1,3-Dienes endocyclic

Allylic bromination of pregnenolone acetate with dibromodi-methylhydantoin affords the 7-bromo compound (155) of undefined stereochemistry. Dehydrobromination by means of collidine followed by saponification affords the 5,7 endocyclic cis,cis-diene, 156. This compound contains the same chromophore as ergosterol, a steroid used as a vitamin D precursor. The latter displays a complex series of photochemical reactions among the known products is lumisterol, in which the stereochemistry at both C9 and Cio is inverted. Indeed, irradiation of 156 proceeds to give just such a product (158). This reaction can be rationalized by... 

These phenomena can be illustrated by the cycloaddition reactions of fulvenes with electron-deficient a-pyrones. In general, the Diels-Alder reactions of electron-deficient dienes such as 458 with 6-alkyl substituted fulvenes favor addition across one of the endocyclic... 

Niggli and Neuenschwander294 studied the reaction of fulvene (461) with cyclopen-tadiene. The main product fraction consisted of three 1 1 adducts, as illustrated in equation 138. Diels-Alder Adducts 462 and 463 resulted from attack of cyclopentadiene at the endocyclic and exocyclic double bonds of fulvene, respectively. The formation of 464 was rationalized by a [6 + 4] cycloaddition reaction followed by two [1,5] hydrogen shifts. It was stated that due to the absence of electron-donating and electron-withdrawing groups on both triene and diene, fulvene may have reacted via its HOMO as well as its LUMO. 

There are other possibilities for selective reduction in the hydrogenation of symmetrically substituted dienes. Raney-nickel afforded 1-alkenes, whereas supported Pd catalysts gave a mixture of 1- and 2-alkenes148. A selective reduction of a terminal double bond was carried out in the presence of an endocyclic double bond, which was trisubstituted149-152. 

The Diels-Alder reaction between a 2-fluoroacrylic acid derivative of 8-phenyl-menthol (83) and cyclopentadiene shows high exo- and jr-diastereofacial selectivity (Scheme 30). The C(2) of endocyclic cross-conjugated 2-(acylamino)-l,3-dienes exerts excellent diastereofacial control on the Diels-Alder addition with electron-deficient dienophiles to produce octahydroquinolines. ... 

The first important point about the reaction is that the diene must be in the s-cis conformation (12) in order to react. Thus dienes with one double bond exocyclic, such as 14, do not give the reaction, whereas endocyclic dienes (15) react rapidly. Reactivity of open-chain dienes, assuming constant substituent R, depends on the equilibrium between the two conformations. A cis substituent favors the s-trans form (17) and retards the addition, whereas a 2-substituent... 

Depending on the nature of both the halogen and the metal, the group 14 metalloles can react with halogens in three different ways (i) addition of 1 or 2 equivalents of halogen to the diene system, with the cyclic structure preserved, (ii) cleavage of one or two endocyclic E-C bonds, with destruction of the cyclic structure, and (iii) substitution of one or two exocyclic substituents, with preservation of the metallole structure. 

The endocyclic and exo-endo diene systems of 2-vinylfuran la participate in cycloaddition reactions with dimethyl acetylenedicarboxylate... 

DMAD), and a 1 1 mixture of the diesters 2a (endocyclic product) and 4a (exo-endo product) was obtained when the reaction was carried out at room temperature. The overall yield (10%) was very low due to polymerization of the 2-vinylfuran. In contrast with reactions with other dienophiles (see Section II,B,1), in which the exo-endo diene of 2-vinylfurans proved to be more reactive than the furan system itself, the reaction with DMAD resulted in reduced selectivity, probably due to equilibration of the more soluble adducts in this case (68TL4589 73AJC1059). 

If (/-orbital participation in bonding occurs to any significant extent in this series of compounds it does so in the 9-oxa-10-dichlorogermyl derivative, which, together with only a 9.5 deviation from ring planarity, exhibits the expected short (1.90 A) endocyclic Ge—C bonds (170). Nonetheless, such a claim is made with justifiable caution in view of the absence of the same phenomena in both the fluorene (68) and 1,4-digermacyclohexa-2,5-dienes (21, 26), whose ring planarity would seem even more favorably disposed to tt bond formation. 

