Heck-dihydroxylation reaction

Scheme 5.4 Example of a tandem Heck asymmetric dihydroxylation reaction.

An interesting combination is that of a Heck coupling with an enantioselective dihydroxylation reaction. To achieve this, a bifunctional catalyst consisting of active palladium and osmium species anchored on silica gel through a mercaptopropyl spacer, and a cinchona alkaloid, respectively, was prepared and applied using N-methylmorpholine N-oxide as a cooxidant This one-pot process of alkene formation and subsequent dihydroxylation afforded diols of type 282 in excellent yields and with high enantiomeric excesses (Scheme 8.59) [385]. 

This catalytic system allows three independent transformations to occur in sequence the Heck reaction, N-oxidation and asymmetric dihydroxylation (AD). The mechanism of the Heck reaction is discussed in the previous section. Here we take a closer look at the last two steps. They are coupled processes, based on the Sharpless asymmetric dihydroxylation reaction [22, 23]. Several recent reviews on Sharpless asymmetric dihydroxylation cover the general synthetic aspects [24-27], together with methods for immobilization of the osmium catalysts [28]. 

An orf/io-directed lithiation allows the conversion of 25 to aryl iodide 40. Reductive ether formation of aldehyde 40 with crotyl alcohol yields compound 41. Intramolecular Heck reaction of 41 affords a mixture of the olefins 42 and 43. The undesired alkene 42 can be isomer-ized quantitatively to the desired enol ether 43 with Wilkinson s catalyst. Sharpless dihydroxylation ee 94 %) of the enol ether 43 provides lactol 44, which is oxidized directly to lactone 45. Finally, the pyridone-O-methyl ester is cleaved under acid conditions (45 — 7). 

Ionic solvents have been used to facilitate the Heck reaction (13-9), the oxidation of alcohols with hypervalent iodine reagents (19-3)," and the catalytic asymmetric dihydroxylation of olefins (15-48) using a recoverable and reusable osmium/... 

It has been shown that supported acrylates could serve as partners in various reactions such as Diels Alder, Heck coupling followed by dihydroxylation of resulting... 

Sequential Heck reaction and hydrosilylation of carbonyl have been carried out with the Grubbs I catalyst. AUyUc alcohols can be S3mthesized via conjugated carbonyl compounds prepared from cross-metathesis in situ by reduction with j-Bu2AlH. An access to enantiomeric 2,3-dihydroxyalkanoic esters is based on cross-metathesis, dihydroxylation and methanolysis. ... 

The concept of SI L catalyst has been developed quickly in the last decade. Holderich et al. [4] added acidic chloroaluminate ILs to various types of supports, and the catalytic activities of the immobilized ILs were found to be higher than those of the conventional catalysts under the same conditions. Inspired by this work, SIL catalysts have been widely used in the coupling reactions for olefin hydroformylation [5], olefin metathesis [6], Heck reactions [7], and hydroamination [8], and so on. SIL catalytic systems have also been reported for some other reactions, such as water-gas shift reaction [9], dihydroxylation of olefins [10], and hydrogenation [1 Ij. The solid supports used include magnetic NPs [12], mesoporous molecular sieves [13], soluble organic ions [14], noncovalently solid-phase [15], IL-functionalized carbon nanotubes [16], polymer cocktail [17], and so on. 

The first asymmetric synthesis of (20 S)-camptothecin using catalytic asymmetric induction was achieved by Fang et al. in 1994 [74], They carried out a catalytic enantioselective synthesis of Comins s intermediate (23) in order to avoid the use of the expensive chiral auxiliary, 8-phenylmenthol, or similar compound. Intramolecular Heck reaction of pyridine derivative (26) gave the cyclic olefins (27) and (28) in a ratio 1 8. The allylic ether (27) can be isomerized to (28) upon treatment with Wilkinson s catalyst [75], Asymmetric Sharpless dihydroxylation of (28) proceeded successfully when 2,5-diphenyl-4,6-bis(9-0-dihydroquinidyl)pyrimidine [(DHQD)2-PYR] was used as the chiral catalyst [76], and subsequent oxidation gave (29) in 94% ee. Treatment of (29) with acid gave the target molecule (23, Scheme 2.5), which was converted to (20S)-camptothecin in 2 steps using the Comins s procedure [73]. 

Allylic esters are among the most versatile substrate in organic synthesis. They are popular substrates for many transition metal-catalyzed reactions, such as dihydroxylation, the Tsuji-Trost reaction, hydrogenation, and isomerization, inter alia In this context, the Heck arylation can be considered a privileged reaction for the formation of such esters in a regio- and stereoselective way. However, this transformation has been a challenging one due to the... 

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