Piperylenes Diels-Alder with

Less reactive dienes such as cyclohexadiene can be employed efficiently, giving the adduct in 90% yield in 93% ee. Acyclic dienes such as piperylene, 2,4-hexadiene, and 1-phenylbutadiene also react with the acryloyloxazolidinone derivative to afford Diels-Alder cycloadducts in high optical yields (Scheme 1.38, Table 1.17). 

Reduction of azirine-2-phosphonates 99 (Scheme 3.32) with NaBH4 in ethanol exclusively gave ris-aziridine-2-phosphonates 100 in 81-82 % yield [86, 87]. A Diels-Alder reaction between azirine-2-phosphonate 101 and trans-piperylene 102... 

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. 

Table 6.8 reports the relative reaction rates of Diels-Alder reactions of 2,5-dimethylbenzoquinone with tran -piperylene in different lithium salt solutions. The data show that the reaction rate depends on the concentration of LT and that in 4.0m LT-AC and 4.0m LT-DE the rate accelerations are comparable to that exhibited in 5.0m LP-DE and 5.0m LP-AC. 

Table 6.8 Relative reaction rates of Diels-Alder reactions of 2,6-dimethylbenzoquinone with Jrawi -piperylene in LiC104 (LP) and LiNTf2 (LT) in acetone (AC) and diethyl ether (DE)...

A number of other specific reactions have been studied. For example, Diels-Alder reactions of the unsaturated 5(4//)-oxazolone derived from piperonal with 2-ferf-butyldimethylsilyloxy-1,3-butadiene, piperylene, 1-acetoxy-1,3-butadiene, and Danishefsky s diene have been described. In these cases, the results are variable and are dependent on the diene with poor yields often obtained even at high temperatures. Moreover, the stereochemical outcome of these reactions has not been determined. " ... 

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 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... 

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

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. 

The first report of a chiral aluminum Lewis acid employed in a heteroatom Diels-Alder reaction utilized Koga s mentholoxy dichloroaluminum catalyst 4 [75]. trans-Piperylene and 1-methoxybutadiene were reacted with n-butyl glyoxalate and diethyl mesoxalate the results are summarized in Sch. 46. The asymmetric induction and chemical yield in these reactions are quite poor but the authors did find that moderate asymmetric induction could be obtained from reactions catalyzed by Eu(hfc)3. 

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... 

Disubstituted cyclohexadienones undergo Diels-Alder reactions more slowly than the unsubstituted counterparts. Thus 12 does not react with piperylene (13) at 180°, but in the presence of SnCl4 the reaction proceeds in 85% yield at 25°. Moreover a complete reversal of face selectivity can be achieved by use of a Lewis acid catalyst. Thus 15 reacts thermally with 13 to give 16, whereas the catalyzed reaction results in 17. Thus the stereochemistry of four asymmetric centers can be controlled.5... 

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. 

Hydrophenanthrenes are known to be useful intermediates in the total synthesis of diterpenes and steroids. The Diels-Alder reaction of heteroannular bicyclic dienone 8 with ( )-piperylene (9a) and 2,3-dimethyl-l,3-butadiene (9b)... 

Phosphonodithioic anhydrides react regioselectively with unsymmetri-cal dienes to give Diels-Alder adducts.24 For example, isoprene and frans-piperylene react to form adducts shown in Scheme 6-1V. Yields... 

On the other hand diphenylketene does not give Diels-Alder products, but only 1,2-cycloadducts, on reaction with cyclopentadiene, piperylenes, iso-prene, and other dienes - . 

Catalysis of a Diels-Alder reaction. The uncatalyzed reaction of methyl tratts-4-oxobutenoate (1) with /rcns-piperylene (2) results in formation of (3) and (4) in about equal amounts. The reaction catalyzed by a Lewis acid favors... 

Diek—Alder reactions. 9-Vinyl-9-BBN is a more reactive dienophile than methyl acrylate, and undergoes Diels-Alder reactions at 25-55°. Although the reaction with isoprenes shows the expected para selectivity, the reaction with /ra/w-piperylene shows an unusual ie/a -selectivily, possibly as a result of steric factors. This reaction also can show high eittto-stereoseleclivity (92 8). 

Diels-Alder Reactions. Bp3-OEt2 is used to catalyze and reverse the regiospecificity of some Diels-Alder reactions, e.g. with pen-hydroxylated naphthoquinones, sulfur-containing conpounds, the reaction of 1-substituted trans-1,3-dienes with 2,6-dimethylbenzoquinones, and the reaction of 6-inethoxy-l-vinyl-3,4-dihydronaphthalene with p-quinones. BFs-OEta has a drastic effect on the regioselectivity of the Diels-Alder reaction of quinoline- and isoquinoline-5,8-dione with piperylene, which produces substituted azaanthraquinones. This Lewis acid is the most effective catalyst for the Diels-Alder reaction of furan with methyl acrylate, giving high endo selectivity in the 7-oxabicyclo[2.2.1]heptene product (eq 35). ... 

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