Esters, conjugated reaction with aldehydes

As with other reactions, silyl esters of phosphorus acids constitute an important and useful category of reagents for conjugate addition reactions. With aldehydes, ketones, and esters, the silyl ester linkage is transferred to the carbonyl oxygen, facilitating the completion of the reaction, generating the free carbonyl or ester upon workup with a protic solvent (Equation 3.25). 

Based on our previous results on the nucleophilic alkenoylation of aldehydes via metallated a, 3-unsaturated aminonitriles [50], we now envisaged an enanti-oselective variant. Thus, the enantiopure a-aminonitriles 60 were metallated with LDA and by reaction with aldehydes the adducts 61 could be obtained. Subsequent cleavage of the aminonitrile function with silver nitrate led to the desired a -hydroxyenones 62 in overall yields of 29-80% and enantiomeric excesses ee of 50-69%. Alternatively, the conjugate addition of the lithiated aminonitrile 63 to t-butyl crotonate led to the y-keto ester 63 in 35% yield and an enantiomeric excess ee of >90% (Scheme 1.1.18). 

Allylic arsonium ylides show a similar pattern of reactivity. Ethoxycarbonylallyl ylides, wherein the ester group is conjugated with the ylidic carbon atom, gave dienes in reactions with aldehydes or ketones , whereas other allylic ylides lacking such an electron-withdrawing substituent gave vinylic epoxides in high yield e.g. equation 8. In the... 

Conjugate additions. Cuprate addition to ynoate esters and trapping with aldehydes (in the presence of EtjAlCl) provide special analogs of Baylis-Hillman reaction products. Trapping with phosphonoacetatic esters is another synthetically valuable variation. Silyl ketene acetals derived from 1,6-addition to allyl alk-4-yn-2-enoates undergo Claisen... 

We have recently found that unsaturated ( )-ester aldehydes 1 (Scheme 1) quantitatively reacted with primary amines, including lysine, within a very short time ( 5 min) in an extremely diluted organic buffer solution (10 -10 M) to yield 1,2-dihydropyridines as irreversible products through the accelerated 6- r-azaelectrocyclization of the intermediary Schiff base [72, 73]. To the best of our knowledge, our reaction where unsaturated aldehyde 1, for instances, reacts with enzyme to modify several Lys residues within 15 min at room temperature is the fastest conjugation reaction with lysines in water (Fig. 1). We wondered if this reaction would work as a new protocol for Lys labeling. 

The Hantsch pyridine synthesis provides the final step in the preparation of all dihydrop-yridines. This reaction consists in essence in the condensation of an aromatic aldehyde with an excess of an acetoacetate ester and ammonia. Tlie need to produce unsymmetrically subsrituted dihydropyridines led to the development of modifications on the synthesis. (The chirality in unsymmetrical compounds leads to marked enhancement in potency.) Methyl acetoacetate foniis an aldol product (30) with aldehyde 29 conjugate addition of ethyl acetoacetate would complete assembly of the carbon skeleton. Ammonia would provide the heterocyclic atom. Thus, application of this modified reaction affords the mixed diester felodipine 31 [8]. 

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. 

Starting from 2,4,6-octatriene and pivaldehyde, the conjugated homoallylic alcohol 8 is obtained as the sole product. Cycloheptatriene-derived complexes react with aldehydes and C02 to afford mixtures of the isomeric 1,3- and 1,4-cycloheptadienyl carbinols or acids, respectively. Interestingly, analogous reactions with methyl chloroformate or dimethyl carbamoyl chloride produce the conjugated dienyl ester 9 or amide 10 as unique products [19,20]. 

The three-component method is applicable to the synthesis of various C(6)- or C(7)-functionalized PGs. Scheme 11 illustrates the tandem conjugate addition-aldol reaction that affords 7-hydroxy-PGE derivatives (18). Both saturated and unsaturated C7 aldehydes can be used as a side-chain units. The aldol adducts can be transformed to naturally occurring PGs (5a, 19) and, more importantly, to a variety of analogues such as tumor-suppressing A7-PGA, (20) or 7-fluoro-PGI2, a stabilized prostacyclin (21). The unique cellular behavior displayed by A7-PGA methyl ester is well correlated to its chemical reaction with thiols (20). 

Conjugate addition of 1 to a,(3-unsaturated esters, ketones, or aldehydes followed by reaction with an alkyl iodide (or bromide) proceeds by stereoselective attack anti to the silyl group, apparently because of electronic effects.2 Protonation of the (3-silyl enolate of 5 proceeds in the same sense to give the opposite diaster-eomer (Chart I). 

Reformatsky first introduced electron-withdrawing substituents on the a-carbon of an organozinc halide, leading to the more reactive and thermally stable a-(alkoxycarbonyl)alkylzinc halides (123).107 Typically these reagents react with aldehydes or ketones to afford 3-hydroxy esters while nitriles afford 3-keto esters (Blaise reaction),l07b>c but 1,4-conjugate additions to select a,3 unsaturated ketones are precedented (Section 1.2.2.2.2). 

The conjugate addition of lithium bis(phenyldimethylsilyl)cuprates to tt./t-unsaUi rated esters forms enolates, which readily react with aldehydes to give 2-substituted 3-silyl esters (equation 129)509. The products are useful as intermediates in the synthesis of allylsilanes and natural products. Yields of this reaction may be significantly increased by... 

A direct asymmetric reductive Mannich-type reaction that allows for the formation of three contiguous stereocentres with high chemo-, diastereo-, and enantio-selectivity (10 1 to 50 1 dr, 96-99% ee ) has been presented (Scheme 4). The reaction commences with the formation of the corresponding iminium ion from aldehyde (122) and prolinol (g) catalyst (125), followed by conjugate reduction with Hantzsch ester (123) to generate an enamine, which then undergoes Mannich reaction with imine (124) to produce (126).179... 

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