Michael addition reaction retro

Several studies have shown that the amide bonds that comprise the PAM AM dendrimer backbone are relatively unstable and begin decomposing at temperatures as low as 75 °C [45,50,52,56-58]. The low onset temperature of dendrimer decomposition is not surprising given that PAMAM den-drimers can undergo retro-Michael addition reactions at temperatures above 100 °C [16]. Far more forcing conditions are required to fully activate the catalysts, which suggests that the dendrimer decomposes into various surface species that continue to poison the nanoparticle surfaces. 

The reaction starts with the formation of a mixed anhydride and an acetate on treatment with an excess of acetic anhydride at 80 °C. There follows a Dieckmann condensation to give 2-590 and an intramolecular rearrangement/Michael addi-hon/retro Michael addition to afford the desired tetracyclic compound 2-592 via 2-591 in an overall yield of remarkable 92%. 

The retrosynthesis involves the following transformations i) isomerisation of the endocyclic doble bond to the exo position ii) substitution of the terminal methylene group by a more stable carbonyl group (retro-Wittig reaction) iii) nucleophilic retro-Michael addition iv) reductive allylic rearrangement v) dealkylation of tertiary alcohol vi) homolytic cleavage and functionalisation vii) dehydroiodination viii) conversion of ethynyl ketone to carboxylic acid derivative ix) homolytic cleavage and functionalisation x) 3-bromo-debutylation xi) conversion of vinyl trimethylstannane to methyl 2-oxocyclopentanecarboxylate (67). 

As in the uncatalyzed reactions with enamines (vide supra), there is potentially more than one point where stereochemical differentiation can occur (Scheme 59). Selectivity can occur if the initial addition of the enol ether to the Lewis acid complex of the a,/J-unsaturated acceptor (step A) is the product-determining step. Reversion of the initial adduct 59.1 to the neutral starting acceptor and the silyl enol ether is possible, at least in some cases. If the Michael-retro-Michael manifold is rapid, then selectivity in the product generation would be determined by the relative rates of the decomposition of the diastereomers of the dipolar intermediate (59.1). For example, preferential loss of the silyl cation (or rm-butyl cation for tert-butyl esters step B) from one of the isomers could lead to selectivity in product construction. Alternatively, intramolecular transfer of the silyl cation from the donor to the acceptor (step D) could be preferred for one of the diastereomeric intermediates. If the Michael-retro-Michael addition pathway is rapid and an alternative mechanism (silyl transfer) is product-determining, then the stereochemistry of the adducts formed should show little dependence on the configuration of the starting materials employed, as is observed. 

Another aspect to Rogers s work on the synthesis of unsymmetrical hydroxylamines (Section 4.2.5.1) is that the elimination step is base catalyzed (occurring at room temperature).16 Salts of amine oxides derived from P-aminopropionic esters 45 or nitriles 46 undergo the reaction, which involves a retro-Michael addition, facilitated by the formal positive charge on nitrogen. 

The proposed reaction mechanism involves initially the activation of cyclohexenone by the thiourea group and subsequently a Michael addition of the tertiary amine at the p-position. The resulting enolate intermediate attacks the aldehyde performing an aldol reaction. Finally, a retro-Michael addition releases the catalyst to afford the product (Scheme 19.22). This mechanism supports the experimental results of the authors diethyl analogue 16b showed similar enantioselectivities, but significant lower yield for the reaction between 2-cyclohexen-l-one and 3-phenylpropionaldehyde, presumably because of the difficulty of the amine to perform the Michael addition due to confined space in the presence of the more flexible ethyl substituents. 

After the failure of the intramolecular 8 2 attack strategy, intramolecular oxa-Michael strategy was used to test the seven-membered ring formation. The retro-synthetic analysis is shown in Fig. 3.21. Key intermediate 3.38 was inversed to its analog 3.46 while compound 3.46 could be synthesized from compound 3.47 through an intramolecular oxa-Michael addition reaction. 

The reaction of 11 with ordinary alkenes with a Cu(OTf)2 catalyst gave 1,2-dihydronaphthalenes. However, reaction with p-methoxystyrene 8 afforded the naphthalene derivative 12 in 83% yield. Probably, retro-Michael addition occurred and the methoxy group was eliminated as methanol [24], This protocol can be applied to the formation of benzo-fused heteroaromatic compounds such as indols 35 and benzofuranes (Scheme 15.13) [25]. 

