Dienes configuration

The reactivities of dienes in the Diels-Alder reaction depend on the number and kind of substituents they possess. The larger the substituents are, or the more of them, at the ends of the conjugated system, the slower the reaction is likely to be. There also is a marked difference in reactivity with diene configuration. Thus trans- i, 3-pentadiene is substantially less reactive toward a given dienophile (such as maleic anhydride) than is cis-1,3-pentadiene. In fact,... 

Various azodicarboxylate esters have been used extensively as dienophiles.129 Several representative examples of this type of cycloaddition are listed in Table 6-III.33-40 As mentioned above, these reactions appear to be concerted processes that retain the 1,3-diene configuration in the adducts.303 The more bulky esters tend to react more slowly with dienes. Thus, dimethylazodicarboxylate reacts 5-6 times more rapidly with cyclopentadiene than does the ethyl ester,423 whereas the fert-butyl compound reacts sluggishly.42b... 

The 2,3-cis, 4,5-frani-diene configuration is important for activity. An acetylenic 4,5 linkage is also effective [23,24]. This linkage has no effect on the overall geometry of the molecule as compared with ABA, in that the carboxyls and 6-methyl group of both... 

Since a number of natural products have cyclohexadiene structure, adduct formation has been used in the past as a means of identifying cisoid diene configuration for the structure determination of the natural products. For example, ergosterol series isomers, namely, lumisterol, pyrocalciferol, and isopyrocalciferol, possess a 1,3-diene system in the B ring. Each of these... 

Type I is a radical reaction arising from chain rupture and leading to the formation of six-membered rings which occurs in the pyrolysis or depolymerization of polymers with double bonds in a 1,5- or 1,6-diene configuration. 

The achiral triene chain of (a//-rrans-)-3-demethyl-famesic ester as well as its (6-cis-)-isoiner cyclize in the presence of acids to give the decalol derivative with four chirai centres whose relative configuration is well defined (P.A. Stadler, 1957 A. Escherunoser, 1959 W.S. Johnson, 1968, 1976). A monocyclic diene is formed as an intermediate (G. Stork, 1955). With more complicated 1,5-polyenes, such as squalene, oily mixtures of various cycliz-ation products are obtained. The 18,19-glycol of squalene 2,3-oxide, however, cyclized in modest yield with picric acid catalysis to give a complex tetracyclic natural product with nine chiral centres. Picric acid acts as a protic acid of medium strength whose conjugated base is non-nucleophilic. Such acids activate oxygen functions selectively (K.B. Sharpless, 1970). 

If a molecule has a centre of inversion (or centre of symmetry), i, reflection of each nucleus through the centre of the molecule to an equal distance on the opposite side of the centre produces a configuration indistinguishable from the initial one. Figure 4.4 shows s-trans-buta-1,3-diene (the x refers to trans about a nominally single bond) and sulphur hexafluoride, both of which have inversion centres. 

Measurement of Unsaturation. The presence of double bonds in a fatty acid side chain can be detected chemically or through use of instmmentation. Iodine value (IV) (74) is a measure of extent of the reaction of iodine with double bonds the higher the IV, the more unsaturated the oil. IV may also be calculated from fatty acid composition. The cis—trans configuration of double bonds may be deterrnined by infrared (59) or nmr spectroscopy. Naturally occurring oils have methylene-intermpted double bonds that do not absorb in the uv however, conjugated dienes maybe deterrnined in an appropriate solvent at 233 nm. 

The dlenophlle, 3-acetyl-2(3H)-oxazolane, Is an attractive Intermediate for the synthesis of vicinal aminoalcohols with cIs configurations. It reacts with 1,3-dienes, even under quite mild conditions, to form (4+2) cycloadducts. Its high reactivity with deactivated 1,3-dienes Is noteworthy. This property is present also in 2(3H)-oxa201one which can be obtained easily through solvolysis of 3-acetyl-2(3H)-oxa2olone In methanol. 3-Acetyl-2(3H)-oxazolone, on UV irradiation In the presence of a sensitizer, combines easily with olefins to form (2+2) cycloadducts, the hydrolysis of which leads to the class of cis-2-aminocyclobutanols. 

From the HH COSY contour plot 12a it can be established which cycloadduct has been produced from l-(iV,iV-dimethylamino)-2-methylbuta-1,3-diene and tra 5-P-nitrostyrene. The Jhh coupling constant in the one-dimensional H NMR spectrum 12b can be used to deduce the relative configuration of the adduct. 

High vacuum sublimation of the 3-monobenzoate of (32) or tosylchloride dehydration gives 3/ -benzoyloxy-7-methylenecholest-5-ene (33). The configuration of (32) is probably as shown since 7a-hydroxy steroids dehydrate readily to yield homoannular dienes. In an analogous reaction ethyl and iso-butyl Grignard reagents give alkylidene compounds as the only isolated products. 

The result of oxidation of 8-hydroxy-2,3-tetrafluorobenzobicyclo[3 2 l]octa-2,6-diene depends on the configuration of the hydroxyl group In the syn isomer, the double bond is not epoxidized by the Jones reagent [5/] (equation 52)... 

The absolute configuration of the cycloaddition product obtained by the reaction of ketones with activated dienes catalyzed by (S)-t-Bu-BOX-Cu(II) (S)-21b points also to an intermediate in which the geometry around the central copper atom is square-planar similar to 26 above, and that the diene approaches the carbonyl functionality in an endo fashion. 

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