Bis-exocyclic 1,3-diene

Palladium catalyzed cycloisomerizations of 6-cn-l-ynes lead most readily to five-membered rings. Palladium binds exclusively to terminal C = C triple bonds in the presence of internal ones and induces cyclizations with high chemoselectivity. Synthetically useful bis-exocyclic 1,3-dienes have been obtained in high yields, which can, for example, be applied in Diels-Alder reactions (B.M. Trost, 1989). 

On the other hand, the cyclization reaction of a vinyllithium onto an acetylenic unit provides an efficient route to five- and six-membered bis-exocyclic 1,3-dienes, which react stereoselectively with a wide range of dienophiles157. The 5-exo carbolithiation reaction of vinyllithiums 369, derived from the corresponding vinyl bromides, is syw-stereospecific giving, after hydrolysis, the /(-isomer of five-membered outer-ring dienes 370 and tolerates aryl-, silyl- or alkyl-substituents at the distal acetylenic carbon (Scheme 97). However, the alkyl-substituted alkynes are far more resistant to rearrangement than the aryl- or silyl-substrates and the addition of TMEDA and longer reaction times are needed for the latter... 

An extension of this intramolecular carbolithiation was recently disclosed for the preparation of bis-exocyclic 1,3-dienes by the 5-exo-dig cyclization of (5-acetylenic vinyllithiums. Through subsequent Diels-Alder chemistry, they serve as useful precursors of polycyclic ring systems [14a] (Scheme 7-13). 

Vinyllithiums have been shown to undergo also an efficient anionic 6-exo-dig cyclization to form a bis-exocyclic 1,3-diene in good yield [24] (Scheme 7-25). 

An alternative preparation of the exocyclic diene (III) was attempted by first thermally decomposing the bromosulfone (I) to yield 2,3-bis(bromomethyl)-1,3-butadiene VI, which, in turn, was allowed to react with aniline following the procedure described by Gaoni (19). 

Cycloaddition reactions are particularly valuable for the construction of sulphur heterocycles, and have been intensively investigated. Thus, Diels-Alder reactions of the thiolan (22), which has an exocyclic cis-diene structure, and which is easily prepared from 2,3-bis(bromomethyl)buta-1,3-diene, have been studied. Addition of SOj gave the cyclo-adduct (23). A series of dithiabicycloheptan-3-ones have been prepared by [3 -i- 2]cycloadditions of the cyclic thiocarbonyl ylide (24) to a variety of dipolarophiles e.g., to formaldehyde to give (25). ... 

Fig. 26) in which the exocyclic carbonyl group is bound in the apical position and the dienophile reacts out of the s-cis conformation. Steric shielding of the Si face by the ligand phenyl group would favor diene attack on the exposed Re face. As with Mg(II)bis(oxazoline) complexes (Sect. 3.1.3), the presence of ligands other than the dienophile appear to be important in the creation of a stereode-fined environment about the metal center. 

---