Diene system and

The Diels-Alder cycloaddition is one example of a pencyclic reaction, which is a one step reaction that proceeds through a cyclic transition state Bond formation occurs at both ends of the diene system and the Diels-Alder transition state involves a cyclic array of six carbons and six tt electrons The diene must adopt the s cis conformation m the transition state... 

Let us now examine the Diels-Alder cycloaddition from a molecular orbital perspective Chemical experience such as the observation that the substituents that increase the reac tivity of a dienophile tend to be those that attract electrons suggests that electrons flow from the diene to the dienophile during the reaction Thus the orbitals to be considered are the HOMO of the diene and the LUMO of the dienophile As shown m Figure 10 11 for the case of ethylene and 1 3 butadiene the symmetry properties of the HOMO of the diene and the LUMO of the dienophile permit bond formation between the ends of the diene system and the two carbons of the dienophile double bond because the necessary orbitals overlap m phase with each other Cycloaddition of a diene and an alkene is said to be a symmetry allowed reaction... 

P-Hydroxy steroids which contain the 5,7-diene system and can be activated with uv light to produce vitamin D compounds are called provitamins. The two most important provitamins are ergosterol (1) and 7-dehydrocholesterol (3). They are produced in plants and animals, respectively, and 7-dehydrocholesterol is produced synthetically on a commercial scale. Small amounts of hydroxylated detivatives of the provitamins have been synthesized in efforts to prepare the metaboHtes of vitamin D, but these products do not occur naturally. The provitamins do not possess physiological activities, with the exception that provitamin D is found in the skin of animals and acts as a precursor to vitamin D, and synthetic dihydroxalated... 

Two attractive routes to thiolene oxide and dioxide are the diene-SO104 and diene-S02298 cycloadditions, respectively. These cycloadditions are highly stereoselective at both carbons of the diene systems and at sulfur (see equation 62 for specifics) which, in the case of sulfoxide formation, proceed via attack of triplet SO on the diene. Equation 112 shows an example of such a cycloaddition104. The overall yields are significantly improved by running the cycloadditions in the absence of oxygen and by the use of excess diene. 

We focused our attention on Tall oil, a by-product of the paper industry, whenever this is prepared according to the KRAFT process. Said material consists of a mixture of highly unsaturated fatty acids (many of which with conjugated diene systems) and terpene derived rosin acids. The rosin acids have the molecular formula C20H30O2 and thus belong to the diterpenes (pimaric and abietic acids). Tall Oil has an iodine number equal to approximately 170 gl2/100 g. 

Study of the photochemistry of myrcene, 23, a triene containing both a conjugated diene system and a remote, unconjugated double bond, has been exceptionally informative. Direct excitation produces a cyclobutene derivative 24, /S-pinene, and a host of minor products287 including 25. In contrast, indirect excitation using a number of sensitizers gives only a derivative of bicyclo[2.1.1]hexane, 25.288... 

Matsumoto, M Nasu, S., Takeda, M., Murakami, H. and Watanabe, N. (2000) Singlet oxygenation of 4-(4-tert-butyl-3,3-dimethyl-2,3-dihydrofuran-5-yl)-2-pyridone non-stereospecific 1,4-addition of singlet oxygen to a 1,3-diene system and thermal rearrangement of the resulting 1,4- endoperoxides to stable... 

The reaction of chlorosulfonyl isocyanate with (7 )-l-isobutenylidene-2-phenylcyclopropane affords a mixture of optically active adducts 21-24, which possess the 1,2-dialkylidene structure and can be separated by column chromatography. The enantiomeric purity was determined by H-NMR techniques. In particular the adducts 21 and 22 contain a chiral, skewed diene system, and further addition of 4-phcnyl-3//-1,2,4-triazole-3,5(4W)-dionc produces a cycloadduct 25 or 26 with a stereogenic center, although with some loss of enantiomeric purity13. 

More recently, the use of benzoyl peroxide catalyst or light (or both) has extended the scope of the reaction. Thus, previously unsuccessful brominations of conjugated diene systems and terminal methyl groups can now be accomplished. ... 

This is a 4-77 electron process. The bonds involved are the tt bonds of the conjugated diene system and the tt bond and the ring-forming electronic configurations and symmetry characteristics of the orbitals involved are as follows ... 

The vinyltin moiety can form part of a diene system and undergo coupling with a dienyl iodide to form a tetraene [23] (Scheme 4-3). Crisp and Glink [24] have carried out a number of couplings directed towards -amino acid side-chain elaboration steps and observed the occurrence of ipso-substitution as a side reaction. 

The addition of mercaptans to thebainone-A is analogous to the addition of isoamyl and phenyl mercaptans to a /3-unsaturated ketones of the benzalacetone type, which has been reasonably explained as a 1 4-addition process [11-12], The formation of [xiv] from /3-ethylthio-codide no doubt occurs by 1 4-addition to the diene system, and from [xii] by addition of mercaptan to the carbonyl group followed by elimination of water, or by the condensation of mercaptan with the enol form of the ketone [3]. 

Matrix photolysis of 2i/-pyran-2-one at 8 K leads to electrocyclic ring opening of the bond O/C-2 and formation of the aldoketene 8 which, by addition of CH3OH, can be intercepted as ester 10. Photolysis at higher temperatures or in ether leads to electrocyclization of the 1,3-diene system and formation of the p-lactone 9 which, by addition of CH3OH, yields the enol ether 12. Further photolysis of the P-lactone 9 causes decarboxylation resulting in cyclobutadiene 11 and its products, e.g. the dimer 14 (CHAPMAN) [12] ... 

Compounds 425 and 426 were transformed into 436 and 441, respectively, in two parallel sequences that involved conversion of the 3-keto moiety present in the A ring into a diene system, and transformation of the butenolide into a furan moiety as depicted in Scheme 51. Both synthetic products show the same spectral data as their natural counterparts. However, the synthetic material 436 has an identical sign of optical rotation as that of natural tubipofurane, indicating that the absolute stereochemistry of both compounds is identical. In contrast, the sign of the optical rotation of the synthetic material 441 is opposite to that of... 

Under some conditions, furan undergoes addition of reactants to the diene system and thus gives nonaromatic products. This occurs by reaction with an electrophile to form the usual carbocation. which then adds a nucleophile rather than eliminating a proton in the usual way of aromatic substitution. As an example, when nitric add in methanol solution is used as the nitration medium, the product is a 5-methoxy-2-nitro addition product (Scheme 7.17). 

The infrared and ultraviolet spectra of a product of the biohydrogenation of linolenic acid in Butyrivibriofibrisolvens displayed a conjugated cis,trans-diene system and the product was shown to be cis-9,trans-l l,cis-15-octadecatrienoic acid (Kepler and Tove, 1967). Wilde and Dawson (1966) had found that linolenic acid was first converted to a trienoic acid in which two of the double bonds were conjugated. This was tentatively identified as either or A -octadecatrienoic acid. Since... 

---