CH3O OH • Lipases can also be effective for deprotection under very mild conditions, as in the case shown below, where conventional methods were unsuccessful. CH 3 CH3 O O O OAc O Lipase MY OH O 0.1 M pH 7.2 buffer Cl CH O Cl CH3 O 3 O O CH OH CH3 28 °C, 4 days 3 O O O Sakaki, J.; Sakoda, H.; Sugita, Y.; Sato, M.; Kaneto, C. Tetrahedron: Asymmetry, 1991, 2, 343. CH3 O OCH 3 O CH3 • A potentially general method for selectively acylating the primary hydroxyl group of a 1,2-diol N makesmakes useuse ofof stannylenestannylene acetalsacetals asas intermediates:intermediates: CH3 HO H OH Bu2SnO, AcCl, Neocarzinostatin Chromophore HO OBn O OBn AcO OBn toluene, 100 °C HO O CH2Cl2, 0 °C HO Myers, A. G.; Liang, J.; Hammond, M.; Wu, Y.; Kuo, E. Y. J. Am. Chem. Soc. 1998, 120, Bu Sn 5319. Bu Review: Hannessian,Hanessian, S.; S.; David, David, S. S. Tetrahedron Tetrahedron 1985, 1985, 41, 41, 643., 643. P. Hogan 4 • When one protective group is stable to conditions that cleave another and the converse is also true, these groups are often said to bear an orthogonal relationship. This concept is illustrated well in the Allyl Carbonate: context of carbonates (and carbamates). O Pd2(dba)3, dppe, Et2NH, THF Summary of methods for deprotecting carbonates: RO ROH O Methyl Carbonate: O Genet, J.P.; Blart E.; Savignac, M.; Lemeune, S.; Lemaire-Audoire, S.; Bernard, J. Synlett 1993, K2CO3, MeOH RO ROH 680. OCH3 2-(Trimethylsilyl)ethyl Carbonate: Meyers, A. I.; Tomioka, K.; Roland, D. M.; Comins, D. Tetrahedron Lett. 1978, 19, 1375. O TBAF, THF RO ROH O 9-Fluorenylmethyl Carbonate: Si(CH3)3 • The pKa of fluorene is ≈ 10.3 O Gioeli, C.; Balgobin, S.; Josephson, S.; Chattopadhyaya, J.
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