Diels-Alder dimerisation

Also the mirror image of the strueture I, eorreetly denoted as exo-3,10-dihydroxy-3,5,8,10-tetra-methyltrieyelo[6.2.2.0 ]dodeea-5,l 1-diene-4,9-dione, would be possible sinee enantiomers are not differentiated by NMR. A retro-Diels-Alder fragmentation of I to CsH/oO explains why the moleeular ion eorresponding to the moleeular formula C16//20O4 is not deteeted in the mass spee-trum. The metabolite I eould be formed by Diels-Alder dimerisation of 1,5-dimethyleyelohexa-l,3-dien-5-ol-6-one J as the primary metabolite which acts as diene and dienophile as well... 

Diels-Alder dimerisation of 1,2-dihydrophosphinine oxides has also been reported, giving new 2-phosphabicyclo[2,2,2]oct-5-ene-2-oxides, e.g.,... 

For the formation of the naphtho[l,8-6c]pyrandione nucleus in these pigments a mechanism involving Diels-Alder dimerisation of a 1,2-benzoquinone precursor can be envisioned (Scheme 22). The dimer (136) may be further oxidised to the intermediate (137) which could then undergo fragmentation to give (138) as indicated by the arrows. Tautomerisation and lactone ring formation then leads to the bad-iones-A or -B. 

Breslow studied the dimerisation of cyclopentadiene and the reaction between substituted maleimides and 9-(hydroxymethyl)anthracene in alcohol-water mixtures. He successfully correlated the rate constant with the solubility of the starting materials for each Diels-Alder reaction. From these relations he estimated the change in solvent accessible surface between initial state and activated complex " . Again, Breslow completely neglects hydrogen bonding interactions, but since he only studied alcohol-water mixtures, the enforced hydrophobic interactions will dominate the behaviour. Recently, also Diels-Alder reactions in dilute salt solutions in aqueous ethanol have been studied and minor rate increases have been observed Lubineau has demonstrated that addition of sugars can induce an extra acceleration of the aqueous Diels-Alder reaction . Also the effect of surfactants on Diels-Alder reactions has been studied. This topic will be extensively reviewed in Chapter 4. 

Synthesis The Diels-Alder reaction is simply the dimerisation of isoprene to giye the naturally occurring terpene A. Now we haye to cleaye one double bond and leaye the other alone. It turns out that epoxidation is selectiye in this case. 

Can polymerise, but its main danger iies in its propensity to dimerise by the Diels-Alder reaction. This reaction takes piace at a temperature starting at 0-40°C, under pressure, if the dimerisation is not controlled, the storage equipments temperature and pressure rise very quickly, which leads to their destruction. Storage temperatures of -80°C have been recommended. 

This is a general situation for thermal, concerted additions those involving Ane + 2ne systems proceed readily, e.g. the Diels-Alder reaction, whereas those involving 2ne + 2ne systems, e.g. the cyclo-dimerisation of alkenes, do not. We might, however, expect that photochemical cyclodimerisation of alkenes would be symmetry... 

Another example is the synthesis of carpanone (23), which takes place by a two-step dimerisation of two identical moieties (22). the second step proceeding via a hetero-Diels-Alder cycloaddition [19] (Scheme 4.5). 

Having made extensive studies of the behaviour of models of the propagation center of 1,3-diphenyl butadiene this monomer would be the most appropriate one to investigate. Unfortunately, it seems that it cannot be isolated because of extremely facile dimerisation via the Diels Alder reaction. A cursory examination has, however, been made of the polymerisation of... 

This double Claisen-Schmidt reaction takes place under the influence of ethano-lic potassium hydroxide (Expt 7.7) and presumably proceeds in the stepwise manner (cf. formulation in Section 6.12.2, p. 1032). The four aryl groups in tetra-cyclopentadienone effectively stabilise the cyclopentadienone system, which otherwise has only a transient existence and readily undergoes dimerisation by way of a diene-dienophile interaction (Diels-Alder reaction, Section 7.6). The use of tetracyclone as a dienophile for the preparation of 3,4,5,6-tetraphenyl-dihydrophthalic anhydride is noted on p. 1121. 

Some molecules are studied for their theoretical interest one being cyclopentadienone 16. But it turns out that this dimerises instantly by a Diels-Alder reaction and cannot be studied. The simplest cyclopentadienone that can be made is the tetraphenyl compound 17. Aldol disconnection gives 18 but we can now do a second aldol disconnection to reveal the two symmetrical starting materials 19 and 20. 

Von besonderer praparativer und theoretischer Bedeutung sind die Er-gebnisse der photosensibilisierten Dimerisation von 1,3-Dienen. Hier konkurrieren C4-Cyclodimerisation und C6-Cyclodimerisation (Diels-Alder-Typ). So bildet sich aus Butadien bei der photosensibilisierten Dimerisation neben cis- und trans-l,2-Divinylcyclobutan (Formeln 16 tmd 17) auch 4-Vinylcyclohexen (Formel 18) (134, 139). Isopren liefert... 

Gut untersucht ist auch die photochemische Dimerisation des Cylohexa-1,3-diens. Nach Schenck et al. (264) bilden sich bei der photosensibilisierten Dimerisation bei 20 °C das anti-C4-Cycloaddukt 33, das syn-C4-Cyclo-addukt 34 und das exo-Diels-Alder-Addukt 35 in einem vom jeweils ver-wendeten Sensibilisator (z. B. Benzaldehyd, Benzophenon, Michlers Keton) praktisch unabhangigen Mengenverhaltnis von etwa 12 3 5. 

The most notable Diels-Alder reaction in which the heteroatom is part of the diene system is the dimerisation of acrolein 6.178 giving the adduct 6.179. As in the reaction of butadienecarboxylic acid 6.154 with acrylic acid 6.155 [see (Section 6.5.2.2) page 229], the two electrophilic carbon atoms are the ones which have become bonded. 

The highest mass peak in the mass spectra of these dimers corresponds to M /2, even at 120 °C and 15 eV. The fragmentation pattern for presecamine (138) is identical to that of compound (136) which has been named secodine. This indicates the occurrence of a facile retro-Diels-Alder reaction such a process can be carried out preparatively at 175°C/0.2mm to give secodine (136) from presecamine itself, and dihydrosecodine (137) from tetrahydropresecamine. The product (137) has been identified by direct comparison with synthetic material. On standing without solvent, (136) and (137) dimerise to give diastereoisomeric mixtures of the corresponding presecamines. 

To date, no known bisindole alkaloid has been shown to be only an artefact. In addition, no experimental evidence exists which undermines the assumption that bisindole alkaloids are actually formed from the completed monomeric partners. Support for this idea is derived from the kind of reactions apparently necessary to effect such dimerisations which are known biogenetic processes amine-aldehyde condensations, Mannich reactions, Michael additions, Friedel-Craft type condensations, Diels-Alder type processes, radical coupling etc. The observation that the skeletal distribution amongst monomeric alkaloids is reflected throughout the dimeric series lends further support. 

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