Piperylenes, cycloaddition with

Table 6. Diels-Alder Reactions of 168 with Piperylene Cycloaddition Conditions Product 173 Product 174...

As both, the cycloaddition of trans-piperylene and trans,trans-hexadiene implies a connection of a methylated unsaturated carbon with the more bulky part of the standard silaethene, a decrease of reactivity of trans.trans-hexadiene on sterical grouds is ruled out. On the other hand, an intermediate formation of a weak 7i-complex of trans-piperylene but not of bulkier trans.trans-hexadiene with the unsaturated silicon atom of silaethene would explain an increased reactivity of trans-piperylene (Scheme 12). In a way, the silaethene would then catalyze its own [4+2] cycloaddition with trans-piperylene but not with trans.trans-hexadiene. This mechanism can also explain the transformation of the [4+2] into a [2+2] cycloaddition by going from trans- to cis-piperylene but not fi om trans,trans- to cis.trans-hexadiene. 

Ahern and Gokel (1979) briefly mention that (jE -arenediazocyanides also react with a variety of dienes (cyclopentadiene, cyclohexadiene, butadiene, ( )-piperylene, etc.) in a [4+ 2]-cycloaddition reaction with formation of tetrahydropyridazines (Scheme 6-31). Here the two azo nitrogen atoms of the diazocyanide react as a dieneophile in a bis-aza Diels-Alder reaction. 

Strong effects of the catalyst on the regioselectivity have been observed in the cycloadditions of a variety of heterocyclic dienophiles. Some results of the BF3-catalyzed reactions of quinoline-5,8-dione (21) and isoquinoline-5,8-dione (22) with isoprene (2) and (E)-piperylene (3) [25], and of the cycloadditions of 4-quinolones (23a, 23b) as well as 4-benzothiopyranone (23c) with 2-piperidino-butadienes, are reported [26] in Scheme 3.8 and Equation 3.2. The most marked... 

An example of stereocontrol by high pressure is given by the regio- and diastereoselective synthesis of hydrophenanthrenones [18] which are useful intermediates for synthesizing diterpenes and steroids, by EtAlCli-catalyzed cycloadditions of heteroannular bicyclic dienone 50 with (E)-piperylene (24) and 2,3-dimethyl-1,3-butadiene (51) (Scheme 5.4). 

Depending on the trapping reagents, various reactions have been observed an ene reaction between propene, isobutene, and 2231 or 27,34 a [2 + 4] cycloaddition between butadiene and 12,21 22,31 and 27,34 or both an ene reaction and a [2 + 4] cycloaddition between 2,3-dimethylbutadiene and 2231 or 27,34 and between piperylene or hexadiene and 22.31 Some of these reactions are summarized in Scheme 8. By contrast, 27 does not react with the C = C double bond of an alkene such as CH2 = CH-OMe.34... 

However, when the same reaction was performed in the presence of piperylene, 1,2-cycloaddition was observed to give naphthalene ring fused cyclobutapyrimidines 473 with high regio- and stereoselectivity <2005CPB258>. 

Cycloaddition and ene reactions. Dienes >C=C—C=C< such as buta-1,3-diene, isoprene, 2,3-dimethylbuta-l,3-diene, fraws-piperylene, cyclopentadiene or anthracene react with 92 in Diels-Alder fashion to give [2 + 4] cycloadducts 410 (equation 128)62. Ene products 411 are formed additionally when the relative reaction rates for the [2 + 4] cycloaddition reaction and the ene reaction are comparable (e.g. for isoprene and 2,3-dimethyl-l,3-butadiene) Alkenes with allylic hydrogen (propene, 2-butene, isobutene) give ene products see equation 129. 

The photocycloaddition of cyclooctyne to benzene [72], producing bicy-clo[6.6.0]tetradeca-l,3,5,7-tetraene can be sensitized (with acetone) and quenched (with piperylene). The unsensitized reaction occurs with very high efficiency (56% yield at 66% conversion). Because transfer of triplet energy from acetone to benzene is improbable, the authors consider the possibility that the acetylene triplet may be the reactive species in the cycloaddition. 

