Dienes 1,4-functionalization

Carbene complexes containing either the dienophile or the diene functionality bonded directly to the carbene carbon undergo intramolecular [4+2] cycloadditions under mild conditions [108] (Scheme 60). 

Cyclopropanation of the 4,6-diene function proceeds selectively at the 5,6-double bond. Thus, reaction of 185 with the ylide from trimethyl sulfonium iodide and sodium hydride, in DMSO, affords predominantly the a-cyclopropyl compound (187) accompanied... 

Compound 316 contains two suitable ene-diene functions that yielded two diastereomeric tetrahydrobenzo[4]qui-nolizidines in quantitative yield and in a 8 92 ratio in a TiCU-catalyzed intramolecular Diels-Alder (IMDA) reaction (Equation 9) <1998T9529>. 

Silyltitanation of 1,3-dienes with Cp2Ti(SiMe2Ph) selectively affords 4-silylated r 3-allyl-titanocenes, which can further react with carbonyl compounds, C02, or a proton source [26]. Hydrotitanation of acyclic and cyclic 1,3-dienes functionalized at C-2 with a silyloxy group has been achieved [27]. The complexes formed undergo highly stereoselective addition with aldehydes to produce, after basic work-up, anti diastereomeric (3-hydroxy enol silanes. These compounds have proved to be versatile building blocks for stereocontrolled polypropionate synthesis. Thus, the combination of allyltitanation and Mukayiama aldol or tandem aldol-Tishchenko reactions provides a short access to five- or six-carbon polypropionate stereosequences (Scheme 13.15) [28],... 

Regio- and stereoselective dihydroxylation of dienes functionalized at the allylic position with a benzene sulfone group has been reported42. Osmylation of dienic sulfones 33, a potential key synthon for forskolin, occurred exclusively on the A6-7 double bound and preferentially from the a-face of the traws-fused bicyclic molecule, presumably due to a combination of steric and electronic factors (equation 25). While the reaction of diene sulfones proceeded sluggishly under catalytic conditions, treatment of 33a with a stoichiometric amount of OSO4 resulted in quantitative yield of diastereomeric diols 34a and 35 in a 9 1 ratio, respectively. Protecting the hydroxy group of the dienol as its t-butyldimethylsilyl ether (33b) affords diol 34b exclusively. 

The allenylidene-ruthenium(arene) catalyst precursors I have been used for the synthesis of macrocycles by the RCM reaction and were revealed as active as the first generation Grubbs catalyst RuCl2(=CHPh)(PCy3)2 [35], depending on the nature of the diene functional groups and macrocyde size [32] (Scheme 8.3).These macrocyde syntheses show that the allenylidene mthenium catalysts I offer functional group tolerance. 

Dehydrobromination with a base such as collidine then establishes the 1,4-diene function to yield fluoroprednisolone acetate (19-3) [13]. 

The resulting tertiary alcohol is then dehydrated and the ketone at 11 reduced to an alcohol by means of lithium aluminium hydride to give the intermediate (22-1). The acetal is next removed by exchange with acetone in the presence of a dilute acid. The requisite 1,4-diene functionality is then put in place by dehydrogenation... 

Where the diene function is attached to or is part of a carbocyclic system, the vinyl cyclopropane formation and subsequent rearrangement affords a reliable approach to the formation of tricyclic carbocycles. Some of the most elegant demonstrations of the use of this methodology in total synthesis of fused cyclopentanoid terpenes come from the work of Hudlicky and coworkers (equations 63-66)90 95"100. In these cases the diazoketones bear a carboxylate-substituted double bond of diene and the intramolecular cyclopropanation requires the combination of CuS04/Cu(acac)2 as catalyst. 

In cyclofunctionalization of some substituted cyclohexadienylethyl alcohols, products of 1,4-addition were observed (equation 16).63 However, the stereochemistry of the addition to the diene functionality depended upon the choice of electrophile. 

Since [4+2] cycloaddition is not very susceptible to steric hindrance, this synthetic concept has meanwhile been increasingly used for the assembly of hyperbranched compounds [41]. A diene combining two dienophile functions in one molecule was used as AB2 monomer for polymerisation (Fig. 4.23). The diene function can then react with a triple bond of a second AB2 monomer to form pentaphenylbenzene units [30]. 

Products F and G were in turn substrates for subsequent pairing reactions to obtain multicyclic compounds J, N, K, O, and H. Finally, products B, J, and H, which all comprise a diene functionality could be further converted by a subsequent Diels-Alder (DA) reaction with 4-methyl-l,2,4-triazolin-3,5-dione to obtain the highly complex products I, M, and L. This example shows that by applying the build/couple/pair strategy a collection of 15 highly diverse (and complex) scaffolds can be obtained in only three to five steps. ... 

Lipoxygenases (LOs) are nonheme, mononuclear iron enzymes that catalyze the regio- and stereoselective conversion of polyunsaturated fatty acids with a di.di-1,4-diene functionality into products having a l-hydroperoxy-tra 5, cM-2,4-diene functionality. The mammalian LOs typically act on arachidonic acid and produce alkyl hydroperoxides that are converted into leukotrienes and lipoxins, which are involved as messengers in the inflammatory response. Plant enzymes act on linoleic acid, but the role of the product alkyl hydroperoxide is less well understood. 

New results are associated with earlier results since both contribute to an explanation of apparently different data (e. g. high efficiency in radical grafting and ability to covulcanize with highly unsatuiated elastomers shown by terpol5nners containing low amounts of conjugated double bonds) which come from some basic reactions involving mainly the chemistry of free radicals and of diene functionality. 

The introduction of conjugated diene functions in polyisobutene chains has been recently acconq>lished throu an indirect approach which utilizes the dehydro-haiogenation of chlorinated butyl rubber, i. e. randomly chlorinated poly(isobutene-co-isoprene), by means of basic agents ... 

The diene functionality introduced in the polymers described in this work can have a different structure, namely ... 

Lastly, they are either general or very specific (for instance they mention topics such as dienes, functionality or molecular distribution). 

Scheme 3-54). This transformation constitutes a cascade of an intramolecular Heck insertion and subsequent heterocyclization. The initially formed arylpalladium species attacks the bridgehead position of the diene functionality in 238 to foim a JT-allylpalladium complex which is trapped by the internal nucleophilic phenol moiety (cf. Scheme 3-26). Since the starting diene 238 can be prepared in both enantiomeric forms by asymmetric reduction of a ketone, this sequence allows the preparation of both the natural morphine and its unnatural enantiomer. 

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