Advanced organic reactions by Stuart W.

By Stuart W.

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Rev. 1986, 86 , 763. NaBH4 reduces ketones and aldehydes LiBH4 reduces ketones, aldehydes, esters and epoxides. THF soluble LiBH4/TMSCl stronger reducing agent. ACIEE 1989, 28, 218. Zn(BH4)2 reduces ketones and aldehydes R4N BH4 organic soluble (CH2Cl2) borohydrides. Synth Commun. 1990, 20, 907 LiEt3BH reduces ketones, aldehydes, esters, epoxides and R-X Li s-Bu3BH reduces ketones, aldehydes, esters and epoxides (hindered borohydride) Na(CN)NH3 reduces iminium ions, ketones and aldehydes Na(AcO)3BH reduces ketones and aldehydes (less reactive) NaBH2S3 reduces ketones and aldehydes REDUCTIONS 41 Sodium Borohydride NaBH4 - reduces aldehydes and ketones to alcohols - does not react with acids, esters, lactones, epoxides or nitriles.

1975, 94 , 449 O O O iPr3SiH, Et2O Si O 2 Pt 2 OSi(iPr)3 O JOC 1994, 59, 2287 O (87%) - Buchwald Reduction JACS 1991, 113 , 5093 - catalytic reagent prepared from Cp2TiCl2 + nBuLi and stoichometric (Et)3SiH in THF will reduce ester, ketones and aldehydes to alcohols under very mild conditions. - α,b− unsaturated esters are reduced to allylic alcohols - free hydroxyl groups, aliphatic halides and epoxides are not reduced REDUCTIONS 54 Clemmensen Reduction Organic Reactions 1975, 22, 401 Comprehensive Organic Synthesis 1991, vol 8, 307.

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.

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