Michael adduct

If a Michael reaction uses an unsymmetrical ketone with two CH-groups of similar acidity, the enol or enolate is first prepared in pure form (p. llff.). To avoid equilibration one has to work at low temperatures. The reaction may then become slow, and it is advisable to further activate the carbon-carbon double bond. This may be achieved by the introduction of an extra electron-withdrawing silyl substituent at C-2 of an a -synthon. Treatment of the Michael adduct with base removes the silicon, and may lead as well to an aldol addition (G. Stork, 1973, 1974 B R.K. Boeckman, Jr., 1974). 

Glycosidic thiol groups can be introduced into glycosyl bromides by successive reactions with thiourea and aqueous sodium disulfite (D. Horton, 1963 M. Cemy, 1961, 1963). Such thiols are excellent nucleophiles in weakly basic media and add to electrophilic double bonds, e.g., of maleic esters, to give Michael adducts in high yields. Several chiral amphiphiles have thus been prepared without any need for chromatography (J.-H. Fuhrhop, 1986 A). 

Alkenyl chlorides are generally not very reactive, bnt vinyl chloride is exceptionally reactive and its carbonyiation with NH3 at 100 "C gave the Michael adduct of acrylamide 506 in high yields[360]. 

After isolation the Michael adduct may be subjected to ester hydrolysis and decar boxylation When a p unsaturated ketones are carried through this sequence the final products are 5 keto acids (8 keto acids)... 

An asymmetric synthesis of estrone begins with an asymmetric Michael addition of lithium enolate (178) to the scalemic sulfoxide (179). Direct treatment of the cmde Michael adduct with y /i7-chloroperbenzoic acid to oxidize the sulfoxide to a sulfone, followed by reductive removal of the bromine affords (180, X = a and PH R = H) in over 90% yield. Similarly to the conversion of (175) to (176), base-catalyzed epimerization of (180) produces an 85% isolated yield of (181, X = /5H R = H). C8 and C14 of (181) have the same relative and absolute stereochemistry as that of the naturally occurring steroids. Methylation of (181) provides (182). A (CH2)2CuLi-induced reductive cleavage of sulfone (182) followed by stereoselective alkylation of the resultant enolate with an allyl bromide yields (183). Ozonolysis of (183) produces (184) (wherein the aldehydric oxygen is by isopropyUdene) in 68% yield. Compound (184) is the optically active form of Ziegler s intermediate (176), and is converted to (+)-estrone in 6.3% overall yield and >95% enantiomeric excess (200). 

The reactions of pyrroles with dimethyl acetylenedicarboxylate (DMAD) have been extensively investigated. In the presence of a proton donor the Michael adducts (125) and (126) are formed. However, under aprotic conditions the reversible formation of the 1 1 Diels-Alder adduct (127) is an important reaction. In the case of the adduct from 1-methylpyrrole, reaction with a further molecule of DMAD can occur to give a dihydroindole (Scheme 48) (82H(19)1915). 

Further investigations revealed that the disubstituted indolo[3,2-fl]carbazoles 117a-b may be conveniently obtained in moderate yields simply by heating 115 with dimethyl or diethyl acetylenedicarboxylate in xylene, while the use of lower-boiling solvents (benzene, toluene) led to the formation of the presumed intermediate Michael adducts 118a-b as the major products. The reaction of 115 with 2-chloroacrylonitrile furnished the indolo[3,2-fl]carbazole derivative 119 where aromatizaticRi did not take place (99T2371). 

Indolo[3,2-fl]pyrrolo[3,4-c]carbazoles 120 have been obtained in one step from indole and the corresponding maleimides in acetic acid, with coformation of the Michael adducts 121 (Scheme 15). This reactitai required careful temperature control in order to obtain the desired product ratios. An alternative independent synthesis of compounds 120 could also be accompKshed from 2,3 -biindolyl (115) andsuitable maleimides in hot acetic acid (99T2363). The system 120 where R = H has also been reported as a minor product during studies toward a synthesis of the alkaloid arcyriaflavin A (95TL2689). 

Addition of p-tert-butylthiophenol 178 to the racemic furanone 168 in dry toluene, and in the presence of quinidine as a chiral catalyst, provided (/ )-168 together with the Michael adduct 179. The enantiomeric excess of the recovered furanone (R)-168 was determined via the addition of (/)-Q -methylbenzylamine This amine addition showed complete diastereofacial control to give the adduct 180 in quantitative yield (Scheme 50) (94T4775). 

Reaction of the mesoionic oxazolone 620 with acetylenedicarboxylic ester 621 gave the cycloadduct 622 in aprotic solvents and the Michael adducts... 

The reaction of conjugated nitroalkenes v/ith a,fi-unsaturated esters, ketones, nitnles, and sulfones is catalyzed by TMG to give the Michael adduct of allyiic nitro compounds fEq. 4.108. -... 

