Dialkoxy-substituted

Scheme 1-8. Examples of soluble, dialkoxy-substituted PPV derivatives.

Horhold et al. and Lenz et al. [94,95]. The polycondensation provides the cyano-PPVs as insoluble, intractable powders. Holmes et al. [96], and later on Rikken et al. [97], described a new family of soluble, well-characterized 2,5-dialkyl- and 2,5-dialkoxy-substituted poly(pflrfl-phenylene-cyanovinylene)s (74b) synthesized by Knoevenagel condensation-polymerization of the corresponding alkyl-or alkoxy-substituted aromatic monomers. Careful control of the reaction conditions (tetra-n-butyl ammonium hydroxide as base) is required to avoid Michael-type addition. 

An alternative method to prepare (Mormyl esters uses different building blocks to assemble the 1,4-dicarbonyl system and is complementary in many cases.10 Base-catalyzed addition of nitromefhane to a, J-unsaturated esters, followed by a variation of the Nef reaction, provides y-dialkoxy-substituted esters. The scope of this sequence has not yet been explored. Another approach involves cuprate additions to norephedrine-derived 2-alkenyloxazolidines this process allows small-scale synthesis of several p-formyl esters in optically active form (ee up to 95%).11... 

It is relevant to note that only cyclic aldo-nitrones tend to react in oxidative alkoxylations to give a,a-dialkoxy-substituted nitroxyl radicals. However, the only exception is methylene nitrone (215), which on oxidative methoxylation gives the a,a,a-trimethoxy-substituted nitroxyl radical (277). This is due to the proton in methylene nitrone (215), which, as in the case of cyclic nitrones, exists in the cis -position to the TV-oxide oxygen (Scheme 2.104) (517). 

Moreover, one should mention that in spite of similar electronic structures, PBN and the isoquinoline nitrone (278) react in a different way. Under no circumstances does PBN give an oxidative methoxylation product, whereas nitrone (278) reacts readily to form a,a-dialkoxy-substituted nitroxyl radical (280) (517). Perhaps this difference might be due to the ability to form a complex with methanol in aldo-nitrones with -configuration. This seems favorable for a fast nucleophilic addition of methanol to the radical cation (RC), formed in the oxidation step. The a-methoxy nitrone (279), obtained in the initial methoxylation, has a lower oxidation potential than the initial aldo-nitrone (see Section 2.4). Its oxidation to the radical cation and subsequent reaction with methanol results in the formation of the a,a-dimethoxy-substituted nitroxyl radical (280) (Scheme 2.105). 

Piperidine derivatives and acyclic substances such as trihalogeno-alkanes and dihalogenoalkylamines may be used as starting materials. The piperidine derivatives are obtained mainly from pyridine compounds. Dihalogenoalkylamines and trihalogenoalkanes are prepared from tetrahydropyran derivatives, dialkoxy-substituted malonic esters, or alkane-tetracarboxylic esters. 

A series of tetraalkoxy and dialkoxy-substituted 21,23-dithiaporphyrins 240c and 21-monothiaporphyrins 241c with methoxy, butoxy, octy-loxy, and dodecyloxy substituents at (5-thiophene carbon atoms were also synthesized (02JCS(CC)2642,04T10671). 

Several general synthetic methods have been developed that utilize the novel reactivity of quinone methides generated by oxidation of phenol precursors. Angle and Ranier have generated 2,6-dialkyl or 2,6-dialkoxy-substituted / -quinone methides 51 by oxidation of the corresponding phenols with Ag20. 

In addition to aryl alkynyl coupling reactions involving dihaloarenes and acetylenes or dialkynylarenes which lead to poly(arylene acetylene)s, it is worth noting that these polymers have also been obtained by the Pd(0)-catalysed heteropolycondensation of dialkoxy-substituted dibromoarenes with bis(tribu-tylstannyl)acetylenes [122]. 

Ng, D., Yang, Z., and Garcia-Garibay, M.A. (2002) Engineering reactions in crystals gem-dialkoxy substitution enables the photodecarbonylation of crystalline 2-indanone. Tetrahedron Letters, 43, 7063-7066. 

The synthesis and some aspects of the photochemistry and photophysics of a 2,6-dialkoxy-substituted anthracene SCLC polymer, 61, have been reported by... 

A family of a,a-dialkoxy substituted benzyl radicals (221) has been shown to undergo [3-cleavage both from their ground and excited states. This is in contrast to the stability of benzyl-type radicals toward permanent photochemical... 

Warman JM, Gelinck GH, de Haas MP. (2002) The mobility and relaxation kinetics of charge carriers in molecular materials studied by means of pulse-radiolysis time-resolved microwave conductivity Dialkoxy-substituted phenylene-vinylene polymers. J Phys Condensed Matter 14 9935-9954. 

The discovery by Stork and Borowitz and Opitz and Adolph that the addition of sulfenes (RCH=S02) to enamines gives good yields of 3-aminothietane-1,1-dioxides (e.g., 132) has been extensively exploited in the synthesis of a variety of thietane sulfone derivatives. Truce, Breiter, Abraham, and Norell at about the same time showed that sulfene can add to ketene acetals to give 3,3-dialkoxy substituted thietane 1,1-dioxides (e.g., 133). The sulfenes are generated by dehydrohalgenation of methanesulfonyl chlorides with triethylamine. [Attempts to add sulfines (e.g., PhCH=SO) to enamines to give thietane 1-oxides were not successful.1 Sulfene additions have been reviewed. ... 

Chen et al. synthesized another type of PPV, dialkoxy-substituted poly[phenylene vinylene]s (dROPPV 1/1, 0.2/1, and 0/1), consisting of two repeating units with different side chain lengths (methoxy and 3,7-dimethyloctyloxy) (see Fig. 1.36) [41]. 

Few examples of chelation-controlled allylation reactions involving bicyclic chelates can be found in literature. Nagano and Azuma have shown that the allylation of a dialkoxy-substituted radical adjacent to a dimethyl acetal proceeds, in the presence of MgBr2 OEt2 (2.5 equivalents), with excellent stereocontrol independent of the synjanti stereochemistry of the radical precursor (Scheme 7) [24]. To best explain the high anti ratio obtained, they proposed bicyclic transition state M in which the allylstannane attacks from the side of the pyramidalized radical chelate. Surprisingly, when the same reaction was conducted at a lower temperature (0°C), poor selectivity was observed. No explanation was proposed to account for this decrease. It should be noted that the allylation gave no selectivity in the absence of Lewis acid. 

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