Trienes Diels-Alder reactions

Heteroatom functionalized terpene resins are also utilized in hot melt adhesive and ink appHcations. Diels-Alder reaction of terpenic dienes or trienes with acrylates, methacrylates, or other a, P-unsaturated esters of polyhydric alcohols has been shown to yield resins with superior pressure sensitive adhesive properties relative to petroleum and unmodified polyterpene resins (107). Limonene—phenol resins, produced by the BF etherate-catalyzed condensation of 1.4—2.0 moles of limonene with 1.0 mole of phenol have been shown to impart improved tack, elongation, and tensile strength to ethylene—vinyl acetate and ethylene—methyl acrylate-based hot melt adhesive systems (108). Terpene polyol ethers have been shown to be particularly effective tackifiers in pressure sensitive adhesive appHcations (109). 

Aqueous hydrofluoric acid dissolved in acetonitrile is a good catalyst for intramolecular Diels-Alder reactions [9] This reagent promotes highly stereoselective cyclizations of different triene esters (equation 8) The use of other acids, such as hydrochloric, acetic, and trifluoroacetic acid, results in complete polymerization of the starting trienes [9] (equation 8)... 

Brmsted acid-assisted chiral Lewis acid 8 was also applied to the intramolecular Diels-Alder reaction of an a-unsubstituted triene derivative. ( , )-2,7,9-Decatrienal reacts in the presence of 30 mol% of the catalyst to afford the bicyclo compound in high yield and good enantioselectivity [lOd] (Scheme 1.17). 

A special method, with only two examples, starts from 1,2,4-triazines.20 21 Diels-Alder reaction with the strained dienophile dimethyl tricyclo[4.2.2.02,5]deca-3,7,9-triene-7,8-dicarboxylate (14) is followed by an elimination of nitrogen via a retro-Diels-Alder process. The formed product, however, cannot be isolated, but reacts via another retro-Diels-Alder reaction and an electro-cyclic reaction to provide the azocine derivative 15. The sequence order of the reactions is not clear, but both pathways lead to the same product. 

The synthesis of highly substituted rigid tricyclic nitrogen heterocycles via a tandem four-component condensation (the Ugi reaction)/intramolecular Diels-Alder reaction was investigated in both solution and solid phase [24]. The Ugi reaction in MeOH (Scheme 4.2) involves the condensation of furylaldehydes 17, benzylamine 18, benzyl isocyanide 19 and maleic or fumaric acid derivatives 20, and provides the triene 21 which immediately undergoes an intramolecular Diels-Alder reaction, affording the cycloadduct 22 in a diastereoisomeric mixture with high yield. 

Solid-phase Diels-Alder reactions of amino acid derived trienes [23]... 

Diels-Alder reaction between the Danishefsky triene 1659 and excess dimethyl-acetylene dicarboxylate or methylpropiolate in boiling benzene proceeds, via 1660 and 1661, with loss of trimethylsilanol 4, to give 1662 a and 1662b in 51 and 37% yield, respectively these are transsilylated with methanol to give 1663a and 1663b [40] (Scheme 10.18). 

The reaction of benzyne with cyclohexadiene has been known for some time 4>, but although a number of steroidal cis-dienes are readily available no reactions with arynes had been reported prior to our beginning such investigations 145>. This was somewhat surprising in view of the number of reports concerning the modification of steroids by means of reactions with carbenes 146 i49) and the known Diels-Alder reactions of steroidal dienes and trienes iso.isi). 

Calorimetric investigation of a Diels Alder reaction between propenal and a triene, which had caused problems on scale-up, showed, after the exotherm due to the Diels Alder reaction, and from a temperature a litle above 200°C, a second, more exothermic, reaction with a very fast pressure rise which burst the ARC can employed. This is presumably aromatisation of the alkenylcyclohexenealdehyde first formed, with probable liberation of hydrogen and carbon monoxide. 

