Stilbenes Diels-Alder reactions

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

A triplex has been proposed as an intermediate in some photoreactions. Yang and co-workers [54] found that addition of 1,3-dienes to anthracene excimers leads to different products than does its adition to monomeric excited arenes. Lewis and co-workers [55, 56] found that stilbene excimers can be intercepted by dimethyl fumarate to give an oxetane through a presumed triplex. More recently, Schuster and co-workers [57,58] studied the Triplex Diels-Alder reaction of 1,3-dienes with enol, alkene, and acetylenic dienophiles. Take 9,10-dicyanonaphthalene (DCN)/indene (IN)/l,3-cyclohexadiene (CHD) system as an example ... 

Clear-cut examples of effects of zeolite pore architecture on the selectivity of Diels Alder reactions are not easily found. For instance, 4-vinylcyclohexene is formed with high selectivity from butadiene over a Cu -Y zeolite however, the selectivity is intrinsically due to the properties of Cu1, which can be stabilized by the zeolite, and not to the framework as such (30-31). A simple NaY has been used in the cycloaddition of cyclopentadiene and non-activated dienophiles such as stilbene. With such large primary reactants, formation of secondary products can be impeded by transition state shape selectivity. An exemplary reaction is the condensation of cyclopentadiene and cis-cyclooctene (32) ... 

Although neutral Diels-Alder reactions of stilbene as a dienophile are unknown, the cation radical Diels-Alder reaction cited above occurs smoothly [51]. Trans stilbenes yields only trans adducts, while cw-stilbenes yield primarily, but not exclusively, cis adducts. The stereospecificity of the cycloaddition step in the latter reaction system was, however, not readily evaluated because of the occurrence of competing cis to trans isomerization of the starting material. It was established, however, that at low conversions, the reaction tends toward stereospecificity. Since cw-stilbene is much less readily ionizable than tran -stilbene, the Diels-Alder cycloadditions in the cw-stilbene system were studied using the p,p -6. vciQ hy derivative. 

A more quantitative criterion for the operation of a cation radical cycloaddition mechanism has been carried out for a Diels-Alder reaction system consisting of various substituted derivatives of /ran -stilbene (as the ionizable dienophile) and 2,3-dimethyl-1,3-butadiene (Scheme 37) [71]. 

Furthermore, y-CD can complex two guests at the same time. For example, two stilbene [135], naphthalene [136], or anthracene [137] moieties can fit in the y-CD cavity. Attractive interactions between the end groups of two included guests enhance the stability of the ICs [136], Because of the close proximity between two included guests, bimolecular reactions like [2+ 2]-cycloadditions [135] and Diels-Alder-reactions [138-140] are strongly accelerated by these ICs. 

Using sealed-tube sample holders, Santoro and co-workers (32-35) investigated a wide variety of organic reactions. Examples are the cis - trans isomerization of stilbene and oleic acid, polymerization of styrene, Diels-Alder reactions, and others. Unstable intermediates in an organic reaction have been detected using DTA techniques by Koch (36). If a solution of an unstable compound is heated, temperature changes characteristic of reactions of the intermediate can be detected. Conversely, the absence of thermal effects indicates that no unstable product is present. 

Synthesis of substituted stilbenes has been reported through a tandem process involving Diels-Alder reaction/Wittig reaction/DDQ oxidation sequence (Scheme 16.21) [22]. The Diels-Alder step is catalyzed by cobalt(II) and Zn(II) salts. 

Tellurobeitzaldehyde. This aldehyde (a) is formed in situ by reaction of Te with (C6H5)3P=CHC6H5 and trapped with 2,3-dimethylbutadiene to form the Diels-Alder adduct 1. In the absence of a diene, a reacts with the Wittig reagent to form stilbene (61% yield). 

When allylic hydrogens are present in the alkene, A-allyl hydrazine derivatives are formed preferentially by the ene reaction (Section 7.6.). Furthermore, the reaction of styrenes, (Ey -phenylpropene, and stilbenes with diazenedicarboxylates (traditionally known as azodicar-boxylates) and triazolediones affords products involving double Diels-Alder or Diels-Alder-ene reactions (Section 7.2.10.3.3.). 

Similarly, the Diels-Alder-type reaction of 374 with reactive dienophiles such as styrene and /ra r-stilbene produced phenyl-substituted thiones 391 and 392, respectively, by thermal reactions and/or photoreaction (Scheme 52) <1996TL7603>. 

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