Vinylpyridines Diels-Alder reactions

A review on fliran and its derivatives in the synthesis of other heterocycles was published <95CHE1034>. Furan decomposes on Pd(lll) at 300 K to form H, CO and CjH, which can dimerize to benzene at 350 K <96JA907>. Again, a considerable number of Diels-Alder reactions with furan and fiiran derivatives was reported. The synthesis of 2-pyridinyl-7-oxabicyclo[2.2.1]heptanes (e.g., 3, 4) was accomphshed via zinc chloride-mediated Diels-Alder reaction of furan with 2-vinylpyridines <96SL703>. 

A European patent granted to Quest International Services BV involves use of pyridine derivatives as fragrance materials. 2-Alkyl-substituted pyridines are specifically mentioned, including all possible stereoisomers of 2-(2,4-dimethylcyclohexyl)pyridine 46, which is made from 2-vinylpyridine and 2-methyl-l,3-pentadiene via the Diels-Alder reaction <2006EP01562904>. 

An interesting application of the chiral cyclopalladated complexes is palladium-promoted asymmetric Diels-Alder reactions of l-phenyl-3,4-dimethylphosphole (Scheme 4). In the original report on the Diels-Alder reaction of the phosphole reported by Nelson and co-workers, the dichloropalladium species was employed as a promoter.f In 1994, Leung showed that a chiral cyclopalladated complex was capable of promoting the Diels-Alder reaction. In this reaction, the chiral palladacycle worked as a chiral auxihary and showed almost perfect diastereoselectivity. It has been revealed that simultaneous precoordination of a diene and a dienophile to the palladium center is essential for the reaction. Thus, both dienes and dienophiles should possess Lewis basic functionahty. The diene l-phenyl-3,4-dimethylphosphole has been the only substrate examined so far, while a variety of dienophiles, such as vinylphosphine, vinyl-sulfoxide, vinylsulfide, acrylamide, vinylarsine, vinylpyridine, vinyl-pyrrole, or methylenequinuchdinone, have successfully been applied to this asymmetric Diels-Alder reaction. 

Vinylpyridine gives in the Meerwein reaction with arene diazonium chlorides, 2-(a-chloro-jS-arylethyl)pyridines, some of which readily lose hydrogen chloride, yielding stilbazoles ia. 2- and 3-Vinylpyridines have been used in Diels-Alder reactions with dienes37i -2 ... 

Only the Diels-Alder-ene products are formed from 4-nitro- and 4-/crt-butylphenylethylene with 4-phenyl- or 4-methyl-6 from 2-vinylpyridines and 4-phenyl-7 and from 1-cyclopropyl-l-phenylethylene and 4-phenyl-3//-l,2,4-triazole-3,5(47/)-dione8. From benzylidenecyclopropane the double Diels-Alder adduct 5 was obtained exclusively on reaction with 4-phenyl-3tf-l, 2,4-t riazole-3,5 (4/f)-d ione9. 

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