Coupling aldehydes

Whilst ketocarboxylic acids can be dimerized satisfactorily (Table 2, No. 9, 10), the corresponding aldehydes couple poorly. Good yields can be obtained in these cases when the acetals, e.g. 17 [133], 18 [134], are electrolyred instead. 

A reaction related to alkene-aldehyde coupling is the alkene-imine coupling. A one-pot cyclization involving such a reaction (Eq. 3.27) proceeds smoothly in a mixture of water-THF. The reaction has been used in the asymmetric synthesis of pipecolic acid derivatives.114... 

Methyl fluoro(diethoxyphosphono)dithioacetate (34) has been prepared from difluorinated precursors [56], Fluorophosphonothioacetamides (35) derived from this dithioester, have been successfully transformed into highly functionalized fluoroalkenes (36). Judicious selection of the aldehyde coupling partner can lead expeditiously to the preparation of fluoroolefin dipeptide isosteres following elaboration of the carboethoxy group and desulfurization (Scheme 11). 

The yield of the Fischer indole reaction was improved dramatically by employing 3-cyanopropanal diethylacetal as the aldehyde coupling partner (Scheme 10). Thus the hydrazone 33 was formed by reacting the hydrochloride salt of 31 with the diethylacetal in aqueous HCl. The hydrazone 33 was treated with polyphosphate ester in... 

Long-range couplings occur between ring protons and hydrogens attached to substituent atoms. For example, in heterocyclic 2-aldehydes couplings of the order of 1 Hz have been observed. 

Enone-aldehyde coupling. A recent synthesis of the ring system 2 of mevino-lin (3), a fungal metabolite used for treatment of hypercholesterolemia, depends on... 

Cross pinacol coupling of aldehydes. Pedersen et al.2 have reported the first general method for stereoselective pinacol coupling of aldehydes by use of this vanadium(II) reagent. Aryl aldehydes couple in the presence of 1 to pinacols in >90% yield and with high diastereoselectivity (dllmeso = 12-100 1). Aliphatic aldehydes under similar conditions couple only slowly, but depending on the struc-... 

Cook s stereospecific synthesis19" of trans-1,3 -disubstituted tetrahydro-/ -carbolines by condensation of iVb-benzyltryptophan methyl ester with aldehydes, coupled with a simple 13C n.m.r. method for differentiating cis- and trans-tetrahydro-/ -carbolines,196 should afford a promising starting point for the synthesis of complex alkaloids. Angenot and his collaborators have also discussed the 13C n.m.r. spectra of a number of / -carboline and 3,4-dihydro-/Tcarboline derivatives.19"... 

Fig. 5.7. Left In situ STM image of non-covaiently immobilized cyt c (low ionic strength). Small molecular-size structures between larger (=100 mol) flat aggregates. Tunnel current 1.25 nA (other details in ref. [51]). Right In situ STM image of cyt c covalently immobilized on polycrystalline platinum by silanization and glutaric aldehyde coupling. Tunnel current 1.78 nA. Substrate potential...

Coupling of aldehydes and acrylates. In the presence of DABCO as catalyst, aldehydes couple with acrylates at the a-position to form 2-(l-hydroxyalkyl)acrylates (equation 1). These products are readily converted into a-methylene-P, y-unsaturated es-... 

In the synthesis of the core structure of the diynene antitumor antibiotics esperamicins and calicheamicins by Danishefsky et al. [108], an acetylene-aldehyde coupling reaction was used to achieve the cyclization. As shown in Scheme 60, reaction of a toluene solution of acetylene aldehyde 182 with potassium hexame-thyldisilazide at —78 °C for 20 min afforded a 52% yield of a 10 1 ratio of 183 and 184. 

CAR was discovered in acetate-producing bacteria based on its ability to catalyze the reverse reaction, the reductive activation of carboxylic acids, though it can also catalyze aldehyde oxidation (Table 2). The acid/aldehyde couple has an extremely low reduction potential, for example. Eg value for acetaldehyde/acetate is -580 mV (SHE) and therefore, aldehyde oxidation is much more thermodynamically favorable than acid reduction. The electron carrier for the enzyme inside the cell is not known. CAR isolated from C.formicoaceticum has molecular... 

Four-component (silyl enolates, a,/3-unsaturated thioesters, amines, and aldehydes) coupling reactions have also been successfully conducted with Sc(OTf)3 as catalyst, to afford the corresponding amino thioester and y-acyl-<5-lactam derivatives stereoselec-tively in high yields (Sch. 3) [12]. 

Galactose Oxidase (GO) from the filamentous wheat-root fungus Fusarium spp. is a mononuclear type 2 copper enzyme that catalyzes the two-electron oxidation of a large number of primary alcohols to their corresponding aldehydes, coupled with the reduction of dioxygen to hydrogen peroxide [1,18] ... 

The enamine geometry 32 is cmcial for the stereocontrol in organocatalytic aldehyde-aldehyde couplings amines of type 31 are convenient catalysts for enantioselective enamine-aldol reactions. Examples are shown in Scheme 24 [126,131,132,133,134,135]. 

The mechanistic features stemming from the rapid aldol-type 0-addition, the slow Cj— C2 aldehyde coupling step, and the lack of growth of the 0-branched alcohols give rise to the selectivity pattern in which ethanol is at minimum and... 

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