Martin and coworkers [93] described a highly efficient enantioselective total synthesis ofmanzamine A (6/1-177) with a concise domino Stille/Diels-Alder reaction to construct the tricyclic ABC ring core in 6/1-177 as the key step. Reaction of 6/1-178 with vinyl tributylstannane in the presence of (Ph3P)4Pd afforded the triene... 

In intramolecular Diels-Alder reactions, two rings are formed in one step. The reaction has been used to synthesize a number of interesting ring systems.29 The intramolecular cyclization of ( )-l-nitrodeca-l,6,8-triene at 80 °C affords an endo cycloadduct with the tram ring fusion preferentially, as shown in Eq. 8.18. In contrast, (Z)-nitroalkenes produce a nearly 1 1 mixture of cis- and tra/w-fused cycloadducts.30... 

Isopropyl-4,6-dimethyltricyclo[4.4.05 9]dec-3-en-2-one (40) with unknown absolute configuration was isolated as a putative male pheromone of a Brazilian predatory stink bug (Tynacantha marginata), and its enantiomers were synthesized by Kuwahara as shown in Scheme 58 [88]. The triene A gave B and C by an intramolecular Diels-Alder reaction, and they could be converted to the enantiomers of 40. 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

Diels-Alder catalysis.1 This radical cation can increase the endo-selectivity of Diels-Alder reactions when the dienophile is a styrene or electron-rich alkene. This endo-selectivity obtains even in intramolecular Diels-Alder reactions. Thus the triene 2, a mixture of (Z)- and (E)-isomers, cyclizes in the presence of 1 to 0° to the hydroindanes 3 and 4 in the ratio 97 3. Similar cyclization of (E)-2 results in 3 and 4 in the ratio 98 2 therefore, the catalyst can effect isomerization of (Z)-2 to (E)-2. Even higher stereoselectivity is observed when the styrene group of 2 is replaced by a vinyl sulfide group (SC6H5 in place of QHtOCT ). 

Bicyclic derivatives. Polyhydroxylated carbo-bicyclic derivatives may be regarded as carbasugars with the rigid structure resulting from the presence of the additional carbocyclic ring. The most convenient way for construction of the bicyclic skeleton consists of the Diels-Alder reaction of properly functionalized trienes (intramolecular version) or dienes and olefins (intermolecular). 

Scheme 16.29 Formation of a 1,3,5-triene and Diels-Alder reaction.

The thermally allowed [8 + 2] cycloaddition reactions may be considered as the 10tt analogs of the Diels-Alder reaction in which the diene component has been replaced by a tetraene component. Like trienes in the [6 + 4] cycloaddition reactions, the 87t tetraenes must satisfy certain requirements concerning geometry in order to be able to participate in an [8 + 2] cycloaddition. For example, tetraenes 518 and 519 can undergo an [8 + 2] cycloaddition, whereas an [8 + 2] cycloaddition with 520 is virtually impossible. Due to its fixed -system, 519 is more reactive in cycloaddition reactions than 518 and is therefore more often encountered in the literature. [8 + 2] Cycloadditions have been applied only... 

The non-nitrogenous carbene precursor (102) was used for the photochemical generation of the carbene (103) without complications due to reactions of diazirine or diazo species. In the presence of alkenes, carbene (103) gave rise to cyclopropanes and in the absence of alkenes was proposed to undergo [1,2]-C shift to form (104), which suffered retro-Diels-Alder reaction to give a triene. 

The asymmetric Diels-Alder cycloadditions of enantiopure (5)-5-(/ -tolylsulfinyl)-1,4-benzoquinones with Dane s diene under thermal and Lewis acid conditions produce tetracyclic quinones after spontaneous elimination of the sulfinyl group.The Diels-Alder reaction of barrelene with o-benzoquinone produces tetracyclo[6.2.2.2 .0 ]tetradeca-9,ll,13-triene-4,5-dione. Under kinetic control, the Diels-Alder cycloaddition of 2,3-dicyano-p-benzoquinone (98) with cyclopentadiene in MeOH produces the single cycloadduct (99) (Scheme 38). ... 

