Cinnamaldehydes Diels- Alder with

An unexpected and potentially useful mode of reactivity was observed in the reaction of cinnamaldehyde and cyclopentadiene catalysed by diarylprolinol silyl ether 39 [73]. Rather than observing a Diels-Alder adduct, the products resulting from an ene reaction were isolated in excellent yield. The transformation was found to be general for a series of P-aryl acroleins (40) with routinely excellent levels of... 

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

Wittig olefination of 2-nitro-Z-cinnamaldehyde (1300) with the phosphonium bromide 1301 led to the diene 1302. The Diels-Alder cycloaddition of 1302 with maleimide (1303), followed by dehydrogenation with DDQ, afforded the phthali-mide 1304. Double deoxygenation of 1304 with triphenylphosphine (PPhs) in collidine gave O-methylarcyiiaflavin B (1305). Finally, heating of 1305 with molten pyridine hydrochloride led to arcyriaflavin B (346) (759) (Scheme 5.215). 

Hetero-Diels-Alder reactions have also been conducted with aldehydes (Scheme 10) <2003JOC2803>. The reaction is very sensitive to the substitution pattern on the aldehyde and on the phosphole. Reaction of 1-phenyl-3,4-dimethylphosphole with benzaldehyde or its ra-derivatives afforded the corresponding adducts 55 in quantitative yields (Scheme 10). An increase in the steric demands of the aldehydes or of the phosphole results in a dramatic decrease in the yield. For example, with 2,4,6-trimethylbenzaldehyde, no reaction takes place and only 30% conversion is observed with 2,6-dimethoxybenzaldehyde. Similarly, using the more sterically hindered 1-phenyl-2,3,4,5-tetraethylphosphole, no cycloadduct formation is observed with benzaldehyde. Note that with decanal, the yield reaches only 40%. The reaction affords a mixture of endo- and o o-phosphinites 55, the major component being the OT o-derivative. With /ra t-cinnamaldehyde, a mixture of adducts 56 and 57 was obtained (Scheme 10), showing that the 1-phosphadiene can react either via the C=0 or via the C=C moieties of a,/3-unsaturated aldehydes <2003JOC2803>. 

Among the few examples of simple 1-azadiene Diels-Alder reactions is a dihydropyridine synthesis using the stable azadiene 39 (prepared from cinnamaldehyde and aniline) with the dienophile 38 prepared from the isoxazole 35 by elimination. This is a reverse-electron-demand cycloaddition, the HOMO of the dienophile 38 combining with the LUMO of the azadiene 39 to give the cycloadduct 40 and hence the dihydropyridine 41 with complete regioselectivity and in very high yield.3... 

The alternative direction ((b) on page 124) is suitable for 1,4-diaryl butadienes (28). These were needed with a variety of substituents for a systematic study of electronic effects on the Diels-Alder reaction. Disconnection to give benzyl phosphonium salts (30) and easily made cinnamaldehydes (29) (see Chapter 18) is best. 

Nitrile, azo, and nitroso groups, and even the oxygen molecule, take part in such reactions, and acetylenic triple bonds in particular confer reactivity as philodiene. As for dienes, so for philodienes the reactivity depends on the constitution. Activating groups particularly favor addition. The most reactive components include <%,/ -unsaturated carbonyl compounds such as acrolein, acrylic acid, maleic acid and its anhydride, acetylenedicarboxylic acid, p-benzo-quinone and cinnamaldehyde, as well as saturated nitriles and <%,/ -unsaturated nitro compounds. Tetracyanoethylene also reacts with dienes.41,42 Conjugation of the double bond to an active group is not absolutely essential for a philodiene, for dienes add under certain conditions also to philodienes with isolated double bonds examples of the latter type are vinyl esters and vinyl-acetic acid. Ketenes do not undergo the Diels-Alder reaction with dienes, but instead yield cyclobutanone derivatives 43,44... 

A simple and efficient method for the preparation of a-diazo- i-hydroxy esters by the condensation of aldehydes with ethyl diazoacetate using chloromethy-lated polystyrene SIL as a heterogeneous catalyst in water was developed [66]. Moderate to excellent yields of the corresponding a-diazo-jl-hydroxy esters were obtained. The catalyst was separated by filtration and reused in five consecutive cycles without any appreciable loss of activity. Later MacMillan s imidazolidinone was noncovalently immobihzed in the pores of siHca gel with the aid of IL 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (Mac-SILC) (SILC, supported ionic liquid catalyst) [67]. The Mac-SILC as an SIL catalyst was utilized for the enantioselective Diels-Alder reaction of cyclopentadiene and cinnamaldehyde. The Mac-SILC was more active, especially to an electron-rich dienophile, than its homogeneous counterparts, and the reaction could proceed under mild conditions. 

Pihko and co-workers reported asymmetric organocatalytic Diels-Alder reactions on solid support (JandaJel -supported or n-propyl-functionalized silica gel) with the advantage of easy recovery and reuse, Scheme 3.22 [36]. For example, synthesized polymer- and siUca-supported chiral imidazolidinone catalysts 55 and 56 were demonstrated to be efficient catalyst for the enantioselective Diels-Alder reactions of cyclopentadiene and cinnamaldehyde. 

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