Diels-Alder reactions with isoprene

Davies and coworkers , for example, used iV-enoyl derivatives of a cw-l-aminoindan-2-ol based L3-oxazolidin-2-one (222) as chiral dienophiles in the Diels-Alder reactions with isoprene (91a) and piperylene (91b) which give 223 (equation 62). Their results have been summarized in Table 6. The reactions proceeded with high endo/exo and regioselectivities. Bidentate co-ordination of the catalyst to both carbonyl groups kept the dienophile in a rigid conformation, which gave rise to the high de values observed. 

To increase the low enantiomeric excess of 40% in the stereoselective Diels-Alder reaction with isoprene, the resin-bound auxiliary was modified. After introduction of an aminohexanoic acid spacer to the chiral auxiliary, the corresponding acrylate 15 showed an enhanced ee (70%) on applying the same reaction conditions and yielded 77% (Scheme 12.8). 

The formal 47t participation of the monomeric 2-arylmethylene-l-tetra-linthiones in Diels-Alder reactions with isoprene and (E,E)-, 4-diphenyl-1,3-butadiene provides 3,4-dihydro-2//-thiopyran derivatives [Eq. (5)].7d... 

In Chapter 10 it is stated that Rosyrane (P33.1) can be prepared from benzaldehyde either by Prins reaction with isoprenol (P33.2) or by a hetero-Diels-Alder reaction with isoprene (P33.3). Might one expect differences in the product from these two reactions (Figure P33). 

The vinyl tetralins 146 obtained from the intermolecular Diels-Alder reaction with isoprene were incorporated into a route for nonstereoselective synthesis of the desired pentacycles. Alkylation of 146 with 141 under the usual conditions gave the 1,1,2,2,-tetrasubstituted tetralin 151 as a stereoisomeric mixture. Thermolysis of the mixture yielded the pentacycle (136 or 150) and its stereoisomer... 

Photochemical reactions can produce highly reactive intermediates for subsequent groimd state reactions. Photoisomerization of the cycloheptenone moiety in 151 led to a highly strained trans enol (152) that readily underwent a Diels-Alder reaction with isoprene to produce a mixture of 153 and 154, both of which were carried forward to a structure related to a naturally occurring tricyclic diterpene, vibsanin E (equation 12.94). Irradiation of the aldehyde... 

Cycloaddition. A cobalt-catalyzed system has delivered highly efficient and regioselective Diels-Alder cycloadditions between trialkylsilylacetylenes and 1,3-dienes. TIPS-acetylene was equally effective as other silylacetylenes (TMS-, TES-, TPS-) for the Diels-Alder reaction with isoprene (eq 33). ... 

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

Sequential radical cyclizations are also featured in an efficient and clever synthesis of the cedrane framework 83 (see Scheme 15).30 Compound 81, the product of a regioselective Diels-Alder reaction between isoprene (79) and nitroethylene (80), participates in a nitroaldol reaction (Henry reaction) with 5-methyl-4-hexenal in the presence of a basic resin to give 82. Because the nitro group in... 

The combination of Lewis-acid catalysis and sc-COi has also been investigated. One of these studies involved the AlCls-catalyzed Diels Alder reaction of isoprene and maleic anhydride in sc-COi at 67 °C and at 74.5-78.5 bar [89]. The reaction rate was enhanced with respect to the uncatalyzed reaction and an unconcerted two-step mechanism was suggested [89]. 

It has been known that aromatic heterocycles such as furan, thiophene, and pyrrole undergo Diels-Alder reactions despite their aromaticity and hence expected inertness. Furans have been especially used efficiently as dienes due to their electron-rich properties. Thiophenes and pyrroles are less reactive as dienes than furans. But pyrroles with A-elecIron-withdrawing substituents are efficient dienes. There exists a limited number of examples of five-membered, aromatic heterocycles acting as dienophiles in Diels-Alder reactions. Some nitro heteroaromatics serve as dienophiles in the Diels-Alder reactions. Heating a mixture of l-(phenylsulfonyl)-3-nitropyrrole and isoprene at 175 °C followed by oxidation results in the formation of indoles (see Eq. 8.22).35a A-Tosyl-3-nitroindole undergoes high-yielding Diels-Alder reactions with... 

Complexes 17-19 can be written in one valence structure as a, /3-unsaturated carbonyl compounds in which the carbonyl oxygen atom is coordinated to a BF2(OR) Lewis acid. The C=C double bonds of such organic systems are activated toward certain reactions, like Diels-Alder additions, and complexes 17-19 show similar chemistry. Complexes 17 and 18 undergo Diels-Alder additions with isoprene, 2,3-dimethyl-1,3-butadiene, tram-2-methyl-l,3-pentadiene, and cyclopentadiene to give Diels-Alder products 20-23 as shown in Scheme 1 for complex 17 (32). Compounds 20-23 are prepared in crude product yields of 75-98% and are isolated as analytically pure solids in yields of 16-66%. The X-ray structure of the isoprene product 20 has been determined and the ORTEP diagram (shown in Fig. 3) reveals the regiochemistry of the Diels-Alder addition. The C-14=C-15 double bond distance is 1.327(4) A, and the... 

Tab. 7.2 Diels-Alder reactions with 2,3-dimethylbutadiene (12) and isoprene (13) by use of the GS/MW process [27, 30, 31],...

Many Diels-Alder [4 + 2] cycloadditions show a powerful pressure-induced acceleration, which is often turned to good synthetic purpose as discussed in Section III.A.2. Table 1 illustrates the effect of pressure on the Diels-Alder reaction of isoprene with acrylonitrile as a representative example. This reaction is accelerated by a factor of 1650 by raising the pressure from 1 bar to 10 kbar28. 