Norbornadiene adds typical dienophiles in a homo-Diels-Alder reaction whereby, for example, the adduct 86 is obtained with tetra-cyanoethylene. Dimethyl acetylenedicarboxylate and dicyanoacetyl-ene add to barrelene (bicyclo[2.2.2]octa-2,5,7-triene) in a similar way. °... 

The cross-conjugated trienes have potential in many different types of diversification strategies. For example, the triene clearly lends itself to inter- and intramolecular Diels-Alder reactions. Incorporation of the hydroxymethyl group on the tether allows attachment of functionality suitable for reactions subsequent to the Alder-ene reactions. As depicted in Scheme 8.5, propargyl tosylamides A, alkynyl silanes B, acrylate esters C, and propargyl ethers D can all be readily prepared from 39... 

The trienes were then subjected to a formal Diels-Alder reaction using conditions developed by Gilbertson and others (Scheme 8.7) [34]. The propargyl tosylamide 40 and the propargyl ether 43 have both afforded the formal intramolecular Diels-Alder adducts 44 and 45 in high yield. To date, the formal cycloaddition of the siHcon-teth-ered alkyne 41 has not been affected and heating triene 42 led to a thermal Diels-Alder reaction to furnish cycloadduct 46, albeit in lower yield than the corresponding rhodium-catalyzed examples. 

Corey later reported that BLA 59a catalyzes the intramolecular Diels-Alder reaction of several triene aldehydes and esters with high asymmetric induction to yield the corresponding 6/5-trons-fused bicyclic structures (Scheme 5.74) [143]. 

Very few methods exist for the stereoselective synthesis of fused bicyclic sulfonamides. One example, developed by Tozer and co-workers, involves an inverse electron demand Diels-Alder reaction (Equation 34 Table 4) <20010L369>. Electron-deficient sulfonamide trienes 216, which are prepared in a few steps from V-BOC-protected methanesulfonamide, react slowly at high temperature in a sealed tube to produce m-217 as the major isomeric cycloadduct (BOG = /-butoxycarbonyl). 

A special case of the preparation of cyclobutanes from 1,5-dienes via valence isomerization is the use of acyclic or cyclic 1,5,7-trienes which give four-membered rings via an intramolecular [7t + 7ts2] cycloaddition (Diels-Alder reaction). This variant is illustrated for monocyclic tricnes 18 and 20 where two 71-bonds are transformed into a-bonds, resulting in tricyclic compounds 1968 and 21.09... 

Oxabicyclo[3.2.0]hepta-l,4,6-triene (289), a planar Sn-electron analog of 4, has been prepared by flow pyrolysis of 288 (both cis and trans) in approximately 10% yield (>95% purity) 289 is an extremely sensitive compound, polymerizing instantaneously on exposure to oxygen. In solution, where it is stable for several days, it slowly dimerizes to give the known compound 291 the pentacyclic intermediate 290 is possibly involved. In Diels-Alder reactions, 289 behaves like an olefin with cyclopentadiene it reacts immediately to give 292. Hydrogenation occurs at the same site. ... 

Intramolecular Diels-Alder reactions of trienes. -10 The thermal cycli/ation of the (f ,L)-triene 1 afl ords trans- and ci.v-perhydroindcnes 2 and 3, with a slight preference for the former isomer. The reaction is markedly catalyzed by Lewis acids such as A1C13, C,II5A1C12, and TiCI4, and results in cyclization exclusively to the... 

A. A. Ghini, C. Bumouf, J. C. Lopez, A. Olesker, and G. Lukacs, Intramolecular Diels-Alder reactions on pyranose trienes. Stereoselective access to bis-annulated pyranosides. Tetrahedron Lett. 37 2301 (1990). 

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