Chemoselectivity carbonyl

Similarly, 5-hydroxymethylfurfural (HMF) was chemoselectively carbonylated at the quasi-benzylic alcohol functionality to afford the previously undescribed... 

Fig. 10.7. Chemoselective carbonyl group reductions, II. A chemoselective reduction of the less hindered ketone takes place on the left side, and a chemoselective reduction of the more strongly hindered ketone takes place on the right side.

Fig. 10.8. Chemoselective carbonyl group reductions, III. Reduction of a saturated ketone in the presence of an unsaturated ketone (left) and reduction of an unsaturated ketone in the presence of a saturated ketone (right).

Fig. 8.2. Chemoselective carbonyl group reductions I. On the left side a chemoselective reduction of the aldehyde takes place, whereas on the right side a chemoselective reduction of the ketone is shown.

The macrosphelides were isolated in the mid-1990s and found to inhibit cell-adhesion [35], bringing these natural products into the chemical biology spotlight Takahashi and coworkers reported a combinatorial synthesis of 122 macrosphelides, including the natural ones based on two palladium-catalyzed chemoselective carbonylations of vinyl halide, first in an intermolecular fashion and then in a... 

Another interesting example of chemoselective carbonyl yhde cycloaddition is the rhodium(II)-mediated reaction of 56 with the bis(phenylallyl-idene)cyclopentanone 145 which furnished the novel 2,5-epoxycyclopenta[fc]-... 

Chemoselective Carbonyl Reductions. Selective reducing agents for aldehydes over ketones which have been reported this year include a formic acid-trialkylamine-ruthenium(II) system (Vol.2, p.115),... 

A convenient synthesis of 3-spiro-fused benzofuran-2(3//)-ones was reported via Pd-catalyzed chemoselective carbonylation of vinyl triflates in the presence of o-iodophe-nols, followed by a regioselective intramolecular Heck reaction (Scheme 20). ... 

Scheme 4.55. Chemoselective carbonyl olefination with the molybdenum-methylidene 56b.

Similarly, 5-hydroxymethylfurfural (HMF) was chemoselectively carbonylated at the quasi-benzylic alcohol functionality to afford the previously undescribed 5-formylfuran-2-acetic acid (FFA) (Figure 10) as the sole carbonylation product, using the Pd(tppts)3 catalyst in a completely aqueous medium under mild reaction conditions (70°C, 5 bar The only byproduct observed was... 

Chemoselective Carbonylation Reactions of Enol Triflates and lodoalkenes... 

Chemoselective carbonylation of a vinyl iodide 34 with alcohol containing a vinyl bromide moiety 35 has been successfully employed for the solid-phase synthesis of a macrosphelide precursor 36 [43]. After the 4-methoxyphenylmethyl (MPM) group was removed, the palladium-catalyzed carbonylative macrolactonization of the vinyl bromide 37 achieved the synthesis of the macrosphelide-supported derivative 38 (Scheme 9.13). The combinatorial synthesis of a 122-member macrosphelide library has been performed by the three-component strategy based on the palladium-catalyzed chemoselective carbonylation/macrolactonization reaction. 

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). 

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). 

In transforming bis-ketone 45 to keto-epoxide 46, the elevated stereoselectivity was believed to be a consequence of tbe molecular shape — tbe sulfur ylide attacked preferentially from tbe convex face of the strongly puckered molecule of 45. Moreover, the pronounced chemoselectivity was attributed to tbe increased electropbilicity of the furanone versus the pyranone carbonyl, as a result of an inductive effect generated by tbe pair of spiroacetal oxygen substituents at tbe furanone a-position. ... 

Chemoselective reduction of a,(3-epoxy carbonyl compounds to aldols and their analogs by organoseleniums and its application to natural product synthesis 98YGK736. 

Me- SiCl also affects the stereoselectivity of 1,2-additions to carbonyl compounds [ 133]. Witli the aid of suitable activators, these mildly reactive reagents show selec-tivities unattainable by the conventional reagents, as ilustrated below for Me- SiCl-dependent Chemoselectivity fEq. 10.13) [134]. 

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... 

Scheme 4 outlines the synthesis of key intermediate 7 in its correct absolute stereochemical form from readily available (S)-(-)-malic acid (15). Simultaneous protection of the contiguous carboxyl and secondary hydroxyl groups in the form of an acetonide proceeds smoothly with 2,2 -dimethoxypropane and para-toluene-sulfonic acid and provides intermediate 26 as a crystalline solid in 75-85 % yield. Chemoselective reduction of the terminal carboxyl group in 26 with borane-tetrahydrofuran complex (B H3 THF) affords a primary hydroxyl group that attacks the proximal carbonyl group, upon acidification, to give a hydroxybutyrolactone. Treat-... 

Inherent in the reduction of asymmetrically substituted cyclic imides is the problem of regiose-lectivity. Imides, in which one carbonyl group is part of a (thio)carbamate or urea function, usually show complete chemoselectivity for reduction of the other carbonyl group, indicated with an arrow. 

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. 

The combination of thionation by Lawesson s reagent [98] of oxoenamino-ketones 96 with normal electron-demand Diels-Alder reaction of conjugated aldehydes allows a variety of thiopyrans 97 to be synthesized by a regio-selective and chemoselective one-pot methodology [99] (Equation 2.28). Thionation occurred at the more electrophilic ketonic carbonyl group. O O... 

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