In the classical Morita-Baylis-Hillman (MBH) reaction an a,P-unsaturated ester (electrophilically activated alkene), is activated by the reversible Michael-addition of a tertiary amine catalyst (e.g. DABCO), producing a zwitterion intermediate, the enolate moiety of which can react with an aldehyde to form an aldolate zwitterion. Retro-Michael-addition then regenerates the catalyst and the MBH-product (Scheme 7.22). The catalyst is sometimes used in high amounts (over stoichiometric) and often the reaction is very sensitive to the Michael acceptor used. 

These amines gave, with methyl propiolate, products of Michael mono- and bis-addition. Adducts underwent further reaction leading to triazolo[4,5-/]quinolones 181, after retro Diels-Alder reaction and acetylene elimination to its methoxycar-... 

The yield of the cyclization step under the influence of a metal template can be increased when the corresponding dialdehyde 19 of the tetrapyrrole 16 is used. The reaction sequence is initiated by insertion of palladium(II) or nickel(II) into the tetrapyrrole to give 20 followed by Michael addition of one acrylaldehyde side chain to the other yielding the macrotetracycle 21 from which in a retro-Michael reaction acetaldehyde is eliminated to give 22. 

Another example of a [4S+1C] cycloaddition process is found in the reaction of alkenylcarbene complexes and lithium enolates derived from alkynyl methyl ketones. In Sect. 2.6.4.9 it was described how, in general, lithium enolates react with alkenylcarbene complexes to produce [3C+2S] cycloadducts. However, when the reaction is performed using lithium enolates derived from alkynyl methyl ketones and the temperature is raised to 65 °C, a new formal [4s+lcj cy-clopentenone derivative is formed [79] (Scheme 38). The mechanism proposed for this transformation supposes the formation of the [3C+2S] cycloadducts as depicted in Scheme 32 (see Sect. 2.6.4.9). This intermediate evolves through a retro-aldol-type reaction followed by an intramolecular Michael addition of the allyllithium to the ynone moiety to give the final cyclopentenone derivatives after hydrolysis. The role of the pentacarbonyltungsten fragment seems to be crucial for the outcome of this reaction, as experiments carried out with isolated intermediates in the absence of tungsten complexes do not afford the [4S+1C] cycloadducts (Scheme 38). 

Besides the Michael addition-initiated domino reactions presented here, a multitude of other anionic domino reactions exist. Many of these take advantage of an incipient SN-type reaction (for a discussion, see above). In addition to the presented SN/Michael transformations [97, 98, 100], a SN/retro-Dieckmann condensation was described by Rodriguez and coworkers, which can be used for the construction of substituted cycloheptanes as well as octanes [123]. Various twofold SN-type domino... 

Crosslinking of amine- or hydroxy-terminated PAMAM dendrimers using cyclic anhydride - amine or cyclic anhydride - hydroxy addition reactions was employed for preparation of crosslinked thin films of very low permeability [73], Polyanhydrides, such as maleic anhydride-methyl vinyl ether copolymers, were used as crosslinking components. In the case of amine-terminated PAMAM, crosslinking and chemical stability were further increased by imidization of the maleamic acid groups retro-Michael eliminations were followed by Michael additions to further crosslink the film. 

The regioselectivity of Michael additions of thiolates to 2,4-dienones can be altered drastically by variation of the reaction conditions and addition of Lewis acids to the reaction mixture. Lawton and coworkers examined the reaction of 2-mercaptoethanol with l-(3-nitrophenyl)-2,4-pentadien-l-one and observed a high regioselectivity in favor of the 1,6-addition product at 45 °C (equation 42)123,124. Lowering of the reaction temperature caused an increase in the amount of 1,4-adduct, and at —40°C, a product ratio of 40 60 was found. These events suggest that kinetic control favors the 1,4-addition product whereas the 1,6-adduct is thermodynamically more stable. If, however, the reaction was carried out with a complex of the dienone and titanium tetrachloride, only the 1,4-adduct was isolated after hydrolytic workup123. Obviously, this product is trapped as a metal chelate which prevents formation of the 1,6-adduct by retro-Michael/Michael addition. In the absence of the chelating Lewis acid, the 1,4-addition product can indeed be converted... 

The structural similarity between claenone (42) and stolonidiol (38) enabled Yamada to exploit an almost identical strategy for the total synthesis of (-)-stolonidiol (38) [40]. A short retrosynthetic analysis is depicted in Fig. 12. An intramolecular HWE reaction of 68 was successfully applied for the macrocyclization. The highly substituted cyclopentanone 69 was made available by a sequence that is highlighted by the sequential Michael-Mi-chael addition between the enolate 53 and the a, -unsaturated ester 70 followed by a retro-aldol addition. However, as is the case for the claenone (42) synthesis, the synthesis of stolonidiol (38) is characterized by numerous functional and protecting group transformations that are a consequence of Yamada s synthetic strategy. 

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