Catalytic system carboxaldehyde

A recent convincing case of chiral amplification opens a wide horizon in stereoselective catalysis [37 c] thus, when a 5-pyrimidyl alcohol with a small (2%) enantiomeric excess is treated with diisopropylzinc and pyrimidine-5-carboxaldehyde, it undergoes an autocatalytic reaction to generate more of the alcohol. The chiral catalyst is formed from the initial alcohol. The ee result of the (5)-isomer was successively increased in the series 2%—> 10%—>57% 81 % 89%. Amplification factors of up to ca. 1700 were recorded with the catalytic system of Scheme 2 [37 c]. This is the first case in which the enantiomeric excess of the product is greater than that of the chiral catalyst [97]. 

The coupling reaction between an electron-deficient alkene and an aldehyde (Baylis-Hillman reaction) usually requires a cata-lyst/catalytic system (typically, a tertiary amine and a Lewis acid) to be successful. The base catalyst is not necessary when pyridine-2-carboxaldehyde is employed as electrophile it is enough with the activation effected by stoichiometric amounts of TMSOTf for the reaction to proceed to give indolizidine derivatives (eq 71). ... 

The N-benzylation of imidazole-4-carboxaldehyde (165) and 4-cyanoimidazole (168) was studied using nano-ZnO and the triethylamine catalytic system under solvent-free conditions to afford the N-benzylated derivatives (166, 167, 169, 170) (Schemes 9.54, 9.55) (Oresmaa et al. 2007). 

Promoted by CuI/2-hydroxybenzohydrazide catalytic system, a variety of pyr-rolo[l,2-a]quinoxalines have been efficiently one-pot synthesized from pyrtole-2-carboxaldehyde and 2-haloanilines in moderate to excellent yields (Table 3.8) (Li et al. 2015). 

In terms of economical synthetic approaches to indoles, the synthesis of this heterocycle from anilines and trialkylammonium chlorides was effected in an aqueous medium (H20-dioxane) at 180°C in the presence of a catalytic amount of ruthenium(III) chloride hydrate and triphenylphosphine together with tin(II)chloride <00TL1811>. Muchowski devised a novel synthetic route to indole-4-carboxaldehydes and 4-acetylindoles 86 via hydrolytic cleavage of W-alkyl-5-aminoisoquinolinium salts 85 to homophthaldehyde derivatives upon heating in a two phase alkyl acetate-water system containing an excess of a 2 1 sodium bisulfite-sodium sulfite mixture <00JHC1293>. 

Bradsher and co-workers have developed a general method for synthesizing the protoberberine ring system. It involves the condensation of a properly constituted 1-isoquinoline carboxaldehyde with the appropriately substituted benzyl bromide. Acid-catalyzed cyclization of the resulting quaternary salts affords the fully aromatic tetracyclic ring system which on catalytic reduction leads directly to the tetrahydro-protoberberine skeleton. Recent modifications 93) of this synthetic method involving the use of the oxime, or better, the ethylene acetal of the 1-isoquinoline carboxaldehyde, have permitted the use of less vigorous conditions for the production of the tetracyclic intermediate... 

More complex branching occurs in mechanisms where selectivities arise. Therefore, consider as an example the overall transformation of a symmetric alkene cyclopentene in the presence of CO and H2 to both cyclopentane carboxaldehyde and cyclopentane, facilitated by the addition of a Group 9 (or CAS systems VIIIB) metal hydride complex HML capable of performing both hydroformylation [27,28] and hydrogenation [29]. Upon addition of the metal hydride to the system, one would expect the steps HML —> HML i —> H(ji-cyclopentene)ML i CsH9ML i where the bold intermediates should be common to both unicycUc catalytic sequence of steps. At this point there is a branching in the network of intermediates, and one of either two paths is followed ... 

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