Butadienes Diels-Alder addition

These circumstances become clear when we consider several common examples. The Diels-Alder addition of ethylene and butadiene is taken as the first and simplest example. Fig. 4.2a indicates the nodal property of HO and LU of ethylene and butadiene and the mode of charge transfer interaction. The ethylene HO is bonding while LU is antibonding. The... 

Also the a-n interaction in Diels-Alder additions, which occurs with sy -fashion with regard to both diene and dienophile, is explained (Fig. 7.38). For the first place, the p-a type interaction is allowed, by the selection rule already mentioned, between the jr-part of butadiene and the ji-part of ethylene. Once this weak p-a type interaction starts, the p AO part forms a six-electron system. The HO of this -part will come from HO of butadiene jr-part interacting with LU of ethylene jr-part will interact with er-LU s of both butadiene and ethylene. The mode of interaction is as indicated in Fig. 7.38. 

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... 

The addition of 2,3-dimethyl-1,3-butadiene to 17 gives only one structural isomer, 21. The pseudo-first order half-life of this Diels-Alder addition reaction is 47.5 minutes at 22.2°C in neat diene solution. The Ea of this reaction is estimated to be 9.2 kcal/mol. This reaction rate is 50 times faster than the rate of addition of this diene to methyl methacrylate (33). 

Complex 19 does not appear to undergo Diels-Alder addition with isoprene additions with 2,3-dimethyl-l,3-butadiene and cyclopentadiene do take place, however. The cyclopentadiene product exists solely as the endo isomer. 

Dimethylenecyclohexane and 1,2-dimethylene-A4-cyclohexene provided tetracyclic derivatives 107 and 108, respectively, as depicted in Scheme 26 (74H649 77H953). Compounds 109, containing a similar tetracyclic skeleton but with varying placement of the bridgehead nitrogen atoms, were prepared by Diels-Alder addition of butadiene and several... 

Relatively small substituents at C-2 and C-3 of the diene exert little steric influence on the rate of Diels-Alder addition. 2,3-Dimethylbutadiene reacts with maleic anhydride about 10 times faster than butadiene, and this is because of the electronic effect of the methyl groups. 2-t-Butyl-1,3-butadiene is 27 times more reactive than butadiene. This is because the /-butyl substituent favors the s-cis conformation, because of the steric repulsions in the s-trans conformation. 

Several cycloaddition reactions of 2,5-dihydrothiophene derivatives have been reported. Compounds possessing an enamine system undergo [2 + 2] cycloaddition with acetylene-dicarboxylic ester (Scheme 215) (77AHC(2l)253). Diels-Alder addition of the 2,5-di-hydrothiophene-3-carboxylic ester (557) with butadiene, followed by desulfurization, leads to the trisubstituted cyclohexane (558) (B-74MI31404). 

Since the first report in 1914 of the formation of a butadiene-sulfur dioxide adduct [538], much work has been carried out on the reaction of conjugated dienes with sulfur dioxide and its applications in synthetic strategies. Two pathways can and have been observed a cheletropic reaction (to 2s + ir 4s) yielding 3-sulfolenes and (4 -ns + tt 2s) hetero-Diels-Alder addition yielding sultines (see [539] and [540] and references... 

Figure 3 State correlation diagrams for (a) the addition of two ethylene molecules to form cyclobutanet (b) the Diels-Alder addition of ethylene and butadiene...

Phenyl-l,2,4-triazoline-3,5-dione acts as a dienophile by in situ reaction with butadiene, cyclopentadiene, cycloheptatriene, and bicyclohepta-diene (62TL615). Thus, it is possible to compare the reactivity of the cis-azo dienophile (110) with trans-azo dienophiles, such as ethyl azodicar-boxylate, which has been observed to undergo alternate modes of reaction when used with less reactive or hindered dienes. Treatment of (110) with several dienes resulted in exclusively Diels-Alder addition. The results are summarized in Table III. 

Another highlight of the period under review is the report of the total synthesis of the tricyclic alkaloid fawcettimine (22) (Scheme 2).9 Lewis-acid-catalysed Diels-Alder addition of butadiene to 2-allyl-5-methylcyclohex-2-enone provided the ds-octalone (23), which was modified as indicated to give the dialdehyde (24). After considerable experimentation, conditions were defined which led to regioselective ring-closure in the desired direction. The unsaturated aldehyde was treated directly with the Wadsworth-Emmons reagent to provide... 

In the first attempt the tetracyclic intermediate (33) was prepared from 3,4-methylenedioxy-ta-nitrostyrene upon Diels-Alder addition of butadiene followed by zinc and hydrochloric acid reduction to an amine eventually converted into 33 by formaldehyde and hydrochloric acid. All other experiments, designed to anticipate the addition of a C6 diene (in order to introduce at once also the C2 carbon unit for ring D formation) or of a four-carbon diene with different functionality, failed. The structure of 33 is based on spectroscopic data, on considerations on the accepted stereochemical courses of this type of... 