TABLE 1. Pressure-induced rate acceleration of the Diels-Alder reaction of isoprene with acrylonitrile at 21 °C (AV = -35.4 cm3 mol-1, AV = -37.0 cm3 mol-1)28... 

Fig. 20 The deuterium and carbon-13 KIEs calculated for the Diels-Alder reaction between isoprene and maleic anhydride using the isotopic enrichment in the unreacted isoprene recovered from a reaction taken to 98.9% of completion. The numbers in parentheses represent the error in the KIE. Reproduced, with permission, from...

Oppolzer and colleagues performed pioneering work on the application of chiral sultam based dienophiles in asymmetric Diels-Alder reactions. The bomanesultam based dienophiles provided excellent de values in the Lewis acid mediated Diels-Alder reactions with a wide variety of dienes179. The efficiency of the simpler toluene-2,a-sultam based dienophiles was also studied180. Chiral auxiliary 282 proved superior to 283 and 284 in the aluminum Lewis acid catalyzed Diels-Alder reactions of its A-acryloyl derivative with cyclopentadiene, 1,3-butadiene and isoprene, affording the adducts with >90% de. 

An unusual Diels-Alder cycloaddition involving the Cp=Cy bond has been described. The reaction took place by treatment of the electron-deficient allenylidene moiety in complex [RuCp(=C=C=CPh2)(CO)(P -Pr3)][BF4] (46) with a 20-fold excess of isoprene at room temperature affording the cycloadduct 90 (Scheme 33) [287]. This Diels-Alder cycloaddition in which the allenylidene moiety acts as a dienophile was completely regioselective, only the Cp=Cy bond of the allenylidene skeleton being implicated. Furthermore, it was also regioselective with regard to the orientation of the diene with the exclusive attack of C(l) and C(4) carbons at the Cp and Cy positions, respectively. Allenylidene 46 also underwent Diels-Alder reactions with cyclopentadiene and cyclohexadiene to afford the... 

Total synthesis of racemic limonene by Diels-Alder reaction of isoprene with methyl vinyl ketone and subsequent Wittig reaction of the resulting ketone with methylene triphenylphosphorane... 

Narasaka reported a highly enantioselective Diels-Alder reaction between isoprene and a fumaric acid derivative 17 using a catalytic amount of a chiral titanium reagent prepared from tartrate-derived chiral 1,4-diol 18 and TiCI2(0-/-Pr)2 [27]. When a stoichiometric amount of the partially resolved chiral diol (25% ee) was used, the corresponding cycloadduct 19 with 83% ee was obtained (Scheme 9.12). In this reaction, white precipitate was formed, which proved to... 

Asymmetric ene and Diels-Alder reactions with methyl glyoxalate. The reaction of methyl glyoxalate with isoprene catalyzed by the BINOL-titanium complex (R)-l provides not only the expected ene product (2), but also the Diels-Alder product (3), both in 97% ee (equation I). This chiral titanium complex is also an... 

The rate constant data for the Diels-Alder reaction of isoprene and maleic anhydride (23,24) may be correlated with IL of phenol blue at 35°C by... 

Cycloaddition reactions of levoglucosenone (2) have been widely investigated and include the Diels-Alder reaction with various dienes such as cyclopentadiene, butadiene, and isoprene.11-14 In addition, 1,3-dipolar cycloadditions have been investigated.15... 

What has just been stated regarding the LCAO coefficients of the dienophile LUMO combined with the rules for the regioselectivity of any one-step cycloaddition leads to the following consequences for the Diels-Alder reactions of isoprene ... 

Table 2 Experimental and theoretical 2H and 13C KIEs for the Lewis acid catalyzed Diels-Alder reaction of isoprene with acrolein, methyl vinyl ketone and ethyl acrylate in toluene at 25°C...

Table 4.6 Diastereoselective Aza-Diels-Alder reactions with IV-galactosyl imines and isoprene...

Further calculations were performed on 2,3-diaza-1,3-butadiene with different heterodienophiles such as ethene, formaldehyde and formaldimine showing the same exo oxygen or nitrogen lone pair preference [49] as well as on the nitro-soethene/ethene system [50]. Recently, ab initio studies have also been performed for the Lewis acid catalysed hetero Diels-Alder reaction of isoprene and sulfur dioxide by Sordo [51]. 

With its concerted mechanism implying little charge distribution change along the pathway, the Diels-Alder reaction has been understood to have little rate dependence on solvent choice. For example, the relative rate of cyclopentadiene dimerization increases only by a factor of 3 when carried out in ethanol. The relative rate for the Diels-Alder reaction of isoprene with maleic anhydride (Table 7.1) varies by only a factor of 13 with solvents whose dielectric constants vary by almost a factor of 10, but the rate acceleration is not a simple function of the solvent polarity. Furthermore, the dimerizations of cyclopentadiene and 1,3-butadiene proceed at essentially identical rates in the gas and solution phases. ... 

Diels-Alder Cycloadditions (AUcene -+ Six-Membered Cycloadduct). lV-Acryloyl-a-methyltoluene-2,a-sultam (3a) participates in highly endo and C(a)-re ir-face selective Lewis acid promoted Diels-Alder reactions with Cyclopentadiene, 1,3-Butadiene, and Isoprene (eq 2 and Table 1). These levels of induction compare favorably with most alternative auxiliaries, including the 10,2-camphorsultam. However, V-crotonyl-a-methyltoluene-2,a-sultam (ent-3b) reacts with cyclopentadiene with only mod-... 

Table 3 Diels-Alder Reactions of Isoprene, 2-Trimethylsilylmethyl- and 2-Ttimethylstannylmethyl-1,3-butadienes with Conjugated Enoyl Compounds (Scheme 43)...

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