Stereochemistry chemoselective

The wM-diacetate 363 can be transformed into either enantiomer of the 4-substituted 2-cyclohexen-l-ol 364 via the enzymatic hydrolysis. By changing the relative reactivity of the allylic leaving groups (acetate and the more reactive carbonate), either enantiomer of 4-substituted cyclohexenyl acetate is accessible by choice. Then the enantioselective synthesis of (7 )- and (S)-5-substituted 1,3-cyclohexadienes 365 and 367 can be achieved. The Pd(II)-cat-alyzed acetoxylactonization of the diene acids affords the lactones 366 and 368 of different stereochemistry[310]. The tropane alkaloid skeletons 370 and 371 have been constructed based on this chemoselective Pd-catalyzed reactions of 6-benzyloxy-l,3-cycloheptadiene (369)[311]. 

Apart from information on stereochemistry, bromine bridging does not provide a priori any rule regarding regio- and chemoselectivity. Therefore, we systematically investigated (ref. 3) these two selectivities in the bromination of ethylenic compounds substituted by a variety of more or less branched alkyl groups (Scheme 4). 

Palladium-catalyzed cyclization of alkenes and alkynes were reported by Balme and co-workers.143 144 Intramolecular carbopalladation occurs to give polycyclic compounds. It has been shown that the nucleophile type has a large influence on the cyclization process. Both 5-exo- and 6-endo-cyclization are observed for substrates with nitrile (116 and 118) and ester (120, 122, and 124) substituents, respectively (Scheme 36). When a mixed nucleophile (CN and C02Me) is used, a mixture of 5-exo and 6-endo products is obtained. The chemoselectivity is controlled by the size of the nucleophile used. The stereochemistry of the initial double bond plays an important role on the stereoselectivity of the cyclization. (Z)-olefins (118 and 120) and (/. )-olefins (116 and 124) afford as- (119 and 121) and trans-cyclization products (117 and 123), respectively. 

Direct epoxidation of compound 20 with Bu OOH in the presence of VO(acac)2 [10], proceeded chemoselectively to give the epoxide 21 which was regioselectively opened with UAIH4 to afford the 1,2-diol 22 (76%). X-ray diffraction analysis of this compound confirmed the assigned stereochemistry for intermediates 20-22. 

The various nucleophiles enumerated in the chemoselectivity section (Section 3.3.2.2) will be considered in turn and classified as to the stereochemistry of addition and, thereby, the mechanism they follow. 

The corresponding /i-amino aldehydes are reduced in situ and the corresponding amino alcohols are isolated in good yield with up to >99 % ee. The Mannich reactions proceed with excellent chemoselectivity and inline formation occurs with the acceptor aldehyde at a faster rate than C-C bond-formation. Moreover, the one-pot three-component direct asymmetric cross-Mannich reaction enables aliphatic aldehydes to serve as acceptors. The absolute stereochemistry of the reaction was determined by synthesis and reveled that L-proline provides syn /i-amino aldehydes with (S) stereochemistry of the amino group. In addition, the proline-catalyzed direct asymmetric Mannich-type reaction has been connected to one-pot tandem cyanation and allylation reaction in THF and aqueous media affording functional a-amino acid derivatives [39, 42]. 

As discussed, in the absence of catalysts catecholborane adds to C=C triple bonds but not to C=C double bonds (see Figure 16.15). Accordingly, if enynes are reacted with catecholborane, the latter will add chemoselectively to their C=C triple bond (Figure 16.21). As expected, the resulting boronic ester features the same regio- and stereochemistry seen in Fig-... 

Selectivity comes in three sorts chemoselectivity, regioselectivity, and stereoselectivity. Chemoselectivity is which group reacts regioselectivity is where it reacts. Stereoselectivity is how the group reacts with regard to the stereochemistry of the product. 

Asymmetric dihydroxylation (Chapter 45) is straightforward though you might like to comment on the chemoselectivity. The diol is converted into the epoxide and you should explain the regio- and chemoselectivity of this step. The next step is perhaps the most interesting what is the mechanism of the cyclization, what is the role of silicon, and how is the stereochemistry controlled ... 

This NADPH reaction is typically stereo- and chemoselective, though the stereochemistry is rather wasted here as tile next step is a dehydration, typical of what is now an aldol product, and occurring by an enzyme-catalysed ElcB mechanism. 

There are a number of synthetically important applications, involving these heterocycles, as unstable intermediates, which are reviewed here. These applications feature the ability of selenium to be readily extruded from seleniranes and selenirenes, neighboring group participation by / -Se to control the stereochemistry of nucleophilic substitution reactions, and facile, chemoselective replacement of Se by H in radical-induced reactions. 

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