Amberlyst Michael addition

The heterogeneous catalytic systems have some advantages over homogeneous re Chemical transformations under heterogeneous conditions can occur v/ith better efficiencies, higher purity of products, and easier work-up Balliniandcoworkers have found that commercial amberlyst A-27 is the best choice for the Michael addition of nittoalkanes v/ith fi-subsdnited alkene acceptors fEq 4111 The reacdon is also carried out by potassium carbonate in the presence of Aliquat 336 under idttasonic irradiadon fEq 4 112 ... 

Die Verwendung von Amberlyst-A21 als heterogenem Katalysator fiihrt bei der Michael-Addition ohne Einsatz von Losungsmitteln ebenfalls zu guten Ergebnissen4. 

Keywords nitroalkane, olefine, Michael addition, Amberlyst A-27, nitro compound... 

The iminothiazocine 583 was synthesized from the ethyl ester 580 by Michael addition to acrylonitrile in DMF to give the nitrile 581 which, treated with EtOH saturated with HC1, gave the iminoether 582. Cyclization of 582 upon treatment with Amberlyst A-21 ion exchange resin gave the 1,5-thiazocine 583 (Scheme 115). 

Recent examples, for instance, of the catalytic application of the commercially available macroporous Amberlyst-15 include the Michael addition of pyrroles to a,P-unsaturated ketones (Scheme 10.4) [48]. In this process, the acid ion exchange resin (dry, 10% w/w) allows on to obtain mono and dialkylated pyrroles 5 and 6 in reasonable yields. Similarly, this catalyst (dry, 30% w/w) can catalyze the aza-Michael reaction of amines with a,P-unsaturated ketones, esters and nitriles to afford 7 in 75-95% yields under solvent-free conditions. Interestingly, yields were significantly lower using typical solvents such as DCM (dichloromethane), CH3CN, THF, DMF or EtOH [49], Recycling the catalyst is possible in both cases, but a smooth decrease in the yield is observed for each new run. 

The synthesis of the C1-C7 fragment, which corresponds to the lactone, starts with the homoallylic alcohol 2 which was prepared from 1. The existing stereocenter and the conjugate addition method of Evans [21] allow the control of the C5 stereocenter. The homoallylic alcohol 2 was oxidatively cleaved and homologated to the trans enoate 3 by a Wittig olefination. Treatment of 3 with benzaldehyde and a catalytic amount of KHMDS provided acetal 4. The internal Michael addition of the hemiacetal intermediate proceeds with complete stereoselectivity [22]. After deprotection and oxidation, the corresponding aldehyde was treated with Amberlyst-15 and then with camphor sulfonic acid (CSA), to yield pyrane 5 as a mixture of (3- and a-anomers (1.8/1). This compound was converted to the thiophenyl acetal 6 (4 steps) as this compound can be hydrolyzed later under mild conditions (Hg +) with subsequent oxidation of the lactol to the desired lactone. Compound 6 represents the C1-C7 fragment of discodermolide (Scheme 1). 

F -Anion exchange resins. Fluoride ion immobilized on strongly basic anion exchange resins, particularly Amberlyst A27 and Dowex MSA-1, promote the various reactions that have been found to be promoted by alkali metal or tetraalkylammonium fluorides, such as C- and O-alkylation, sulfenylation, and Michael additions. 

Other examples of Michael additions are the reaction of nitroolefins with acetone on L-proline-modified Mg-Al mixed oxides (51) or with diethyl malonate on metalorganic MCM-41 (52), or the addition of nitromethane to unsaturated carbonyl compounds on KF-modified alumina (53). Furthermore, solid catalysts such as montmorillonite clays and hydroxyapatite have recently been employed for the construction of S—C bonds by /lia-Michael addition of thiols to unsaturated ketones and other carbonyl compounds (54,55). Similarly, aza-Michael additions of amines to alkenes activated by -CN, -COOR, or -COR groups, lead to C—N bond containing products on Amberlyst-15 or polyaniline supported Cul (56,57). 

A procedure by using enamino ester and alkynone as the substrates was developed as well [2]. 2,3,4,6-Tetrasubstituted pyridines were prepared in a single step. Various acids, such as acetic acid, Amberlyst 15 ion exchange resin, zinc(II) bromide or ytterbium(III) triflate, can be applied as promoter for the cycUzation step of the Michael addition adduct. 4-(3-Oxoalkyl)isoxazoles found could be applied as starting material for pyridine synthesis as well [3],... 

Alkylation of j8-diketones using polymers as supports for the intermediate enolate anion has also been reported (Gelbard and Colonna, 1977). Reaction of several cyclohexyl j3-diketones with Amberlite IRA-9(K) formed a resin-linked j8-diketonate that could be readily alkylated. Similarly, it was shown that alkylation of phenoxide anions can be readily effected when the anions are supported via an ionic bond with the resin (Gelbard and Colonna, 1977). Alkylations of /3-diketones were also shown to occur if a fluoride-substituted, strongly basic resin (Amberlyst A26, A27, or Dowex MSA-1) is used (Miller et at., 1978). In this case, the presence of the fluoride ion was necessary before reaction would proceed. Considering the report on the use of Amberlite IRA-900— a very similar resin— the necessity for a fluoride ion is puzzling. In the same publication, the 0-alkylation of phenols, the sulfenylation of /3-diketones, and the Michael addition of a thiol to an ot,/8-unsaturated ketone were also investigated. 

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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

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