Advanced Organic Reactions by Rizzo C.J.

By Rizzo C.J.

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Solvent. 5 mol % Catalyst 50 : 1 150 : 1 33 : 1 52 : 1 OH REDUCTIONS 34 Olefin Isomerization: CH3 (3 : 1) OH OH olefin isomerization O OH major product - Conducting the hydrogenation at high H 2 pressures supresses olefin isomerization and often gives higher diastereoselectivity. Other Lewis basic groups can direct the hydrogenation. (Ir seems to be superior to Rh for these cases) OMe CO2Me OMe Ir CO2Me + Ir+ 99 : 1 O O CO2H Ir+ 7 : 1 Rh+ 1 : 1 O O N O O N O 130 : 1 1:1 Acyclic Examples Rh+ (2 mol %) OH Me CH 3 JCSCC 1982, 348 H2 (15 psi) Me L L M OH (97:3) H O OH H H Ph H 3C H H L CH3 Ph M L Ph anti H H O H 32 : 1 CO2H > 99 : 1 O > 99 : 1 Rh+ OH 1,2-strain Ph syn REDUCTIONS L L R3 H OH R3 35 H O M R2 R1 OH favored R3 H syn R1 R2 R1 R2 R3 H L H R1 R2 M OH disfavored R3 1,2-strain R1 R2 O H L anti - Supression of olefin isomerization is critical for acyclic stereocontrol !

Kocienski, Protecting Groups, Georg Thieme Verlag, 1994 1. 2 3. 4. Hydroxyl groups Ketones and aldehydes Amines Carboxylic Acids - Protect functional groups which may be incompatible with a set of reaction conditions - 2 step process- must be efficient - Selectivity a. selective protection b. selective deprotection Hydroxyl Protecting Groups Ethers Methyl ethers R-OH → R-OMe Formation: Cleavage: - difficult to remove except for on phenols CH2N2, silica or HBF4 NaH, MeI, THF AlBr3, EtSH PhSe Ph2P Me3SiI OMe O OH O AlBr3, EtSH TL 1987, 28 , 3659 O O OBz Methoxymethyl ether MOM R-OH → R-OCH2OMe OBz stable to base and mild acid Formation: - MeOCH2Cl, NaH, THF - MeOCH2Cl, CH2Cl2, iPr2EtN Cleavage - Me2BBr2 TL 1983, 24 , 3969 PROTECTING GROUPS Methoxyethoxymethyl ethers (MEM) R-OH → R-OCH2OCH2CH2OMe stable to base and mild acid Formation: - MeOCH2CH2OCH2Cl, NaH, THF - MeOCH2CH2OCH2Cl, CH2Cl2, iPr2EtN Cleavage - Lewis acids such as ZnBr2, TiCl4, Me2BBr2 TL 1976, 809 S B Cl MEM-O HO S C5H11 O-Si(Ph)2tBu O C5H11 TL 1983, 24 , 3965, 3969 O O-Si(Ph)2tBu - can also be cleaved in the presence of THP ethers Methyl Thiomethyl Ethers (MTM) R-OH → R-OCH2SMe Stable to base and mild acid Formation: - MeSCH2Cl, NaH, THF Cleavage: - HgCl2, CH3CN/H2O - AgNO3, THF, H2O, base Benzyloxymethyl Ethers (BOM) R-OH → R-OCH2OCH2Ph Stable to acid and base Formation: - PhOCH2CH2Cl, CH2Cl2, iPr2EtN Cleavage: - H2/ PtO2 - Na/ NH3, EtOH Tetrahydropyranyl Ether R-OH (THP) O H + , PhH Formation Cleavage: R O O Stable to base, acid labile - DHP (dihydropyran), pTSA, PhH - AcOH, THF, H2O - Amberlyst H-15, MeOH Ethoxyethyl ethers (EE) JACS 1979, 101 , 7104; JACS 1974, 96 , 4745.

1970, 48 , 735. Lalancette Reduction 42 NaBH4/ NiCl 2 Chem. Pharm. Bull. 1981, 29 , 1159; Chem. Ber. 1984, 117 , 856. Ar-NO2 → Ar-NH2 Ar-NO → Ar- NH2 R2C=N-OH → R2CH-NH2 NaBH4 / TiCl 4 Synthesis 1980, 695. R-COOH → R-CH2-OH R-COOR' → R-CH2-OH R-CN → R-CH2-NH2 R-CONH2 → R-CH2-NH2 R2C=N-OH → R2CH-NH2 R-SO2-R' → R-S-R' NaBH4 / CeCl 3 Luche Reduction reduced α,β-unsaturated ketones in a 1,2-fashion OH NaBH 4/ CeCl3 R R NaBH 4 R R O OH O R + R R R R R R H R JCSCC 1978, 601 JACS 1978, 100 , 2226 O OH CO 2Me NaBH 4/ CeCl3 C 5H11 CO 2Me MeOH C 5H11 OH OH - selective reduction of ketones in the presence of aldehydes.

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