Trienes conjugated

The precise value of the resonance energy of benzene depends as comparisons with 13 5 cyclohexatriene and (Z) 13 5 hexatriene illustrate on the compound chosen as the reference What is important is that the resonance energy of benzene is quite large SIX to ten times that of a conjugated triene It is this very large increment of resonance energy that places benzene and related compounds m a separate category that we call aromatic... 

Figure 11.15. Typical chemical groupings in a sulphur-vulcanised natural rubber network, (a) Monosulphide cross-link (b) disulphide cross-link (c) polysulphide cross-link (j = 3-6) (d) parallel vicinal cross-link (n = 1-6) attached to adjacent main-chain atoms and which have the same influence as a single cross-link (e) cross-links attached to common or adjacent carbon atom (f) intra-chain cyclic monosulphide (g) intra-chain cyclic disulphide (h) pendent sulphide group terminated by moiety X derived from accelerator (i) conjugated diene (j) conjugated triene (k) extra-network material (1) carbon-carbon cross-links (probably absent)...

Leukotriene B4, formed by enzymic hydrolysis of LTA4, is chemotactic for macrophages and neutrophils at concentrations as low as 1 ng/ml. The stereochemistry of the conjugated triene subunit was established by synthesis which also made LTB4 available in quantity for biological research. 

We reach a similar conclusion when comparing benzene with the open-chain conjugated tr-iene (Z)-l,3,5-hexatriene. Here we compare two real molecules, both conjugated trienes, but one is cyclic and the other is not. The heat of hydrogenation of... 

Nine-membered ring systems are potentially accessible via a TMM cycloaddition with conjugated trienes in a [6-1-3] fashion. However, tropone is the only reported system that undergoes such a reaction. Interestingly, these cydoadditions are remarkably selective in that only nine-memhered ring products are formed. 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

Note that a conjugated diene and a conjugated triene react with, opposite stereochemistry. The diene opens and doses by a conrotatory path, whereas the triene opens and closes by a disrotatory path. The difference is due to the different symmetries of the diene and triene HOMOs. 

Conjugation (Chapter 14 introduction) A series of overlapping p orbitals, usually in alternating single and multiple bonds. For example, 1,3-butadiene is a conjugated diene, 3-buten-2-one is a conjugated enone, and benzene is a cyclic conjugated triene. 

A distinguishing feature of the Nicolaou synthesis of rapamycin is the use of a palladium-mediated tandem inter-/intramolecular Stille coupling to construct rapamycin s 31-membered macrolide ring and conjugated triene moiety. This maneuver was unprecedented in the macrolide field,9 and it can be applied to a fully deprotected seco substrate (vide infra). 

Scheme 27).43b A regiospecific monohydroboration of enyne 107 with disiamylborane furnishes (A)-vinylborane 108, a substance that combines stereospecifically with vinyl iodide 109 under the indicated conditions to give conjugated triene 110 (52% overall yield). Trisporol B benzyl ether (111) is obtained after acid-induced hydrolysis of the dioxolane ketal functions.

Our strategy is based on the premise that the 31-membered ring and the conjugated triene array of the natural product could be fashioned simultaneously by a tandem inter-/intramolecular Stille coupling. Moreover, the mild conditions under which Stille couplings can be performed fueled hopes that the crucial stitching cycliza-tion could be conducted on a fully deprotected seco bis(vinyl iodide) (see 145, Schemes 40 and 54) the stitching cyclization would thus be the final operation in the synthesis. 

The first example of successful diene-diene RCM to construct a macrocyclic conjugated triene was disclosed by Wang and Porco in the first total synthesis of oximidine II (258) [121]. Oximidine II belongs to the family of salicylate en-... 

The orbital phase is continuons in the linear conjugate triene and discontinuous in the cross-conjugate trine. The electron delocalization between the terminal bonds is favored in the linear triene and disfavored in the cross-conjugate triene. The linear triene is more stable. The continuity-discontinuity of orbital phase underlies the thermodynamic stabilities of non-cyclic conjugated molecules. 

The leukotrienes are a family of conjugated trienes formed from eicosanoic acids in leukocytes, mastocytoma cells, platelets, and macrophages by the lipoxyge-... 

With the above procedure, dienoic fatty acid peroxidation products yield a conjugated triene chromophore with second-derivative absorption minima iocated at 258, 269.5 and 281 nm, whilst trienoic fatty-acid peroxidation products give rise to a conjugated tetraene chromophore with minima at 278, 289, 303 and 318 nm. The arrows in (c) denote second-derivative absorption minima corresponding to the conjugated triene adduct arising from linoleate-derived peroxidation products (conjugated hydroperoxydienes, hydroxydienes and oxodienes). 

One of the first examples of the formation of the bicyclo[3.1.0]hex-2-ene structure from an acyclic conjugated triene was the formation of suprasterol I (24) and suprasterol II (25) from vitamin D2<131) ... 

Cycloheptatriene, as an example of a conjugated triene, is mainly cyclopro-panated at an outer double bond (Scheme 6). This is true for Rh2(OAc)4, Cu(OTf)2 and Pd(OAc)2, but the highest yield is obtained again with the rhodium catalyst72>. Twofold cyclopropanation occurs to only a minor extent, as long as an excess of olefin is applied. With equal amounts of diazo ester and cycloheptatriene, double cyclopropanation increases and even traces of the triply cyelopropanated triene are found with Rh2(OAc)4 and Cu(OTf)2. This behavior essentially parallels the earlier... 

We start with the simplest conjugated triene, 1,3,5-hexatriene, for which there are the two isomers, the (Z)- and ( )-, species 79 and 80, respectively. Nearly 30 years ago57, the enthalpy of combustion of the former, as liquid, was reported. From this number,... 

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