1.3- diketones Michael reactions

The Michael reaction is of central importance here. This reaction is a vinylogous aldol addition, and most facts, which have been discussed in section 1.10, also apply here the reaction is catalyzed by acids and by bases, and it may be made regioselective by the choice of appropriate enol derivatives. Stereoselectivity is also observed in reactions with cyclic educts. An important difference to the aldol addition is, that the Michael addition is usually less prone to sterical hindrance. This is evidenced by the two examples given below, in which cyclic 1,3-diketones add to o, -unsaturated carbonyl compounds (K. Hiroi, 1975 H, Smith, 1964). 

A synthetically useful reaction known as the Michael reaction, or Michael addition, involves nucleophilic addition of carbanions to a p unsaturated ketones The most common types of carbanions used are enolate 10ns derived from p diketones These enolates are weak bases (Section 18 6) and react with a p unsaturated ketones by conjugate addition... 

Stabilized anions exhibit a pronounced tendency to undergo conjugate addition to a p unsaturated carbonyl compounds This reaction called the Michael reaction has been described for anions derived from p diketones m Section 18 13 The enolates of ethyl acetoacetate and diethyl malonate also undergo Michael addition to the p carbon atom of a p unsaturated aldehydes ketones and esters For example... 

The Michael reaction occurs with a variety of a,/3-unsaturated carbonyl compounds, not just conjugated ketones. Unsaturated aldehydes, esters, thio-esters, nitriles, amides, and nitro compounds can all act as the electrophilic acceptor component in Michael reactions (Table 23.1). Similarly, a variety of different donors can be used, including /3-diketones, /3-keto esters, malonic esters, /3-keto nitriles, and nitro compounds. 

A Michael reaction involves the conjugate addition of a stable enolate ion donor to an o,/3-unsaturated carbonyl acceptor, yielding a 1,5-dicarbonyl product. Usually, the stable enolate ion is derived from a /3-diketone, jS-keto ester, malonic ester, or similar compound. The C—C bond made in the conjugate addition step is the one between the a carbon of the acidic donor and the (3 carbon of the unsaturated acceptor. 

Figure 23.8 The Stork reaction between cyclohexanone and 3-buten-2-one. Cyclohexanone is first converted into an enamine, the enamine adds to the a -unsaturated ketone in a Michael reaction, and the conjugate addition product is hydrolyzed to yield a 1,5-diketone.

The Robinson annulation is a two-step process that combines a Michael reaction with an intramolecular aldol reaction. It takes place between a nucleophilic donor, such as a /3-keto ester, an enamine, or a /3-diketone, and an a,/3-unsaturated ketone acceptor, such as 3-buten-2-one. The product is a substituted 2-cyclohexenone. 

The first step of the Robinson annulation is simply a Michael reaction. An enamine or an enolate ion from a jS-keto ester or /3-diketone effects a conjugate addition to an a-,/3-unsaturated ketone, yielding a 1,5-diketone. But as we saw in Section 23.6,1,5-diketones undergo intramolecular aldol condensation to yield cyclohexenones when treated with base. Thus, the final product contains a six-membered ring, and an annulation has been accomplished. An example occurs during the commercial synthesis of the steroid hormone estrone (figure 23.9). 

In this example, the /3-diketone 2-methyJ-l,3-cyclopentanedione is used to generate the enolate ion required for Michael reaction and an aryl-substituted a,/3-unsaturated ketone is used as the acceptor. Base-catalyzed Michael reaction between the two partners yields an intermediate triketone, which then cyclizes in an intramolecular aldol condensation to give a Robinson annulation product. Several further transformations are required to complete the synthesis of estrone. 

The conjugate addition of a carbon nucleophile to an a./3-unsiituratcd acceptor is known as the Michael reaction. The best Michael reactions take place between unusually acidic donors (/3-keto esters or /3-diketones) and unhindered n,/3-unsaturated acceptors. Knamines, prepared by reaction of a ketone with a disu Instituted amine, are also good Michael donors. 