The Diels-Alder reaction of 168 with piperylene 171 was investigated, in order to study the endo selectivity of the cycloaddition (Scheme 44).102 The resulting... 

In fact, the cycloaddition of butadiene to ethylene, as well as cycloadditions of similar non-polar dienes to non-polar alkenes seem experimentally to be cases where concerted and stepwise (biradical or biradicaloid) mechanisms compete. We have recently discussed a number of cases, such as the dimerization of butadiene, piperylene, and chloroprene, the cycloadditions of butadiene or methylated dienes to halogenated alkenes, and others, where non-stereospecificity and competitive formation of [2 + 2] adducts indicate that mechanisms involving diradical intermediates compete with concerted mechanisms10). Alternatively, one could claim, with Firestone, that these reactions, both [4 + 2] and [2 + 2], involve diradical intermediates1 In our opinion, it is possible to believe that a concerted component can coexist with the diradical one , and that both mechanisms can occur in the very same vessel 1 ). Bartlett s experiments on diene-haloalkene cycloadditions have also been interpreted in this way12). 

The rate of cycloaddition increases - also in agreement with organic Diels-Alder reactions - when electron donating substituents like methyl groups are introduced into the dienes. For example, isoprene is four times, trans-piperylene three times as reactive as butadiene (Scheme 11) [23]. Of course, these... 

Whereas in the case of fra j-piperylene a [4+2] cycloadduct is formed with the standard silaethene, no such product is formed in the case of c/j-piperylene but only a [2+2] cycloadduct (Scheme 11) [26]. Obviously, the [4+2] cycloaddition proceeds here more slowly than the unfavored [2+2] cycloaddition. In this sense, silico Diels-Alder reactions are influenced by conformative effects of the organic dienes like Diels-Alder reactions [27] a displacement of the transoid/cisoid equilibrium in the direction of the transoid isomer on steric grounds decreases the rate of cycloadditions. 

In general, die influence of diene conformation on the rate of silico Diels-Alder reaction speaks for a concerted mechanism of the [4+2] cycloaddition. This mechanism is confirmed by the small influence of solvent polarity and radical trt s on the reaction rate, which speaks against a pronotmced ionic or radical multiple step mechanism. On the other hand, trans-piperylene reacts over 1000 times faster with the standard silaethene than does tra s,tra s-hexadiene, wh eas the opposite is true for ethenes [27]. This fact contradicts dramatically the carbon-analogy of silaethenes, fiom which either trans-piperylene reacts too fast, or trans.trans-hexadiene too slow with the silaethene. 

Tricyclic furanquinones were also obtained from cycloaddition reactions. 3-Methylbenzofuran-4,7-dione reacts with piperylene in a Diels-Alder reaction to give a mixture of maturinone and its isomer, a 3,8-dimethyl derivative of 84(69TL1929). 

Diels-Alder reactions (6, 65-66). 2-Methoxy-5-methylbenzoquinone shows no regioselectivity in reactions with piperylene or isoprene. These cycloadditions are catalyzed by both BF3 and SnCl4, but they favor different adducts in each case. The difference is believed to arise from different types of complexes with the quinone.1... 

Site selectivity in ketene cycloadditions is also explained by the frontier orbitals. Diphenylketene reacts with isoprene 6.397 mostly at the more substituted double bond to give the cyclobutanone 6.398 as the major product.890 In contrast, it reacts with cw-piperylene 6.399891 and with cw-butadiene-l-nitrile 6.400890 at the less substituted double bond. In all three cases the site of attack is the double bond having the largest coefficient in the HOMO. 

The cycloaddition of simple alcohols, acrylates la-d with cyclopentadiene (2)7 and 1,3-penta-diene (7 piperylene)8 succeeds only with moderate optical yields. 

Levchenko and Balon" have examined the regiochemistry of cycloaddition of several symmetrical bis(aryl)sulfonyl sulfur diimides with ( )-piperylene and isoprene which afforded the C-3 and C-5 substituted adducts, respectively (Scheme l-III). These results were confirmed by Wucherpfennig and Kresze, who also found that chloroprene gave the 5-substituted product. ... 

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