Truxinic acid

The dichlorophenyl group also functions as a sufficiently strong anchor to allow formation of solid solutions from pairs of structurally similar potentially reactive olefins. A series of such pairs are 170 to 173. All were found to yield the expected (3-truxinic-acid-type photodimers, 75 (135). 

It is well established that cinnamic acid and some substituted cinnamic acids (including frans-p-coumaric acid but not ferulic acid) can be dimerized in vitro by sunlight to truxillic and truxinic acids and their derivatives... 

Diquercetin 3-galactoside ester of tetrahydroxy-p.-truxinic acid (monochaetin, 13.6) Monochaetum multiflorum leaves Melastomataceae 360... 

A great number of olefinic compounds are known to photodimerize in the crystalline state (1,2). Formation of a-truxillic and / -truxinic acids from two types of cinnamic acid crystals was interpreted by Bernstein and Quimby in 1943 to be a crystal lattice controlled reaction (5). In 1964 their hypothesis on cinnamic acid crystals was visualized by Schmidt and co-workers, who correlated the crystal structure of several olefin derivatives with photoreactivity and configuration of the products (4). In these olefinic crystals the potentially reactive double bonds are oriented in parallel to each other and are separated by approximately 4 A, favorable for [2+2] cycloaddition with minimal atomic and molecular motion. In general, the environment of olefinic double bonds in these crystals conforms to one of three principal types (a) the -type crystal, in which the double bonds of neighboring molecules make contact at a distance of -3.7 A across a center of symmetry to give a centrosymmetric dimer (1-dimer) (b) the / -type crystal, characterized by a lattice having one axial length of... 

Keywords /ram-cinnamic acid, double salt formation, [2+2]photodimerization, truxillic acid, truxinic acid, cyclobutane... 

Scheme 3 Photodimerisation of trans-cinnamic acid (9) and three of the possible diastereomers, the a-truxillic acid (10), the p-truxinic acid (11) and the 5-truxinic acid (12)...

This polymer was able to convert more than 50% of the tnmv-cinnamic acid in 8-truxinic acid (12), an isomer that is never formed with the catalyst in solution. This demonstrated how the cavity of the imprinted polymer could control the stereochemistry of the reaction in a direction which did not occur at all using the free catalyst in solution (Scheme 3) [9]. 

Figure 5 Solid-state photoreaction of -cinnamic acid to produce (i-truxinic acid.

Schmidt et al. found that a distinct product phase appeared at less than 10% conversion in some cases (indicating heterogeneous reaction), while in others none was evident at greater than 70% conversion [63]. Certain derivatives, (3-truxinic acids in particular, were formed in a modification different from that into which they could be recrystallized. Schmidt interpreted these results in terms of a phase separation of the forming product which occurs when it reaches a limiting solubility in the lattice of the reactant. He further pointed out that recrystallization may be necessary to improve yields in the dimerization of 0-cinnamic acids in certain instances where monomer molecules may be stranded in nonreactive sites such as M2.. M.. M2 (Section IV.B.5.). 5-Bromo-2-hydroxy-cinnamic acid is an example of a case where there is no evidence for phase separation and reaction is found to be slow and proceed in low yield [63,108]. 

After 2 days P-monomer after 6 days mainly 2,2 -diethoxy-P-truxinic acid... 

Stdrmer, H. and Laage, E. (1921). Truxillic acids. (III). Natural and artihcial truxihic and truxinic acids. Berichte, 54B, 77-84. [234]... 

In contrast to solutions, solid drugs have a fixed conformation resulting in topochemical reactions. The majority of photoreactions in the solid state, described in the literature, deal with lattice-controlled examples and photodimerizations. A precondition for these reactions is the parallel position of the double bond of two adjacent molecules in the crystal lattice as shown by the example of the trimorphic, frans-cinnamic acid. Irradiation of the a- and the 5-modifications causes the formation of a-truxillic acid and (i-truxinic acid, respectively, whereas the y-modification is photostable due to the distance of the double bonds fixed by the lattice (Fig. 8) (10). 

FIGURE 18.1. Solid-state reactions that occur on irradiation of cinnamic acids. The reaction products are truxillic or truxinic acids, depending on the relative orientations of the molecules in the crystal. The distance between C=C bonds in the solid state must lie in the range 3.6-4.1 A. 

Intramolecular Additions - The truxinic acid derivative (23) is formed on irradiation of (24) in methanol using a Pyrex filter. The reaction is quantitative and has a quantum yield of 0.55. Similar reactivity is observed when crystals of (24) are irradiated. The use of a benzene ring as a constraint has provided a method for the formation of a single stereoisomer in high yield this is seen best of all with the yield this is cinnamate ester (25). Irradiation of (25) for 21 h in methylene chloride solution affords an 88% yield of (26). A less efficient reaction... 

Study, dimerization would not occur (Scheme 1-a.). Therefore, the molecules of the 5 reactant pair must undergo a rotation so that a favorable overlap between the orbitals can occur (scheme 1-b) and lead to the formation of the 5 dimer. It is difficult to rationalize why the 5 dimer is formed only from cation 1 and the e dimer is formed only in homogeneous solution from cation 2. There is precedent for the formation of the 5 dimer in the solid state. Previous work in our laboratory indicates that the dioctadecyl ester of S-truxinic acid is formed by irradiation of C crystals of the octadecyl ester of trans cinnamic acid (32). 

It has been tempting for researchers to assume that the crosslinking takes place when an excited double bond adds to one of its neighbors to form cyclobutane-type structures. Parallels from the poly(vinyl cinnamate) system have been drawn to the well-known dimerization of cinnamic acid to truxillic and truxinic acids. A study of a number of substituted cinnamic acids revealed that in order for efficient dimerization to take place the double bonds of two neighboring cinnamate structures must lie within 3.6—4.1 A of each other 25>. The regularity of structure in the crystalline state is not preserved in solution or in the polymeric medium. It has been difficult, therefore, to conclude that cyclobutane-type structures are formed predominantly during the photocrosslinking of poly (vinyl cinnamate). 

The second type of photoreactive arrangement of monomer molecules in crystals with mirror symmetry leads to polymers with a stereochemistry at the cyclobutane ring corresponding to / -truxinic acid as shown in Reaction 4. So far, crystallographic evidence for this type of polymerization is lacking but should be coming in near future. 

Recent work of Green and Schmidt (1) shows how the packing can be influenced to obtain either of the two reaction products. As they have reported, derivatives of 2,4-dichlorocinnamic acid always pack in a head-to-head arrangement, giving rise to derivatives of jS-truxinic acid. Thus, polymers with the same stereochemistry as V (Reaction 4) should be produced by using the 2,4-dichlorophenyl residue as the guiding group. 

Intramolecular Cycloadditions. - Intramolecular (2 + 2)-photocycloaddi-tion has been reported within diene 1,1-dicarbonitriles. Both direct and sensitized irradiation is effective. Unlike the singlet process, where an exciplex is involved, the sensitized process proceeds via 1,4-biradicals. The singlet and triplet reactivity has been investigated for a series of 1-alkene-1,1-dicarbonitriles with additional unsaturation at the 5, 6 or 7 positions. The cyclophane moiety shown in the derivative (23) has been suggested as a useful reaction control system. The irradiation of this cinnamate derivative affords the (3-truxinic acid derivative (24), which can be uncoupled from the paracyclophane. ... 

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