Dehydrodimers

The presence of the aliphatic amino group complicates the course of the reaction. Thus, the oxidative coupling of 4-ethynyl-1,3-dimethyl-5-aminomethylpyrazole in mild conditions (20°C, CuCl, pyridine, O2) leads to only 20% of butadiyne. However, acylic protection eliminates these complications, and 4-ethynyl-1,3-dimethyl-5-(acetyl)aminomethylpyrazole forms a dehydrodimer in 95% yield (Scheme 66) (86TH1). 

Upon changing the applied strategy from the completely random approach (cf. Scheme 22) to a block assembly, a complete change of the product distribution is observed in the preparation of the protected expanded pericyclinones of type 89 Only the three macrocycles 123,125 and 176 were isolated after the oxidative cyclooligomerization of the dehydrodimer 175 (Scheme 32) [39]. 

ANDREASEN M F, CHRISTENSEN L p, MEYER A s, HANSEN A (2000) Content of phenoHc acids and ferulic acid dehydrodimers in 17 rye (Secale cereale L.) varieties,VoMr a/ of Agricultural and Food Chemistry, 48, 2837-42. 

Ralph, J. Quideau, S. Grabber, J. H. Hatheld, R. D. Identification and synthesis of new ferulic acid dehydrodimers present in grass cell walls. J. Chem. Soc., Perkin Trans. 1994, 1, 3485-3498. 

In the photoaddition of 2-pyrrolidone the 5-alkyl isomer (69) always predominates, usually in a ratio of 2 1. The formation of anti-Markovnikov 1 1 adducts, telomers, and dehydrodimers of structure (71) supports a free radical mechanism. Similarly, formamide undergoes olefin addition under... 

Electron-rich olefins with substituents Y = phenyl, vinyl, amino, or alkoxy can be coupled by anodic oxidation to tail-tail dimers being either deprotonated to dienes and/or substituted a to Y, depending on Y and the reaction conditions (Eq. 6). Alkyl substituted arenes can be dehydrodimer-ized to diphenyls or diphenylmethanes depending on the kind of substitution (Eq. 7). 

Anodic oxidation of 1,2,3-trimethoxybenzene in acetone containing dilute sulphuric acid gives 2,6-dimethoxybenzoquinone but in contrast 1,2,4-trimethoxy-benzene affords the dehydrodimer 11 in good yield [82]. Dehydrodimerization becomes an important process in the oxidation of methoxybenzenes in dichlo-... 

Oxidation of the series of a,co-diarylalkanes 27 gives both the cyclised product 28 and a dehydrodimer type product 29 depending on the length of the alkane... 

Anodic oxidation of triphenylamine gives the radical-cation which is rapidly converted to the dehydrodimer, tetraphenylbenzidine. A para-substituent prevents this dimerization and gives radical-cations very stable in acetonitrile solution against nucleophiles [155], This class of compound was first prepared as the per-... 

More satisfactory separations were obtained when reverse-phase liquid chromatography was used. The separation of the standard dimeric mixture was carried out on an LC-18 column with refractometry as the mode of detection and acetonitrile/acetone (1 1) as the mobile phase (system II). Separation proceeded according to the polarity of the various dimers, and complete separation of all dimers, except those of the thermal dimer of methyl linoleate and the dehydrodimer of methyl oleate, were obtained at a flow rate of 0.5 ml/min. The resolution of the two unresolved peaks would be increased by using another LC-18 column in series, but a sacrifice in the analysis time would have to be made. 

Acetonitrile was used in the mobile phase when ultraviolet detection of the carbonyl group at 205 nm was used for the various dimers. Separation of the thermal dimer of methyl linoleate and the dehydrodimer of methyl oleate was again incomplete. The sample at concentrations as low as those required for UV detection (1-5 mg/ml) was sparingly soluble in the mobile phase, resulting in increased retention times. Finally, incomplete separation was obtained when various systems of acetonitrile/methyl chloride were used as the mobile phase in HPLC with infrared detection of the carbonyl group at 5.72 /xm. 

Tetrahydrofuran gives only the 2,2 -dehydrodimer expected from exclusive a-deprotonation of the ether cation radical. 

Aside from being UV-protectants, in a number of species certain stilbenes act as phytoalexins. Resveratrol (6.24 trans-3,5,4 -trihydroxy-stilbene), its cA-isomer, as well as their glucosides and dehydrodimer mm-e- iniferin (6.25) are present in grape leaves and berries and play a role in the defense against gray rot caused by the fungal pathogen Botrytis cinerea. 

Viniferin is synthesized by a grape peroxidase (Morales et al., 1997). The fungus in return is able to inactivate resveratrol through the action of a laccase-like stilbene oxidase (Breuil et al., 1998). This results in the formation of resveratrol tram-dehydrodimer (6.26), as well as the corresponding cA-dehydrodimer (6.27), both of which structurally resemble viniferin. 

Resveratrol fraas-dehydrodimer frans-S-Viniferin frans-e-Viniferin... 

A13. Andreasen, M. F., Landbo, A. K., Christensen, L. P., Hansen, A., and Meyer, A. S., Antioxidant effects of phenolic rye (Secale cereale L.) extracts, monomeric hydroxycinnamates, and ferulic acid dehydrodimers on human low-density lipoproteins. J. Agric. Food Chem. 49, 4090—4096 (2001). 

As previous reactions have already shown, this reaction seems to be a free radical chain reaction. The experimental evidence so far available for such a mechanism is the following (a) the formation of dehydrodimers in the absence of an olefin (b) the formation of the anti-Mar-kovnikov 1 1 adduct as the major product when a terminal olefin is employed as the addend, and (c) the telomeric products obtained. Thus, reaction sequence can be summarized as follows ... 

A dehydrodimer isolated from this reaction was shown to have the following structure... 

Thermolytic Reactions. At the higher temperatures, nonoxidative reactions also occur as for example the formation of tetrasubstituted cyclohexenes via Diels-Alder reaction, or the formation of dehydrodimers and mono- or polycyclic dimers via combination of alkyl free radicals or free radical attack on double bonds. 

Andreason, M.F. Christensen, L.P. Mayer, A.S. Hansen, A. 2000. Content of phenolic acids and ferulic acid dehydrodimers in 17 rye Secale cerreale L.) varieties. J. Agric. Food Chem. 48 2837-2842. 

Numerous stilbenes such as e-viniferin (Landrault et al. 2002) and 5-viniferin, two resveratrol dehydrodimers (Vitrac et al. 2005), the resveratrol dimer pallidol (Vitrac et al. 2001), a-viniferin, a trimer of resveratrol (Pryce and Langcake 1977)... 

Gasparro EP, Eresco JR. Ultraviolet-induced 8,8 adenine dehydrodimers in oligo- and poly-nucleotides. Nucleic Acids Res. 1986 14 4239-4251. 

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