Exocyclic dienes

The decomposition of N-phenyl-l,3,4,6-tetrahydrothieno(3,4-c)-pyrrole-2,2-dioxide (II) was carried out in a sublimator at elevated temperatures ( 160°C) to expel SO. and to give the exocyclic diene (III). 2... 

In addition to monomer III two major by products (IV) and (V) were detected (18). Compound (IV) was a dimer which resulted from Diels-Alder reaction between two exocyclic dienes (III) and was found at the bottom of the sublimator after thermal decomposition of the sulfone II. The GC/mass spectrum of this sample showed a molecular ion peak of 342 m/e that was in agreement with dimer molecular weight. The compound (V) had a pink tint and was found in a trace quantity in the crude N-pheny1-3,4-dimethylenepyrrolidine (III) by NMR (6 1.9 - 1.8, -CH, s). It could also be readily formed when N-pheny1-3,4-dimethyrenepyrrolidine (III) solution was exposed to air for a period of time. 

When the decomposition of N-phenyl-1,3,4,6-tetrahydrothieno-(3,4-c)-pyrrole-2,2-dioxide (II) was carried out in a sublimator a relatively high yield (80-95%) was obtained. However, under identical conditions, the decomposition of 5-(carboethoxyphenyl)-l,3,4,6-tetrahydrothieno(3,4-c)-pyrrole-2,2-dioxide yielded only 15% of diene product. This observation was found in agreement with the results reported by Alston (18). It was suggested that the yield from these sulfones depended on the relative volatility of the exocyclic diene formed since these dienes could undergo dimerization readily at the decomposition temperature of lbO C. 

An alternative preparation of the exocyclic diene (III) was attempted by first thermally decomposing the bromosulfone (I) to yield 2,3-bis(bromomethyl)-1,3-butadiene VI, which, in turn, was allowed to react with aniline following the procedure described by Gaoni (19). 

Figure 3. Alternative Preparation of Exocyclic Diene (III) Monomer.

This study has demonstrated that a cyclic pyrroline polymer could be prepared by a free radical initiation of the corresponding exocyclic diene monomer. The polymerization was shown to proceed predominantly by 1,4-addition as expected from a free radical initiator with diene monomers. 

Exocyclic dienes 509<1998HCA828> and 511 <1997JOM(532)181, 1997T12369> give the expected products 510 and 512 in good yields (Equations 61 and 62). 

A new entry to exocyclic dienes was reported by Sha who uncovered that a radical cyclization of the vinyl iodide 100 can lead to the formation of an exocyclic dienes fused with a tetrahydrofuran ring. The cyclization is proposed to proceed in a 5-(n-exo)-exo-dig fashion <00OL2011>. 3,4-Disubstituted tetrahydrofurans can also be constructed via the cyclization of O-stannyl ketyls and allylic 0-stannyI ketyls onto electron-rich or electron-poor alkenes <00TL8941>. 

A series of conjugated polyenes capped with chromophores and containing an androstane spacer were synthesized by Wittig or Wittig-type olefinations from epi-androsterone 5150. For example, vinyl carboxaldehyde 52, prepared from 51 in 60% yield as shown in equation 32, was treated with 9-anthrylmethylphosphonium bromide and n-butyllithium to give diene 53. Exocyclic diene 53 was subsequently oxidized to vinyl carboxaldehyde 54. The androsterone vinyl aldehyde intermediate could either be treated with a tetraphenylporphyrinpolyenyl phosphonium ylide, or, as shown below, the phosphonium salt of the androsterone (55) could be reacted with TPP polyeneal 56. The desired all-(E) isomer, 57, was obtained from the ( )/(Z)-isomeric mixture by chromatographic purification. 

The preparations of acyclic and exocyclic diene complexes of ZrCp2 and HfCp2 were reported almost simultaneously by Erker and coworkers10 and by Nakamura and coworkers52a. These complexes may be prepared (Scheme 4) (1) by direct complexation... 

Fig. 30 Tandem Ugi-IMDA processes that exploit exocyclic dienes...

For the first time, application of sequential Diels-Alder reactions to an in situ-generated 2,3-dimethylenepyrrole was shown with various dienophiles 548 to afford 2,3,6,7-tetrasubstituted carbazoles (549). This novel tandem Diels-Alder reaction leads to carbazole derivatives in two steps, starting from pyrrole 547 and 2 equivalents of a dienophile, and is followed by 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) oxidation of the intermediate octahydrocarbazole. Mechanistically, the formation of the intermediate octahydrocarbazole appears to involve two sequential [4+2] cycloadditions between the exocyclic diene generated by the thermal elimination of acetic acid and a dienophile (529) (Scheme 5.17). 

The addition of allene and a secondary amine to O-propargyl-2-iodophenol led to the sequential intramolecular insertion of the acetylene bond, followed by the intermolecular insertion of allene and finally the capture of the formed allylpalladium complex by the secondary amine to give benzofurane analogues bearing an exocyclic diene system (3.31.). 