Fig. 15.31. Transition state structures of Diels—Alder additions of butadiene A, side view of the addition of acrylic add ester and B, Newman projection of the addition of ethene. 

Wiberg and coworkers published relative rate constants and the products of reaction of silene 6 with a number of alkenes and dienes in ether solution at 100 °C6 106-108. These data are listed in Table 2 along with an indication of the type of product formed in each case. As is the norm in Diels-Alder additions by more conventional dienophiles, the rate of [2 + 4]-cycloaddition of 6 to dienes increases with sequential methyl substitution in the 2- and 3-positions of the diene, as is illustrated by the data for 2,3-dimethyl-1,3-butadiene (DMB), isoprene and 1,3-butadiene. The well-known effects of methyl substitution at the 1- and 4-positions of the diene in conventional Diels-Alder chemistry are also reflected with 6 as the dienophile. For example, lruns-1,3-pen tadiene reacts significantly faster than the f/.v-isorrier, an effect that has been attributed to steric destabilization of the transition state for [2 + 4]-cycloaddition. In fact, the reaction of c/s-l,3-pentadiene with 6 yields silacyclobutane adducts, while the trans-diene reacts by [2 + 4]-cycloaddition108. No detectable reaction occurs with 2,5-dimethyl-2,4-hexadiene. The reaction of 6 with isoprene occurs regioselectively to yield adducts 65a and 65b in the ratio 65a 65b = 8.5 (equation 50)106,107. 

Diepoxides from Diels-Alder addition products of quinone with two molecules of butadiene, which exist as several isomers (f.p., 179°, 186°, 216°, 260°, and 320°C. 

A re-investigation of the Diels-Alder addition of butadiene to (-)-carvone has led to a reassignment of the structures of the major adduct (126) and the minor adduct (127).216 The isolation of the diol (128) on treating the ketone (129) with methyl-magnesium iodide is the result of a [2,3] sigmatropic rearrangement to (130) 217 this rearrangement may find application in artemisyl synthesis. 

Conjugated dienes (e., butadiene) react according to a Diels-Alder addition with cyaito n and 2-cyanopyndines are formed together with some 2,2-bipyridyl [35]. 

AlCls-promoted Diels-Alder addition of butadiene to another sugar-derived dienqf le (290) proceeded, as expected, only from the face 0 site to the allylic ethoxy substituent, giving cycloadduct (291) (Scheme 70). ... 

EtAlClz- or Me2AlCl-promoted Diels-Alder addition of butadiene or isoprene to (363a) also proceeded readily at -78 or -94 C to give, after recrystallization, -100% pure (S)-cyclohexenes (370) (Scheme 90, Table 22, entries 8-10). Reductive cleavage of the cycloadducts (370) with LiAlHa refurnished the sultam (362) (89-95% after crystallization) and gave the pure alcohols (372) on simple bulb-to-bulb distillation. Alternatively, saponification of adduct (370a) with LiOH afford acid (373a) (a potential precursor for a synthesis of (-)-shikimic acid, 347) without epimerization. 

Table 22 Asymmetric Diels-Alder Additions of Butadiene and Isoprene to N-Aciyloylbotnane 10 -Sultams...

Some studies directed at the synthesis of the b + c + d rings of gibberellic acid, and in particular the fragment (134), have been reported.The Diels-Alder addition of butadiene to the cyclopentenone (135) afforded (136), which was converted via its iodo-lactone (137) into the tricyclic compound (138). However, the synthesis broke down at the removal of the ring D substituents. l-Hydroxy-7-methylenebicyclo[3,2,l]octane (139) provides a model for the gibbane-steviol c/d ring system. A synthetic route involves the photoaddition of allene to 1-cyclopentene-l-aldehyde to give l-formyl-7-methylenebicyclo-... 

An elegant total synthesis of the semiprotected form of lincosamine was realized by Marshall and Beaudoin [116]. An aldehyde derived from destomic acid (6-amino-6-deoxy-L-g/yc ro-D-ga/acto-heptonic acid) was derived in a similar way from a L-serinal derivative via hetero Diels-Alder addition to 1-ethoxy-3-[(trimethylsilyl)oxy]-4-benzyloxy-1,3-butadiene [142]. A similar method was applied to the preparation of a semiprotected form of anhydrogalantinic acid, a component of the antibiotic galantin I [142]. 

The octosyl acids are isolated from Streptomyces cacaoi they are part of a broader group of polyoxin antifungal nucleosides [349]. Danishefsky and co-workers [350] have reported a total synthesis of octosyl acid A featuring the hetero-Diels-Alder addition of ( )-l-methoxy-3-(trimethylsilyloxy)butadiene to a protected D-ribose-derived aldehyde. 

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