Diene moiety

Diene moieties, reactive in [2 + 4] additions, can be formed from benzazetines by ring opening to azaxylylenes (Section 5.09.4.2.3). 3,4-Bis(trifluoromethyl)-l,2-dithietene is in equilibrium with hexafluorobutane-2,3-dithione, which adds alkenes to form 2,3-bis-(trifluoromethyl)-l,4-dithiins (Scheme 17 Section 5.15.2.4.6). Systems with more than two conjugated double bonds can react by [6ir + 2ir] processes, which in azepines can compete with the [47t + 27t] reaction (Scheme 18 Section 5.16.3.8.1). Oxepins prefer to react as 47t components, through their oxanorcaradiene isomer, in which the 47r-system is nearly planar (Section 5.17.2.2.5). Thiepins behave similarly (Section 5.17.2.4.4). Nonaromatic heteronins also react in orbital symmetry-controlled [4 + 2] and [8 + 2] cycloadditions (Scheme 19 Section 5.20.3.2.2). 

Intermolecular [4C+2S] cycloaddition reactions where the diene moiety is contained in the carbene complex are less frequent than the [4S+2C] cycloadditions summarised in the previous section. However, 2-butadienylcarbene complexes, generated by a [2+2]/cyclobutene ring opening sequence, undergo Diels-Alder reactions with typical dienophiles [34,35] (Scheme 59). Also, Wulff et al. have described the application of pyranylidene complexes, obtained by a [3+3] cycloaddition reaction (see Sect. 2.8.1), in the inverse-electron-demand Diels-Alder reaction with enol ethers and enamines [87a]. Later, this strategy was applied to the synthesis of steroid-like ring skeletons [87b] (Scheme 59). 

Scheme 78 Novel ring-contraction metathesis of macrolide antibiotics with a 1,3-diene moiety [164]...

Inner-outer-ring dienes are very useful in the synthesis of polycyclic molecules. Their reactivity in the Diels-Alder reaction depends on the type of ring (carbo-cyclic, heterocyclic, aromatic) that bears the ethenyl group or on the electronic effects of substituents at the diene moiety [30]. 

Arylethenes are inner-outer-ring dienes in which the vinyl group is linked to an aromatic system. These dienes are poorly or moderately reactive the presence of electron-donating substituents in the diene moiety markedly increases their reactivity. Their cycloadditions are usually accelerated in order to be carried out under mild conditions. 1-Vinylnaphthalene is more reactive than 2-vinyl-naphthalene and styrenes. 

Because of the presence of the 1,5-diene moiety in many naturally occurring compounds, a great deal of effort has been expended in searching for methods to... 

Very recent work111 has shown that the predominant formation of the endo adduct in the reaction between cyclopropene and isotopically substituted butadiene could be attributed to an attractive interaction between a C—H bond of cyclopropene and the jt bond being formed in the diene moiety. 

In this molecule the diene moiety assumes the C2V symmetry and the prominent feature of its near-UV absorption spectrum is the 240 nm band (fimax 3500), assigned6 to the same transition r - jt L, now characterized by xAi - lB2 symmetry in the C2V point group (see Appendix, Section V). 

In 4 and 5 the chromophore is planar and the optical activity arises from the lack of a vertical symmetry plane (i.e. that bisecting the diene moiety), owing to the presence of the Ci—CH3 bond, which has no counterpart in the other half of the molecule11. 

This point of view finds its justification in the following observations. Compounds 8 (pyrocalciferol) and 9 (isopyrocalciferol), having opposite absolute configurations of the stereogenic centres near the dienes, show lA —> 1B Cotton effects at about 275 nm of the same sign and intensity. The reason for this is that only the twist of the chromophore determines the optical activity in fact the diene moieties are distorted in the same sense in 8 and 9, as found by X-ray diffraction16. 

On the other hand, from a spectroscopic point of view it is more correct to consider the diene moiety as a whole. Therefore, it seems that the allylic contribution can be evaluated using two different points of view the olefin-picture (the former) and the diene-picture (the latter). This difference has important consequences. In fact, in a vast majority of cases these two pictures lead to the same result, whilst in some instances the two predictions can be opposite. Let us consider the three possible different geometries of... 

An interesting series of s-trans planar dienes is that due to Walborsky and co-workers44,45. They prepared and studied the chiroptical properties of several compounds where the diene moiety is linked to a chirally substituted ring, such as 42 and 43. 

The Diels-Alder adduct of sulpholene and cyclopentadiene is a useful starting material for substituted diene synthesis121. The diene moiety is unmasked by retro-Diels-Alder reaction and sulphur dioxide extrusion under flash vacuum pyrolysis conditions (equations 74 and 75)122,123. 

An electron-enriched 1,3-diene moiety as in the substrate 381 can act as a nucleophile toward an activated alkyne moiety (Scheme 94). Iwasawa340 has reported an elegant synthesis of a diquinane framework 382, which is catalyzed by various metals and the rhenium(i) complex appears to be the best catalyst among the metal complexes examined. Minor product 384 presumably is formed through an insertion of a carbenoid species into the neighboring activated benzylic C-H bond. The same carbenoid species can undergo a 1,2-H shift to give the major product 383. 

The palladium-catalyzed addition of silylstannanes to 1,6-diynes invokes intramolecular cyclization, giving rise to 1,2-dialkylidene cyclopentanes (Equation (110)).269 The resulting Z,Z-l-silyl-4-stannyl-l,3-diene moiety fixed in an i -m-conformation makes the molecule axially chiral. Rapid equilibrium between the two helical forms is observed by NMR spectroscopy.270,161... 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

The selective diene hydrogenation of monoterpenes such as myrcene, which contain both isolated monoene and diene moieties, forms a particular challenge [84]. The catalyst [RhH(CO)(PPh3)3] (60) has been reported to perform remarkably well for such hydrogenation reactions, and the diene moiety was shown to be selectively reduced to the monoene, while the isolated double bond remained unaffected under the reaction conditions used (Scheme 14.20). The rates of reaction expressed as average TOF (determined at ca. 80% conversion) ranged from ca. 640 (in benzene, 20 atm H2 at 100 °C) to 7600 mol mol 1 h 1 (in cyclohexane, 20 atm H2 at 80 °C). The hydrogenation in benzene solution resulted in... 

Based on the method used in Kishi s synthesis, a diene moiety can be connected to 77 through routine synthetic chemistry, and this finishes the C-15 to C-19 subunit of intermediate 66. Conversion of 77 to 66 is briefly depicted in Scheme 7-19. 

Simplification of the diene moiety of pyrethrin I resulted in finding allethrin (4) by Schechter [3], which was the first pyrethroid commercialized for household use. Matsui et al. made an extensive effort on the process of allethrin, which resulted in the launch of the first pyrethroid in Japan in 1954. As the propynyl analog of pyrethrin I, prallethrin (11) was commercialized by Sumitomo Chemical in 1988 in the most insecticidally active form [4]. Allethrin and prallethrin have been used widely for control of mosquitoes. 

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