The hydrocarbons 52a-52e and 52g contain four G=C double bonds. In addition to the conjugated diene unit, two isolated bonds are present. The exocyclic one is parallel and the endocyclic one is perpendicular to the local plane of symmetry of the diene portion. When the hydrocarbons 52a-52e and 52g are recoordinated to chromium by the reaction with tris(acetonitrile)-tricarbonylchromium(0) (55) [Eq. (32)], the complexes 46 are not obtained... 

A preference for the coordination of an exocyclic C=C double bond in place of an endocyclic one was also observed for 9-methylene-bicyclo[4.2.1]nona-2,4,7-triene. With 55 this hydrocarbon forms [Cr(CO)3-(fj4 2-C10H10)], in which comparable stereochemistry with 56a is found. However, the diene portion shows Cr—C distances of 220 and 240 pm, as does the exocyclic C=C-double bond (80,81). Hence the differences of bond strengths are not as pronounced as in 56a. 

Random ethylene/propylene copolymers are amorphous and represent an interesting class of synthetic elastomers. The introduction of double bonds, useful for sulphur vulcanisation in the copolymer, can be achieved by copolymerisation of ethylene and propylene with non-conjugated dienes containing only one double bond capable of insertion for instance, 1,4-hexadiene, dicy-clopentadiene and 5-ethylidene-2-norbornene (endocyclic double bond)... 

The selective consecutive cycloisomerization/isomerization of 1,6-dienes to form exocyclic and then endocyclic olefinic compounds is not a common transformation. At 80 °C, with RuCl2(p-cymene)(N-mesitylmethylbenzi-midazole) activated by reaction with diphenylpropynol in the presence of AgOTf as the catalyst, as an attempt to generate allenylidene intermediate, diallyl tosylamide is selectively transformed into 3-mclhyl-4-mclhylene-N-tosylpyrrolidine. When this catalytic cycloisomerization is completed, the isomerization into 3,4-dimethyl-AMosylpyrrolidine starts without external modification of the catalytic system (Scheme 27) [68]. 

Enyne metathesis catalyzed by RuCl2(= CHPh)(PCy3)2 under ethylene is a very efficient reaction to produce conjugated dienes with one endocyclic C = C bond and an exocyclic vinylic bond. The addition of a diazo com-... 

Hexakis(2,4,6-triisopropylphenyl)tetragermabuta-l,3-diene 182 was obtained and converted upon treatment with sulfur into thiatetragermacyclopentene 183 with an endocyclic Ge=Ge bond. The reaction of 183 with dry air furnished a 2,4,7,8-tetraoxa-l,3,5,6-tetragermabicyclo[4.1.1]octane derivative 184 (Scheme 33). Structures of 183 and 184 were confirmed by X-ray crystallographic analysis <20030M1302>. 

Similar decomposition patterns can be proposed for secondary radicals derived from 2 in the initiating hydrogen abstraction Step 1. Thermal aromatization of 6-ring cyclic dienes containing one exocyclic and one endocyclic double bonds is a facile process at 550°-600°C. The reaction involves fast double bond isomerization to a conjugated cyclohexadiene, followed by dehydrogenation (30, 31,32). 

An important ancfllary study with signifrcant synthetic implications revealed that electron-rich dienes attached to fulvenes by a multicarbon chain also exhibited a high degree of preference for forming the corresponding [6 -i- 4] adducts. For example, when substituent X in compound (60) was either diethyl-amino or trimethylsiloxy, the [6 -I- 4] adduct (61) was the major product-type isolated. However, if no substituent was present on the diene portion of the molecule, a mixture consisting of [6 -i- 4], exocyclic [4 -I- 2] (62) and endocyclic [4 -I- 2] (63) cycloadducts was obtained. ... 

A variety of other conjugated dienes participate in the [4 + 1] annulation with stereochemistry in accord with the predictions of equation (20). The 1-vinylcycloalkenes (87)-<89) can be viewed as ( )-sub-stituted dienes, and are thus expected to yield the exo-alcohols (90)-<92). In each of these cases carbene addition generates a mixture of four intermediate 2-vinylcyclopropyl ethers, each of which then rearranges to produce exclusively the predicted stereoisomer. Application of the annulation to the endocyclic (Z,Z)-dienes (93) and (94) also generates in each case a single bicyclo[/i.2.1]alkenol, and the ( , )-diene (97) similarly reacts to afford exclusively the cyclopentenol (98 Scheme 7). 

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