There are only few examples of organic reactions catalysed effectively by Lewis acids which can be carried out in pure water without any organic co-solvent. While water can be used successfully for the uncatalysed Michael addition of 1,3-diketones (Table 4, entry D)22, the corresponding reaction of /i-kctocsters does not give satisfactory results. On the other hand, the Yb(OTf)3 catalysed Michael reaction of various /i-ketoesters (Table 21, entry A)257 and a-nitroesters (Table 21, entry B)258 takes place. 

These are reacted together in basic solution. It can be deduced that the 1,3-diketone is more acidic than the monoketone substrate, so will be ionized by removal of a proton from the carbon between the two carbonyls to give the enolate anion as a nucleophile. This attacks the a,P-unsaturated ketone in a Michael reaction. It is understandable that this large nucleophile prefers to attack the unhindered -position rather than the more congested ketone carbonyl. 

Sodeoka and coworkers reported a chiral Pd-catalyzed enantioselective Michael reaction of 1,3-diketones and (3-ketoesters to enones, [Eq. (13.18)]. It was proposed that the catalyst 29 exists in equilibrium with 30 through liberating H2O... 

Aldol reactions are often used to close five- and six-membered rings. Because of the favorable entropy (p. 211), such ring closures generally take place with ease, even where a ketone condenses with a ketone. An important example is the Robinson annulation reaction which has often been used in the synthesis of steroids and terpenes. In this reaction a cyclic ketone is converted to another cyclic ketone, with one additional six-membered ring containing a double bond. The substrate is treated with methyl vinyl ketone (or a simple derivative of methyl vinyl ketone) and a base.551 The enolate ion of the substrate adds to the methyl vinyl ketone in a Michael reaction (5-17) to give a diketone that undergoes or... 

If we combine decarboxylation with Michael reaction (frames 281-5) we get a general synthesis of 1,5—diketones ... 

Michael reaction. In the presence of (C6H5)3CC104, silyl enol ethers undergo Michael addition to a,p-enones. The adducts can be isolated or rearranged to 1,5-diketones by base. The intermediates cannot be isolated from reactions catalyzed by TiCl4 or CsF. 

When subjected to the action of propanethiol under basic conditions (pH 9.2), jatrophone (2) undergoes a Michael reaction across its C8-C9 double bond, followed by facile transannular cyclization to give the tetracyclic diketone 3.2,9 The susceptibility of this enone part structure to conjugate addition has been proposed to constitute the event responsible for the pronounced biological activity of 2.9... 

The kinetic control approach to constructing the CD-spiroacetal segment envisaged a hetero-Michael cyclisation of the dihydropyrone 28 (derived from 26 by sequential oxidation to the P-diketone, PMB removal and cyclisation to provide the D-ring), where axial attack might be favoured (Scheme 7). In practice, treatment of 28 with DBU led to installation of the C-ring via hetero-Michael reaction, with a small preference (60 40) for formation of the desired spiroacetal 29 over 30. Despite the modest selectivity observed in this mode of spiroacetal formation, the endeavour highlighted an important new means for... 

The preparation of (83) (Expt 8.29) is an example of the Hantzsch pyridine synthesis. This is a widely used general procedure since considerable structural variation in the aldehydic compound (aliphatic or aromatic) and in the 1,3-dicarbonyl component (fi-keto ester or /J-diketone) is possible, leading to the synthesis of a great range of pyridine derivatives. The precise mechanistic sequence of ring formation may depend on the reaction conditions employed. Thus if, as implied in the retrosynthetic analysis above, ethyl acetoacetate and the aldehyde are first allowed to react in the presence of a base catalyst (as in Expt 8.29), a bis-keto ester [e.g. (88)] is formed by successive Knoevenagel and Michael reactions (Section 5.11.6, p. 681). Cyclisation of this 1,5-dione with ammonia then gives the dihydropyridine derivative. Under different reaction conditions condensation between an aminocrotonic ester and an alkylidene acetoacetate may be involved. 

