1.3- Dienes acetylene derivs. 2 molecules

Ortho photocycloaddition was first reported in a U.S. patent [1] dated September 3, 1957. Irradiation of benzonitrile in the presence of various alkenes resulted in the formation of derivatives of l-cyanobicyclo[4.2.0]octa-2,4-diene. The first ortho photocycloaddition to benzene was reported in 1959 by Angus and Bryce-Smith [2], who discovered that benzene and maleic anhydride react to form a stable adduct at 60°C under the influence of ultraviolet radiation. This 1 2 adduct was formed from one molecule of benzene and two molecules of maleic anhydride. Two years later, Bryce-Smith and Lodge [3] found that acetylenes could also be photoadded to benzene. The isolated products were cyclooctatetraenes, formed by ring opening of the primarily formed bicyclo[4.2.0]octa-2,4,7-trienes. Since those early years, hundreds of examples of ortho photocycloadditions of alkenes to the benzene ring and many mechanistic investigations have been reported and they will be discussed in this chapter. 

Generally two main classes of unsaturated sugars in which (1) the anomeric carbon atom is involved in a double bond (glycals) and (2) the unsaturation is placed between other carbon atoms are known. One has to discuss also derivatives with the exo- and enclo-unsaturated bonds (either oleflns/dienes or acetylenes). Another class of unsaturated monosaccharides is represented by the open-chain sugars with the double bond(s) and/or triple bond present in the molecule. The examples of different types of unsaturated sugars are shown in O Fig. 2. Compounds such as glycals will not be discussed here, unless they are used as substrates for the preparation of other, non-anomeric, unsaturated derivatives. 

From the slow, noncatalyzed reactions, derivatives of 7-oxabicyclo[2.2.1]-hepta-2,5-diene (18) may be obtained. UV irradiation does not reverse the addition but allows an easy entry into the 3-oxaquadricyclane series (19)45 When furan adds to an acetylenic diester the product is still reactive enough to add another molecule of furan, a somewhat unexpected feature being that the product (20) is obtained only at lower temperatures. Above 100 C dissociation must occur and the less active double bond takes up the alkyne to give the thermodynamically more stable isomer (21)43 Whatever the addition, mixtures of endo and exo configurations result. The result is thought to reflect the fact that geometrical overlap of orbitals at the primary reaction centers is more important than secondary interactions as bond formation... 

Nitrile, azo, and nitroso groups, and even the oxygen molecule, take part in such reactions, and acetylenic triple bonds in particular confer reactivity as philodiene. As for dienes, so for philodienes the reactivity depends on the constitution. Activating groups particularly favor addition. The most reactive components include <%,/ -unsaturated carbonyl compounds such as acrolein, acrylic acid, maleic acid and its anhydride, acetylenedicarboxylic acid, p-benzo-quinone and cinnamaldehyde, as well as saturated nitriles and <%,/ -unsaturated nitro compounds. Tetracyanoethylene also reacts with dienes.41,42 Conjugation of the double bond to an active group is not absolutely essential for a philodiene, for dienes add under certain conditions also to philodienes with isolated double bonds examples of the latter type are vinyl esters and vinyl-acetic acid. Ketenes do not undergo the Diels-Alder reaction with dienes, but instead yield cyclobutanone derivatives 43,44... 

In the 1950s two very effective pesticides were launched and their names were Dieldrin and Aldrin. As you may guess they were made by the Diels-Alder reaction. Aldrin is derived from two consecutive Diels-Alder reactions. In the first, cyclopentadiene reacts with acetylene to give a simple symmetrical cage molecule norbornadiene (bicyclo[2.2.1]hepta-diene). Norbornadiene is not conjugated and cannot take part in a Diels-Alder reaction as a diene. However, it is quite strained because of the cage and it reacts as a dienophile with perchlorocyclopentadiene to give Aldrin. 

Condensation of y-(trialkylsilyl)allylboranes and allenylboranes with simple aldehydes and conjugated allenic, acetylenic, and enynyl aldehydes followed by a stereoselective elimination of trialkylsilanol produced 1,3-butadienes, diene-allenes, enyne-allenes, enediynes, and dienediynes with high isomeric purity. The trimethyltin chloride-induced transformations of 1-alkynyltrialkylborates also provided easy access to a variety of unsaturated compounds. High energy intermediates, such as o-isotoluenes, biradicals, o-quinodimethanes, benzocyclobutadienes, and enyne-ketenes, were derived from these unsaturated molecules for subsequent synthetic elaborations. 

Phenylacetylene and Fe3(CO)i2 in inert hydrocarbon solvents at 80°C give, as the main products isomeric triphenyltropone iron tricarbonyl complexes. The triphenyltropone ligand can be derived from three acetylene molecules and one carbonyl group (cf. the formation of cyclopenta-dienone complexes p 229). The X-ray structure of one isomer 9.6, shows that the tropone ring is attached to the iron atom by a diene system, the third double bond being bent out of the plane of the diene carbon atoms, away from the iron (cf. structure of GgH8Fe(CO)3 p 166). 

Negishi finds that the palladium-catalysed coupling reaction of -1-alkenylzirconium derivatives (71) with alkenyl halides gives 1,3-dienes with high regio- and stereo-selectivity. Furthermore, the conditions will tolerate functionality in the molecule, such as THP ethers. In this respect, the procedure may offer a wider scope of application than the corresponding organoalanes derived from acetylenes. 

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