C chemoselectivity

Collomb, D., Chantegrel, B., and Deshayas, C., Chemoselectivity in the rhodium(ll) acetate catalysed decomposition of a-diazo-P-keto-y.S-alkenyl-S-aryl compounds. Aromatic C-H insertion reaction or Wolff rearrangement-electrocyclization. Tetrahedron, 52, 10455, 1996. 

Figure 16.2-1. Enzyme-catalyzed oxidations of alcohols. Reactions are grouped according to the feature mainly exploited in the preparative application. A-C Chemoselectivity (e. g. Sects.

Boyer, N. Gloanec, R De Nanteuil, G. Jubault, R Quirion, J.-C. Chemoselective and stereoselective synthesis of gcm-difluoro-P-aminoesters or gcm-difluoro-P-lactams from ethyl bromodifluoroacetate and imines during Reformatsky reaction. Tetrahedron 2007, 65(50), 12352-12366. 

Reetz, M.T. Frombgen, C. Chemoselective reduction of halo-nitro aromatic compounds by 3-cyclodextrin-modi-fied transition metal catalysts in a biphasic system. Synthesis 1999, (9), 1555-1557. 

Second, functional group selectivity is observed - i.e. reactions with aldehydes proceed at —78 °C whereas those with ketones proceed at elevated temperatures (ca. 0 °C). Chemoselectivity is observed with acceptors having two different kinds of carbonyl function, for example aldehyde and ketone or ester, in the same molecule. Treatment of phenylglyoxal with silyl enol ether 38 at —78 °C affords a-hydroxy-y-diketone 39 (Eq. (20)) [20b]. The reaction of ketoesters 40 other than j5-ketoesters with silyl enol ether 38 gives hydroxyketoesters 41 as sole products (Eq. (21)) [23]. 

Stratakis, M. and Rabalakos, C., Chemoselective hydroperoxidation of alkenylarenes within thio-nin-supported zeoHte Na-Y, Tetrahedron Lett., 42, 4545, 2001. 

Palladium catalyzed cycloisomerizations of 6-cn-l-ynes lead most readily to five-membered rings. Palladium binds exclusively to terminal C = C triple bonds in the presence of internal ones and induces cyclizations with high chemoselectivity. Synthetically useful bis-exocyclic 1,3-dienes have been obtained in high yields, which can, for example, be applied in Diels-Alder reactions (B.M. Trost, 1989). 

Chemoselective C-alkylation of the highly acidic and enolic triacetic acid lactone 104 (pAl, = 4.94) and tetronic acid (pA, = 3.76) is possible by use of DBU[68]. No 0-alkylation takes place. The same compound 105 is obtained by the regioslective allylation of copper-protected methyl 3,5-dioxohexano-ate[69]. It is known that base-catalyzed alkylation of nitro compounds affords 0-alkylation products, and the smooth Pd-catalyzed C-allylation of nitroalkanes[38.39], nitroacetate[70], and phenylstilfonylnitromethane[71] is possible. Chemoselective C-allylation of nitroethane (106) or the nitroacetate 107 has been applied to the synthesis of the skeleton of the ergoline alkaloid 108[70]. 

Sodium cyanoborohydride is remarkably chemoselective. Reduction of aldehydes and ketones are, unlike those with NaBH pH-dependent, and practical reduction rates are achieved at pH 3 to 4. At pH 5—7, imines (>C=N—) are reduced more rapidly than carbonyls. This reactivity permits reductive amination of aldehydes and ketones under very mild conditions (42). 

Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

A stereochemical issue of great importance presents itself here. In the chemoselective reduction of the two ketone carbonyls at C-5 and C-8 in 6, the addition of hydride takes place on the same side... 

It has been reported that an allylic C-Si bond can be cleaved by tetrabutylammonium fluoride to give an anionic allylic species, which chemoselectively adds to carbonyl compounds (nitriles, esters, and epoxides failed) to form homoallylsilyloxy compounds13. 

Chemoselective alkynylation in the C-3 position of N-substituted 3,5-dichloropyrazin-2(lH)-one has been described by Van der Fycken et al. [27]. When a mixture of 3,5-dichloro-l-benzylpyrazin-2(lH)-one and pheny-lacetylene in a mixture of NFts and DMF, using Pd(OAc)2/PPh3 as precatalyst and Cul as a co-catalyst, was irradiated for 15 min at 120 °C, the desired l-benzyl-5-chloro-3-phenylethynylpyrazin-2(lH)-one could be isolated with a 77% yield (Scheme 50). Although 2.4 equiv of alkyne were used, no trace of the 1-benzyl-3,5-diphenylethynylpyrazin-2(lH)-one could be detected. 

The high chemoselectivity for the Baeyer-Villiger process was utilized in the synthetic elaboration of another hetero-bicyclic substrate. The biooxidation only provides the expected unsaturated lactone in a desymmetrization reaction without compromising the olefin functionality. The biotransformation product was then converted to pivotal intermediates for C-nucleosides like showdomycin, tetrahydro-furan natural products like kumausyne, and goniofufurone analogs in subsequent chemical operations (Scheme 9.17) [161]. 

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