Chemoselective method

Addition to Carbonyl Compounds. Unlike Grignard and alkykitliium compounds, trialkylboranes are inert to carbonyl compounds. The air-catalyzed addition to formaldehyde is exceptional (373). Alkylborates are more reactive and can transfer alkyl groups to acyl halides. The reaction provides a highly chemoselective method for the synthesis of ketones (374). 

Eash KJ, Pulia MS, Wieland LC, Mohan RS (2000) A simple chemoselective method for the deprotection of acetals and ketals using bismuth nitrate pentahydrate. J Org Chem 65 8399-8401... 

Direct insertion into an X—H a bond constitutes the highlight of dioxirane chemistry . Besides the insertion of a dioxirane oxygen atom into an alkane acH bond, for practical purposes a most valuable oxyfunctionalization, also the more facile insertion into the asiH bond is known, a convenient and chemoselective method of preparing silanols. 

Cyclotrimerization of acetylenes to form benzene derivatives is studied intensively mainly because of the challenge of chemo- and regioselectivities. A regiose-lective and highly chemoselective method for preparing substituted benzenes was described via PdCl2-catalyzed cyclotrimerization in the presence of CuCl2 537... 

Fig. 2. Schematic representation of the chemoselective method for glycopeptide synthesis.

A sulfur-based chemoselective method was applied to the synthesis of a conjugate polysaccharide vaccine against Haemophilus influenzae type b.37 The... 

A practical synthesis of 1,3-OX AZEPINES VIA PHOTOISOMERIZATION OF HETERO AROMATIC V-OXIDES is illustrated for 3,1-BENZOXAZEPINE. A hydroboration procedure for the synthesis of PERHYDRO-9b-BORAPHENALENE AND PERHYDRO-9b-PHEN-ALENOL illustrates beautifully the power of this methodology in the construction of polycyclic substances. The conversion of LIMONENE TO p-MENTH-8-EN-YL METHYL ETHER demonstrates a regio-and chemoselective method for the PHOTOPROTONATION OF CYCLOALKENES. An efficient method for the conversion of a ketone to an olefin involves REDUCTIVE CLEAVAGE OF VINYL PHOSPHATES. A mild method for the conversion of a ketone into the corresponding trimethylsiloxy enol ether using trimethylsilyl acetate is shownforthe synthesis of (Z)-3-TRIMETHYLSILOXY-2-PENTENE. 

The most commonly employed diazenes, e.g., diethyl, bis(2,2,2-trichloroethyl), and di-/m-butyl diazenedicarboxylate, and 4-phenyl-3/7-1,2,4-triazole-3,5(4/7 )-dione, are commercially available. However, various new reagents or methods recently introduced for the preparation of specific diazenes from hydrazines can be successfully applied to other hydrazines. Especially important is the development of chemoselective methods for converting hydrazines to diazenes in the presence of unsaturated substrates, for example in intramolecular cycloaddition reactions (Section 7.2.10.3.10.2.), where either the (di)ene group and other functions present in the substrate are sensitive to the (oxidizing) reagent employed. 

Control of Chemoselectivity. The reaction of allylic alcohols with r-butyl hydroperoxide and vanadium acetylacetonate or titanium tetraisopropoxide provides a highly chemoselective method for the preparation of epoxides, as exemplified below. Catalysis by vanadium is envisioned to involve a complex of r-BuOOH, the OH group of the allylic alcohol, and the metal. 

Functional group transformations classical and chemoselective methods for oxidation and reduction of organic substrates, and the availability and utilization of regio-, chemo-, and stereoselective agents for reducing carbonyl compounds... 

Fig. 30 Highly chemoselective methods for intra- and intermolecular allylic C-H oxidation...

Other Reductions. The reductive removal of 2-oxazolidinone chiral auxiliaries with NaBH4 is a mild and chemoselective method that provided the corresponding alcohols in good to excellent yields (75-95%), with complete preservation of stereochem-... 

Chemoselective methods for oxidation and reduction, and protecting groups to help control chemoselectivity, will appear throughout this book, and we shall return in detail to peptides and their biological functions in Chapter 42. Before then we will address in detail stereoselectivity (in Chapters 32,33, and 41) but the very next chapter will deal with the other aspect of selectivity—regioselectivity. 

However, Joshi et al. (2010) reported a chemoselective method for the synthesis of 1,3-imidazoles (73) by the reaction of substituted aldehydes (72) with o-phenyl-enediamine (71) catalyzed by 5 mol% of tetrabutylammonium fluoride (TBAF) as an inexpensive and relatively nontoxic phase-transfer catalyst in water as a green solvent under ultrasonic irradiation at room temperature (Scheme 8.23). High yields... 

Cross-coupling involving alkyhnetals and/or alkyl electrophiles other than those containing aUyl, benzyl, and or propargyl have been comprehensively summarized [230]. Pd-catalyzed alkylation of alkenyl and aryl electrophiles with alkylzincs is a generally satisfactory and chemoselective method of alkylation, especially when... 

The importance of controlling the direction of ring closure for the unsymmetrical di-ester condensation has prompted researchers to introduce different chemoselective methods for this reaction. The mono-thiol version of the condensation was first developed for that particular purpose by Yamada, Hosaka, and co-workers in 1981. An example is the ring formation of compounds 22. The ring closure occurs using an appropriate base such as lithium diispropylamide (LDA) in dry THF at -30 °C in the case of X = CH2 or sodium hydride at room temperature in case of X = N (or S) to form the corresponding carbocyclic or heterocyclic rings 22a-c in 74-77% yields. 

The direct photolysis of alkyl or aryl halides in solution to form carbon-centered radicals is rarely used in organic synthesis." Alkyl iodides usually afford mixtures of radical and ionic products, while alkyl bromides can produce radical-derived products but in low yield. A notable exception is the photocycliza-tion of haloarenes, which has been shown to produce carbon-centered radicals that can add to aromatic rings. A similar reaction has recently been observed on irradiation of iodoheterocycles, with substituted benzenes or electron-poor alkenes, to form arylated or alkylated heterocycles in good yield. Related reactions have also been reported on irradiation of 4-chloroanilines in the presence of (electron-rich) alkenes, although in this case, the alkylations appear to involve the formation of a phenyl cation. An alternative approach to form carbon-centered radicals is to irradiate the alkyl iodide or bromide in the presence of triethylamine this is proposed to form an amine-haHde exciplex, which cleanly breaks down to give a carbon-centered radical and a halide anion. Cossy and co-workers have shown this to be a fast, convenient, and chemoselective method of radical generation, which has recently been used to prepare the bicyclic core of ( )-bisabolangelone (Scheme 1). ... 

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