Chemoselective transformations

Diverse activation methods of bis(iodozincio)methane (3) for the methylation reactions lead to chemoselective transformations. For instance, the a-keto group is methylated selectively by 3 in the presence of a simple ketone (equation 22)32. The opposite selectivity is observed in the reaction using 3 and /3-TiCl3 (equation 23)32. Aldehyde is methyle-nated selectively by 3 in the presence of simple ketone (equation 24)9. Both aldehyde... 

In summary, alkyne metathesis seems about to become one of the key reactions frequently employed in the synthesis of increasingly complex natural products. Recently, Trost et al. and Fiirstner et al. reported new hydrosilylation protocols for the convenient, chemoselective transformation of alkynes to E alkenes which will extend the value of alkyne metathesis for natural product synthesis [32],... 

ChemoselectiviQr is indeed a very complex issue in the hydroxylation of steroids, but microorganisms capable of chemoselective transformations can often be selected on precedent. 

Transesterification can be used to cleave the acyl group from an ester to release the alcohol. The mildness of the reaction conditions enables chemoselective transformation. A siloxy group /S to a ketone group was not eliminated (Eq. 225) [524], and formation of an epoxide from the unprotected bromohydrin did not occur (Eq. 226) [525]. Similarly, in the synthesis of an avermectin derivative, delactonization was carried out by the titanium-based method as shown in Eq. (227) [526]. 

As mentioned in Sect. 3.2, the carbon-carbon triple bond has a superior reactivity in hydrosilylation compared to a double bond, presumably due to a faster insertion step. This creates an opportunity to perform regio- and chemoselective transformations of enynes [68]. Regioselective transformations of 1,6- and 1,7-enynes proceed via preferred insertion of the triple bond (24). 

Fluorine substituents on the C—C double bond greatly accelerate Claisen rearrangement. For instance, difluorovinyl prenyl ether 83 rearranges at 100°C, while nonfluorinated vinyl prenyl ether rearranges at 200°C [ 1 ]. The chemoselective transformation of 76 to 75 reveals also a favorable fluorine-directed rearrangement [2]. 

This example demonstrates that exploitation of the innate reactivity [13] of a functional group (combination) may allow for chemoselective transformations and may thus render the use of protecting groups obsolete. 

Chiral monoesters, obtained either from a prochiral diol or diester, may be converted by a suitable series of chemoselective transformation to either enantiomer of a given target compound (enantiodivergent synthesis) (Scheme 11.1-13)110 40l... 

The microbial hydrolysis of nitriles to amides or carboxylic acids often does not proceed with high enantioselectivity, but it offers a valuable and mild alternative method to the harsh reaction conditions usually required for this conversion using traditional methodology. As a consequence, it has proven to be a very useful and competitive method in the chemoselective transformation of nitriles on an industrial scale. 

With Beller s work having demonstrated the necessity for a slow dosage of acetone cyanohydrin (145) in order to successfully cyanate aryl halides [49], it was thought that this difference in reactivity could be exploited to chemoselectively transform vinyl halides into nitriles in the presence of aryl halides. Initially, the reaction conditions were applied to bromobenzene (165), and pleasingly no reaction was observed to occur (Table 7.7, Entry 1). 

Application of the cyanation protocol to the dibromo compounds 167,169 and 171 accordingly led to the chemoselective transformation of the vinyl bromide, leaving the aromatic bromide untouched in the process, to afford 168, 170 and 172, respectively (Table 7.8, Entries 2-4). 

Chemoselective Transformations of Compounds containing a Boronic Add (Ester) Substituent... 

The wide range of options to use silane groups of different reactivity for chemoselective transformations is nicely demonstrated by an example from the benzleukodienes (33) [16]. 

The copper-catalyzed coupling of glycolic acids with aryl iodides has been accomplished using common copper catalysts (Scheme 2.54) [79], This was a chemoselective transformation that resulted in the preferential formation of the alkyl aryl ether instead of the ester. The carboxylic acid was retained in the product and was able to be further transformed in subsequent reactions. The electronic composition of the aryl iodide did not significantly influence the outcome of the reaction, and a range of electron-rich and electron-poor arenes were well tolerated by the chemistry. 

The 5 -zincate could not react directly with alkenes [89]. However, in the presence of TM catalyst, the silylzincation could also take place at terminal alkenes. The first such example was reported in 2005 by our group (Table 11) [90]. Catalyzed by Cp2TiCl2, SiSiNOL-zincate and terminal alkenes could be regio- and chemoselectively transformed into allylsilanes. It seemed that in the metalation, the silyl group was specifically introduced at the 1-position and the elimination at the 3-position was favored to form E- or Z-alkenes. Many active groups could be compatible with these reactions. 

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