The Diene Route

At this point, we had exerted an immense amount of effort to overcome a number of difficult transformations and their unprecedented challenges (1) oxidation at C3 and C3a, (2) reduction at C8a, and (3) homologation at C5a. We learned to truly respect the unique structural topology that ABD-tricycle 2 possesses. What remained for the total synthesis was the C-ring construction, which posed serious challenge of its own. 

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Comments The diene A is symmetrical so it doesn t matter which double bond is attacked by the carbene. On the other hand, it may be difficult to stop carbene addition to the second double bond. The only control over the stereochemistry will be that the trans compound we want is more stable. Japanese chemists have recently synthesised optically active trans chrysanthemic acid by this route (Tetrahedron Letters. 1977, 2599). 

Now consider strategy b. How would you make the diene acid B, what reagent would you use for the carbene synthon, and how do you rate the chances of this route ... 

The carbene synthon might be difficult, but since the olefin is conjugated with a carbonyl group we could try a sulphur ylid as a nucleopliilic carbene equivalent (as in frame 283). Synthesis The diene could be made by this route ... 

Since the six carbons shown above have 10 additional bonds, the variety of substituents they carry or the structures they can be a part of is quite varied, making the Diels-Alder reaction a powerful synthetic tool in organic chemistry. A moment s reflection will convince us that a molecule like structure [XVI] is monofunctional from the point of view of the Diels-Alder condensation. If the Diels-Alder reaction is to be used for the preparation of polymers, the reactants must be bis-dienes and bis-dienophiles. If the diene, the dienophile, or both are part of a ring system to begin with, a polycyclic product results. One of the first high molecular weight polymers prepared by this synthetic route was the product resulting from the reaction of 2-vinyl butadiene [XIX] and benzoquinone [XX] ... 

Another attractive route to the diene (107) is by pyrolysis of the phenyl-carbamate or ethylcarbonate ester. 

Hydroformylation of nitrile rubber is another chemical modification that can incorporate a reactive aldehyde group into the diene part and further open up new synthetic routes to the formation of novel nitrile elastomers with a saturated backbone containing carboxyl or hydroxyl functionalities. 

An example of a rhodium(I) complex with a tridentate phosphine is shown in Figure 2.16 it is formed by the usual route, reaction of the phosphine with [RhCl(cycloocta-1,5-diene)]2. 

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. 

ADMET is quite possibly the most flexible transition-metal-catalyzed polymerization route known to date. With the introduction of new, functionality-tolerant robust catalysts, the primary limitation of this chemistry involves the synthesis and cost of the diene monomer that is used. ADMET gives the chemist a powerful tool for the synthesis of polymers not easily accessible via other means, and in this chapter, we designate the key elements of ADMET. We detail the synthetic techniques required to perform this reaction and discuss the wide range of properties observed from the variety of polymers that can be synthesized. For example, branched and functionalized polymers produced by this route provide excellent models (after quantitative hydrogenation) for the study of many large-volume commercial copolymers, and the synthesis of reactive carbosilane polymers provides a flexible route to solvent-resistant elastomers with variable properties. Telechelic oligomers can also be made which offer an excellent means for polymer modification or incorporation into block copolymers. All of these examples illustrate the versatility of ADMET. 

Treatment of the sulpholene (19) with hydroxylamine in refluxing ethanol has led to the formation of isothiazol-3-sulpholenes (21) which presumably progressed via the oxime intermediate (20). Subsequent heating of (21) in toluene at 185°C in a sealed tube led to the generation of the diene (22) which could be trapped with 1 -phenylmaleimide and with DMAD, thus providing a new route to 1,2-benzisothiazole derivatives <96TL4189>. 

The influence of iron(III) salts on coupling reactions of alkynes and aldehydes (Scheme 10, routes B and C) was also explored. In these routes, a new stereoselective coupling of alkynes and aldehydes was unmasked, which led to ( ,Z)-1,5-dihalo-1,4-dienes (route B, Scheme 10) and/or ( )-a,p-unsamrated ketones (route C, Scheme 10) [27]. 

Diene (VI) was reacted with aniline in methanol in the presence of sodium carbonate to yield N-pheny1-3,4-dimethylenepyrrolidine. However, the purified yield was poor (-20%). The alternate route was a more direct method to prepare the monomer (III) without the side reactions on elimination that could take place in the first synthetic approach. Nevertheless, due to the poor yield, all the monomer (III) used for this work was prepared via the first route. The alternative method does have applicability in the preparation of substituted N-phenyl derivatives as these could be less volatile and more difficult to recover by sublimation. 

An expeditious route to amidoboratabenzenes and alkylboratabenzenes has been developed that starts with commercially available 1,3-pentadiene (Scheme 9).18 Deprotonation of the diene, followed by quenching with BCl(NMe2)2, affords a 2,4-pentadienylborane. Treatment with LiNEt2 then leads to ring closure, thereby generating the amidoboratabenzene. 

As to the first route, we started in 1969 (1) in investigating unconventional transition metal complexes of the 5 and 4f block elements of periodic table, e.g., actinides and lanthanides as catalysts for the polymerization of dienes (butadiene and isoprene) with an extremely high cis content. Even a small increase of cistacticity in the vicinity of 100% has an important effect on crystallization and consequently on elastomer processability and properties (2). The f-block elements have unique electronic and stereochemical characteristics and give the possibility of a participation of the f-electrons in the metal ligand bond. 

Other than this system, metallated polysilanes contain the metal in low-valent oxidation states. Such systems have been reported by two groups. In 1995, an alternative functionalization route starting from poly[methyl(H)silylene] or poly[methyl(H)silylene-fo-methylphenylsilylene], 37, was reported, in which the polysilane Si-H moiety was hydro-silated using 1,3,5-hexatriene, affording the diene-modified polymer 67, which was metal functionalized using triiron dodecacarbonyl to give the iron tricarbonyl-polysilane coordination complex, 68.177... 

One such typical transformation is the thermal isomerization of the spiropentane derivative 76 into triene 80 which is assumed to occur via the diene intermediate 78 with the intermediate participation of the cyclopropyl-trimethylenemethane (TMM) 77 and the vinyl-TMM 79 diradicals (equation 29)44. It was shown by using deuterium labels that the diradical 79 forms the triene 80 by 1,6-hydrogen shift. The pathway 76 — 80 which occurs via tetramethylene-ethane diradical was recognized as a less probable route. 

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