Diene syntheses norbornene derivs

Strained cyclohexenes, such as norbornene derivatives, can undergo retro-Diels-Alder reactions even at relatively low temperatures, and this reaction can be used to prepare 1,3-dienes and alkenes (e.g. synthesis of cyclopentadiene by thermolysis of... 

A similar approach to enantiomericaUy pure norbornene derivatives was developed by Nouguier et al. who employed l,3 2,4-di-0-methylene acetals of pentitols as chiral templates [62]. Hence, the 5-(5-acryloyl-D-arabinatol derivative 82 underwent highly stereoselective Lewis acid catalyzed cycloaddition with cyclopentadiene, giving 83 (Scheme 10.28). The stereochemical outcome of the reaction was explained in terms of the chelate complex 84, in which the chair-like dioxane ring and the acrylic moiety are fixed in two parallel planes, forcing the diene to approach the cisoid acrylate from the ii-face. The synthesis and utihty of various methylene protected glycosides have also been reported by this group [63-66]. 

In view of the successful palladium-catalyzed [2 + 2 + 2] assembly of a cyclo-hexa-1,3-diene derivative from one molecule of norbornene and two molecules of /i-bromostyrene [30 a], a rationally designed synthesis of dihydrotrifoliaphane 33 as a precursor to 29 appeared to be plausible. However, when a 1 2 mixture of [2.2]paracyclophane-l-ene (32) and its 1-bromo derivative 4 was subjected to the reported conditions, the novel hydrocarbon 34 with a different array of three [2.2]paracyclophane units was obtained in 62% yield [30b]. Trifoliaphane 29, arising from oxidation of the primarily formed dihydro derivative 33, was obtained as a minor by-product (2 % yield) in this coupling. The constitution of 34 was established by an X-ray crystal structure analysis. 

B3LYP/6-311G (PCM) calculations have shown that the 4 + 2-cycloaddition of cyclopentadiene and -/ -nitrostyrene in nitromethane solution proceeded in a stepwise mechanism. Sulfanyl-methylene-5(4//)-oxazolones and -sulfanyl-a-nitroacrylates have been used as dienophiles in the Diels-Alder reactions to synthesize norbornene/ane amino acid derivatives suitable for peptide synthesis. Organoammonium salts of chiral triamine catalyse the Diels-Alder reaction of a-(carbamoylthio)acroleins with acyclic 1,3-dienes to produce cyclohexenes with sulfur-containing chiral quaternary carbons.The helical-chiral hydrogen-donor... 

---