The same reactions, carried out with potassium carbonate as base in place of a secondary amine, yield exocyclic dienes in good yield, although double-bond isomerization sometimes occurs (equation 38).93 Inclusion of tetra-zi-butylammonium chloride in the reaction mixture stops the double-bond isomerization. Thus, the reaction in equation (38) with the chloride yields only the bis(exomethylene) product in 45% yield in a slow reaction. Some N- and O-heterocyclic products, also, have been prepared by the intramolecular vinyl substitution reaction.94 A 16-membered ring lactone was made by the ring closure of a vinylic iodide group with a vinyl ketone group. The yield, based upon the reactant, was 55% but a stoichiometric amount of bis(acetonitrile)palladium dichloride was employed. The catalyst was prereduced with formic acid so that the reaction proceeded at 25 C (equation 39).95... 

The 2-(l-trimethylsilyloxyvinyl)furan 112 reacts with AZ-phenylmale-imide (NPMI) also through the exocyclic diene to afford a 1 1 cycloadduct which was rearomatized by air oxidation to give compound 113 (83H1933). 

This reagent converts diynes separated by two- to five-carbon chains into (E,E)-exocyclic dienes containing four- to seven-membered rings in 40-89% isolated yields. 

The cationic hexamethylbenzene ruthenium complex 254 can be depro-tonated by treatment with t-BuOK to give the ruthenium complexes 255 and 256 in which the r 4-o-xylylene ligand is coordinated via its exocyclic diene (Scheme 23). Protonation of 255 gives the dication 15c, whereas protonation of 256 produces the fluxional tj3-benzyl derivative 257 which is stabilized by an agostic C—H bond (156,157). 

The mechanism of the formation of compound 1137 appears to be two sequential [4+2] cycloadditions between the exocyclic diene of compounds 1139 and 1141 and a dienophile (Scheme 223). The 2,3 d ethylenepyrrole required for the Diels-Alder reaction can be generated by the thermal elimination of acetic acid to form compound 1139, which is observed by mass spectroscopy. There are two possible pathway by which diene 1139 can proceed to tricycle 1137. The first is the elimination of a second molecule of acetic acid from diene 1139 to form 5-benzyl-aza[5]radialene 1140, which is also observed by mass spectroscopy. Attempts to improve the yield of compound 1137 by accelerating the elimination of acetic acid by acid or base catalysis failed, resulting in the decomposition of compound 1136 < 20000L73>. 

E,E-Exocyclic dienes J A titanium reagent (1) consisting of (CsHsljTiCI, CHjPCCaH,), and sodium amalgam in the ratio 1 1 2 effects cyclization of diynes (2) in which n = 3, 4, or 5 to (E,E)-exocyclic dienes (3) (equation 1). Highest yields (80%) are obtained when... 

The radical cation Diels-Alder reaction has been the subject of many mechanistic and theoretical investigations and has been shown to have much synthetic potential. With regard to heteroaromatics, the reaction has been exploited by Steckhan in the cycloaddition of indoles and 1,3-dienes. This reaction occurs smoothly upon photosensitization by triarylpyrrilium tetrafluoroborates. The reaction is satisfactory rationalized as involving addition of the indole radical cation to electron-rich dienes (Scheme 35), and the regioselectivity is in accord with theoretical predictions [104]. The reaction with exocyclic dienes has been developed for the synthesis of carbazole derivatives such as 52 and 53 [105]. 

The Diels-Alder reaction of the exocyclic diene derivative 920 obtained from 919, with 1,4-naphthoquinone, gave 921 and thus allowed a further functionalization of this molecule by another anthraquinone moiety (Scheme 139) <1999CG2433>. 

Modified T) -titanium compounds have been prepared from exocyclic dienes. The compound (123, n = 3) is an example it is prepared (equation 47) by addition of Bu MgCl to a solution of 1-vinylcyclopen-tene (122, n = 3) and (T) -C5H5)2TiCl, Its reaction with propanal takes place regiospecifically at the carbon atom belonging to the cycle, in 86% yield, affording the sole erythro compound (124, n = 3) (equation... 

Alkylation of hydrazine with a, 3-unsaturated carbonyl derivatives or carbonyl derivatives with a leaving group in the p-position provides pyrazole derivatives. For example, treatment of the tosylate (77), obtained from L-serine, with anhydrous hydrazine gives racemic pyrazolidinone (78). It appears that py-razolidinone (78) or one of the intermediates suffers base-catalyzed racemization (equation 32). Starting from P-lactam (79) seven-membered cyclic hydrazine (80) has been formed by ring closure in an unusually high (84%) yield (equation 33). Reaction of ( ir-allyl)palladium complex (81) with dimethyl-hydrazine produces exocyclic diene (82) in a modest (29%) yield, but this is still more efficient than the reaction of 2,3-bis(chloromethyl)butadiene (83) with dimethylhydrazine (equation 34). ... 

Exocyclic dienes. Hexacarbonyldicobalt complexes of 1,6- and 1,7-enynes are readily formed. In refluxing toluene these complexes release dienes. 

In the same way, exocyclic dienes 165, 166, 167 and 168 (Chart 3) react readily with naphthoquinones to give products with the anthracyclinone skeleton. 

The cycloaddition of tetrahalocyclopropenes to exocyclic dienes leads to bicyclo[4.1.0]heptanes which are readily dehydrohalogenated to analogs of benzocyclopropene (Table 14). 

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