4- Arylaldehydes

Metalated SAMP- or RAMP-hydrazones derived from alkyl- or arylethyl ketones 3 add to arylaldehydes both diastereo- and enantioselectively. Substituted / -hydroxy ketones with relative syn configuration of the major diastereomer are obtained with de 51-80% and 70-80% ee. However, recrystallization of the aldol adducts, followed by ozonolysis, furnishes diastereo- and enantiomerically pure (lS, S )-. yn-a-mcthyl-/3-hydroxy ketones 5 in 36-51% overall yield. The absolute configuration of the aldol adducts was established by X-ray crystallographic analysis. Starting from the SAMP- or RAMP-hydrazone either enantiomer, (S,S) or (R,R), is available using this methodology16. 

Protected cyanohydrins may be employed as acyl anion equivalents in 1,4-additions in the presence of HMPA129. For instance cyanohydrins prepared from arylaldehydes add in a 1,4-fashion under thermodynamic control (THF or THF/HMPA) to cyclohexenone, isophorone and decalone systems in the latter case c/.s-octahydro-2(l/f)-naphthalenones are exclusively obtained 130-131. 

Scheme 5-26 Titanium alkoxide-catalyzed asymmetric hydrophosphonylation of arylaldehydes...

Scheme 6.49 Transformation of arylaldehydes to nitriles by hydroxylamine hydrochloride-clay.

Z- Configuration is typical of the majority of a-aryl(hetaryl)-/V-alkylaldo-nitrones. The isolation of -isomers in the condensation of aromatic aldehydes with iV-j3-]ihenyletli Tliydroxylamine has been described (155). The synthesis of a, N -diary lnitrones gives best results if acidic catalysis is employed (156), or when clay is used as a catalyst (157). Significant reduction of reaction time and increase in the yields of nitrones can be achieved if microwave irradiation is used (158, 159). On the basis of polymeric arylaldehydes, the synthesis of polymeric a,-diarylnitrones has been described (160). 

More recently, a new and straightforward one-pot approach was reported by Bu et al. <2003JOC5415>. This reaction involves the cyclocondensation of l,2-dipyridin-2-yl-ethane-l,2-dione 120 and arylaldehydes, in the presence of ammonium acetate. Imidazo[l,5- ]pyridines 121 were obtained in reasonable yields, but competitive formation of imidazoles 122 was observed. The amount of ammonium acetate used in this reaction was also shown to strongly influence the yield of the cyclization (Scheme 38). 

A versatile synthesis of triazolopyrazines has been elaborated by Akritopoulou-Zanze et al. <2004TL8439>. The method involves two steps a multicomponent Ugi reaction and a subsequent ring closure. Thus, arylaldehydes, propargylamine, cyclohexylisocyanide, and azidoacetic acid yielded the intermediate 406 which easily underwent cyclization to give the partially saturated derivative 407 in excellent yields. 

As mentioned in Section 11.20.5.1 discussing the reactivity of these ring systems, the ring closure reactions to 53-55 have been coupled in several cases with subsequent condensation reactions with arylaldehydes to yield benzylidene derivatives of these ring systems (e.g., 13) which transformations can be considered as cascade reactions. 

Arylalkanoic acid esters, 72 166 Arylaldehydes, microwave-accelerated transformation to nitriles, 76 580-581 Aryl alkyl ethers, acid cleavage of, 70 569 Arylalkylsulfones, 9 273 Arylamine dye intermediates, 9 272t... 

Claisen condensation org chem 1. Condensation, in the presence of sodium ethox-ide, of esters or of esters and ketones to form p-dicarbonyl compounds. 2. Condensation of arylaldehydes and acylphenones with esters or ketones In the presence of sodium ethoxide to yield unsaturated esters. Also known as Claisen reaction. klas-3n kand-on sa-shon ... 

Scheme 6.161 Product range for the 163-catalyzed enantioselective Henry reaction of arylaldehydes with nitromethane.

Stobbe was the first to observe photochromism in fulgides (1.33) when he synthesised them by the condensation of an arylaldehyde or ketone with a substituted methylene succinate. However, it was not until the 1970s, during the course of the extensive work carried out by Heller and his collaborators, that their chemistry and use in photochromism was truly exploited. Heller s work showed, amongst many other things, the importance of R in (1.33) being a 5-membered ring heterocycle, e.g. furan. 

Unlike the 13C-NMR method, H-NMR spectra are not applicable to 3-alkylaldehydes. For 3-arylaldehydes, the chemical shifts of 2-H, 3 -H, and 5-H appear deshielded in oxazolidines derived from chiral aldehydes with configuration A, where Rz is the aryl group. 

Nach diesen Vorstellungen erfolgt in der ersten Stufe eine Reduktion von Nitro-ethan mit Ammoniak zu Hydroxylamin, das mit dem Arylaldehyd iiber ein Aldoxim ein Arylnitril bildet. Die weiteren Reaktions-schritte bestehen dann in einer Addition von Hydroxylamin an dieses Nitril zum Carbonsaure-amid-hydroximid, aus dem mit Essigsaure iiber die O-Acetyl-Derivate 1,2,4-Oxadiazole gebildet werden31. 

Condensation of /3-phenylpropionamides with arylaldehydes, and then cyclization to l-aryl-2-benzazepin-3-ones (Pictet-Spengler reaction) is successful in PPA. The reaction fails, however, with aliphatic aldehydes. An analogous condensation between /3-(m-hydroxyphenyl)propylamine and then cyclization in hot butanol yields l-aryltetrahydro-2//-... 

The apparent fickleness of the acyl-pyrroles and -indoles in their reaction with carbanions to form new C—C bonds arises from the contribution made by the zwitterionic structure, e.g. (410b), to the resonance hybrid and the choice of the reaction conditions is critical for a successful nucleophilic reaction. Thus, formyl-pyrroles and -indoles do not normally undergo the Cannizzaro reaction nor do they form stable cyanohydrins or undergo benzoin-type reactions. However, surprisingly, 2-formylpyrrole reacts with arylaldehydes in the presence of potassium cyanide to yield (428), which is easily oxidized to (429) (B-77MI30505). It is noteworthy that the presence of an ester substituent adjacent to the formyl group modifies the mesomeric interaction to such an extent to allow the formation of (430) in low yield, as a result of an initial benzoin-type self-condensation (Scheme 76) (68BSF637). 

Fe(Por)X x = cr, cio Por = TPP, TMP Arylsubstituted diols + 02 Arylaldehydes, arylalkanes Solvent DMSO, DMF, pyridine, CH2C12 Xir, > 420 nm 19 diols studied the products are selectively formed in more than 90% yield in 10 cases. [263]... 

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