The a, p-unsaturated diketone moiety in the curcuminoids is a Michael reaction acceptor, which belongs to the major class of phase II enzyme inducers... 

Scheme 8.14 Michael reaction of p-diketones (X = alkyl) and p-oxo esters (X = alkoxy) with methyl vinyl ketone (41a, MVK).

According to the classical Hantzsch synthesis of pyridine derivatives, an a,(5-unsaturated carbonyl compound is first formed by Knoevenagel condensation of an aldehyde with a P-dicarbonyl compound. The next step is a Michael reaction with another equivalent of the P-dicarbonyl compound (or its enamine) to form a 1,5-diketone, which finally undergoes a cyclocondensation with ammonia to give a 1,4-dihydropyridine with specific symmetry in its substitution pattern. 

Combining aldol and Michael reactions in one sequence is very powerful, particularly if one of the reactions is a cyclisation. The Robinson annelation9 makes new rings in compounds like 73 that were needed to synthesise steroids. Disconnection of the enone reveals triketone 74 having 1,3- and 1,5-dicarbonyl relationships. The 1,3-disconnection would not remove any carbon atoms but the 1,5-disconnection at the branchpoint gives a symmetrical 3-diketone that should be good at conjugate addition. 

An efficient preparation of 1,5-diketones as precursors to D-ring annulated heterosteroids was elaborated by R.C. Boruah et al. [96] (Scheme 24). Readily available 16-dehydropregnenolone acetate (16-DPA) was used in a Michael reaction with enamines. 

Our retrosynthetic analysis of generalised pyridine 5.4 commences with an adjustment of the oxidation level to produce dihydropyridine 5.5. This molecule can now be disconnected very readily. Cleavage of the carbon-heteroatom bonds in the usual way leaves dienol 5.6 which exists as diketone 5.7. The 1,5-dicarbonyl relationship can be derived from a Michael reaction of ketone 5.8 and enone 5.9, which in turn can arise from condensation of aldehyde 5.10 and ketone 5.11. 

Apart form the aforementioned highly enantioselective hetero-Diels-Alder reactions, that proceed with very low catalyst loadings, the catalytically accessible enolates have also been used for related intramolecular Michael reactions (Philips et al. 2007) and for the desym-metrization of 1,3-diketones yielding cyclopentenes via an intramolecular aldol reaction (Wadamoto et al. 2007). The formation of cyclopentenes, however, presents a special case, so—depending on the stereochemical nature of the enone substrates (s-cis or s-trans) and the stereochemistry of the final products—two different mechanisms are discussed in the literature. Whereas /ran.v-cycl open (cries are proposed to be available upon conjugate addition of a homoenolate to chalcones,... 

The Robinson annulation is a combination of two reactions covered in this chapter. First, a Michael reaction takes place between a nucleophilic donor (the diketone in this problem) and an a,(3-unsaturated carbonyl compound (the enone shown). The resulting product can cyclize in an aldol reaction. The base catalyzes both reactions. 

This sequence of reactions consists of an alkylation of a 1,3-diketone, followed by a Robinson annulation. The carbon-carbon double bond appears where the second carbonyl group of the diketone used to be and is the site of the ring-forming aldol reaction. A Michael reaction between the diketone and the Michael acceptor 3-buten-2-one adds the carbon atoms used to form the second ring, and an alkylation with CH3I adds the methyl group. 

When you need to synthesize a p-hydroxy ketone or aldehyde or an a,p-unsaturated ketone or aldehyde, use an aldol reaction. When you need to synthesize a p-diketone or p-keto ester, use a Claisen reaction. When you need to synthesize a 1,5-dicarbonyl compound, use a Michael reaction. The Robinson annulation is used to synthesize polycyclic molecules by a combination of a Michael reaction with an aldol condensation. 

Michael reaction (Section 23.10) the 1,4-addition reaction of a stabilized enolate anion such as that from a 1.3-diketone to an a,p-unsaturated carbonyl compound. 

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