The net effect of the Stork reaction is the Michael addition of a ketone to an cn/3-unsaturated carbonyl compound. For example, cyclohexanone reacts with the. cyclic amine pyrrolidine to yield an enamine further reaction with an enone such as 3-buten-2-one yields a Michael adduct and aqueous hydrolysis completes the sequence to provide a 1,5-diketone (Figure 23.8). 

The aldehyde or ketone functionalities in the Michael adducts are restored by ozonolysis of the hydrazone moiety resulting in am/-3,4-disubstituted-5-oxoalkanoates 1. 

The addition of a-(acylamino) esters to 3-aryl-2-propenoates, with sodium ethoxide in ethanol or sodium hydride in benzene as base, is a frequently ultilized procedure9-" A The initial Michael adducts cyclize to 3-aryl-5-oxo-2-pyrrolidinecarboxylic acids with modest to high trans diastereoselectivities 10°. 

A very efficient method for annulations158 is based on the addition of lithium or silyl enolates to a-silylated enones as a key step. The diastereoselective 1,4-addition is followed by an aldol condensation. This procedure allows Michael additions under aprotic conditions, whereby the silyl substituent stabilizes the enolate of the Michael adduct preventing polymerization of the enone, 59 l63. 

Oxo esters are accessible via the diastereoselective 1,4-addition of chiral lithium enamine 11 as Michael donor. The terr-butyl ester of L-valine reacts with a / -oxo ester to form a chiral enamine which on deprotonation with lithium diisopropylamide results in the highly chelated enolate 11. Subsequent 1,4-addition to 2-(arylmethylene) or 2-alkylidene-l,3-propanedioates at — 78 °C, followed by removal of the auxiliary by hydrolysis and decarboxylation of the Michael adducts, affords optically active -substituted <5-oxo esters232 (for a related synthesis of 1,5-diesters, see Section 1.5.2.4.2.2.1.). In the same manner, <5-oxo esters with contiguous quaternary and tertiary carbon centers with virtually complete induced (> 99%) and excellent simple diastereoselectivities (d.r. 93 7 to 99.5 0.5) may be obtained 233 234. 

Consecutive Michael additions and alkylations can also be used for the diastereoselective synthesis of 5- and 6-membered ring systems. For instance when 6-iodo-2-hexenoates or 7-iodo-2-heptenoates are employed the enolate of the Michael adduct is stereoselectively quenched in situ to provide the cyclic compound with trans stereochemistry (>94 6 diastereomeric ratio). As the enolate geometry of the Michael donor can be controlled, high stereoselectivity can also be reached towards either the syn or anti configuration at the exocyclic... 

The reductive cleavage of the alkylcobalamine is facilitated by light irradiation and can then proceed at a much more positive potential. A demonstration photoelec-trochemical reactor for the Bij-catalyzed photoelectrochemical synthesis of Michael adduct 17, the alarm pheromone of the ant atta texana (Scheme 9) has been constructed where the complete device is driven solely by solar energy . Hopefully, mediated photoelectrochemical reactions of this type will also be realized at chemically modified electrodes. 

Surprisingly, the 7t-system geometry in a substrate has a notable influence in the enzymatic aminolysis of esters. The reaction of diethyl fumarate with different amines or ammonia in the presence of CALB led to the corresponding trans-amidoesters with good isolated yields, but in the absence of enzyme, a high percentage of the corresponding Michael adduct is obtained (Scheme 7.9). Enzymatic aminolysis of diethyl maleate led to the recovery of the same a, P-unsaturated amidoester, diethyl fumarate, and diethyl maleate. The explanation of these results can be rationalized via a previous Michael/retro-Michael type isomerization of diethyl maleate to fumarate, before the enzymatic reaction takes place. In conclusion, diethylmaleate is not an adequate substrate for this enzymatic aminolysis reaction [23]. 

The mechanism of formation of pyridines from a,P-unsatuiated nitriles and active cyano compounds has been investigated. These processes proceed through a Michael adduct which undergoes a regioselective cyclization to the corresponding pyridine <96H(43)33>. 

Thiolates, generated in situ by the action of ammonium tetra-thiomolybdate on alkyl halides, thiocyanates, and disulfides, undergo conjugate addition to a, (1-unsaturatcd esters, nitriles, and ketones in water under neutral conditions (Eq. 10. II).29 Conjugate addition of thiols was also carried out in a hydrophobic ionic liquid [bmim]PF6/water-solvent system (2 1) in the absence of any acid catalyst to afford the corresponding Michael adducts in high to quantitative yields with excellent 1,4-selectivity under mild and neutral conditions (Eq. 10.12). The use of ionic liquids helps to avoid the use of either acid or base catalysts... 

Scheme 5.5 The dual catalyst system gives access to intermediate 4, which gives the otherwise inaccessible Michael adduct 6 after addition of dimethyl malonate (DMM) instead of the dinitro product 5 [19].

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