Cinnamides, formation

Formation of mixtures of the above type, which is common with internal olefins, do not occur with many functionalized alkenes. Thus, tertiary cinnamates and cinnamides undergo cycloadditions with benzonitrile oxides to give the 5-Ph and 4-Ph regioisomers in a 25-30 75-70 ratio. This result is in contrast to that obtained when methyl cinnamate was used as the dipolarophile (177). 1,3-Dipolar cycloaddition of nitrile oxides to ethyl o -hydroxycinnamate proceeds regiose-lectively to afford the corresponding ethyl fra s-3-aryl-4,5-dihydro-5-(2-hydro-xyphenyl)-4-isoxazolecarboxylates 36 (178). Reaction of 4-[( )-(2-ethoxycarbo-nylvinyl)] coumarin with acetonitrile oxide gives 37 (R = Me) and 38 in 73% and 3% yields, respectively, while reaction of the same dipolarophile with 4-methoxy-benzonitrile oxide affords only 37 (R = 4-MeOCr>H4) (85%) (179). 

N-Methyl-E-cinnamide and p-chloro- -cinnamide were consequently tested as suitable additives. Both cause cinnamide crystals to grow as thin 100 plates (Figure 4c). In the presence of the former, the TV-methyl group of the additive molecule, which invariably adopts the synplanar conformation 3a (SO), prevents formation of the dimer, thus inhibiting the attachment of additional cinnamide molecules along a. The additive p-chloro-E-cinnamide inhibits the deposition of oncoming 100 layers, due to steric hindrance induced by the bulky chlorine atom. 

The driving force for growth of the crystal in the b direction is the energy released by formation of the NH O bonds of the ribbon motif. E-Cinnamic acid in the stable synplanar conformation 2a can replace a E-cinnamide molecule at the end of the ribbon however, at the site of the additive, the attractive NH O bond (- 6 kcal/mol) is replaced by repulsion between the adjacent oxygen lone-pair electrons of the bound additive molecule and of the oncoming cinnamide molecule (1-2 kcal/mol), which leads to an overall loss in energy of 7-8 kcal/mol at the site of the additive (Scheme 6 on page 16). 

An asymmetric photosynthesis may be performed inside a crystal of -cinnamide grown in the presence of E-cinnamic acid and considered in terms of the analysis presented before on the reduction of crystal symmetry (Section IV-J). We envisage the reaction as follows The amide molecules are interlinked by NH O hydrogen bonds along the b axis to form a ribbon motif. Ribbons that are related to one another across a center of inversion are enantiomeric and are labeled / and d (or / and d ) (Figure 39). Molecules of -cinnamic acid will be occluded into the d ribbon preferentially from the +b side of the crystal and into the / ribbon from the — b side. It is well documented that E-cinnamide photodimerizes in the solid state to yield the centrosymmetric dimer tnixillamide. Such a reaction takes place between close-packed amide molecules of two enantiomeric ribbons, d and lord and / (95). It has also been established that solid solutions yield the mixed dimers (Ila) and (lib) (Figure 39) (96). Therefore, we expect preferential formation of the chiral dimer 11a at the + b end of the crystal and of the enantiomeric dimer lib at the —b end of the crystal. Preliminary experimental results are in accordance with this model (97). 

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