Butadiene, Siloxy

Scheme 9 1-Siloxy- 1,3-butadienes as synthetic equivalents of bishomoenolate of bu-tyraldehydes...

No Lewis acid catalyst is required for the Diels-Alder reaction between an aldehyde and the more-reactive l-amino-3-siloxy butadienes such as 18 (3.24). Treatment of the cycloadduct 19 with acetyl chloride provides the desired dihydro-4-pyrone. 

Butadienes bearing trialkylsiloxy group at Cl and/or C2 positions also undergo the homoallylation with benzaldehyde with high regio- and stereoselectivity (Table 4). C2-siloxy-1,3-butadienes react at the Cl position and provide 1,3-anti isomers exclusively (runs 1-3, Table 4), while Cl-siloxy-1,3-butadienes react regioselectively at the distal C4 position and provide 1,3-anti isomers exclusively (runs 5 and 6, Table 4). 

Table 4 Ni-catalyzed homoallylation of benzaldehyde with siloxy-1,3-butadienes promoted by Et3B...

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... 

Several other examples of regioselective cyclopropanation of 1- and 2-substituted butadienes in the presence of copper catalysts are known (Scheme 5). 2-Trimethyl-siloxy-1,3-butadiene parallels the behavior of other 2-substituted butadienes (see Table 9) in that the electron-rich double bond is cyclopropanated 60. With the 1-methoxy-, acetoxy- or trimethylsilyloxy-substituted butadienes 17, 18 and 19, both double bonds are cyclopropanated, thus giving rise to sometimes unseparable mixtures of regio- and stereoisomers 79). Perhaps, the yields of separated and isolated regioisomers in some cases do not reflect the true regioselectivity as considerable... 

The scope and efficiency of [4+2] cycloaddition reactions used for the synthesis of pyridines continue to improve. Recently, the collection of dienes participating in aza-Diels Alder reactions has expanded to include 3-phosphinyl-l-aza-l,3-butadienes, 3-azatrienes, and l,3-bis(trimethylsiloxy)buta-l, 3-dienes (1,3-bis silyl enol ethers), which form phosphorylated, vinyl-substituted, and 2-(arylsulfonyl)-4-hydroxypyridines, respectively <06T1095 06T7661 06S2551>. In addition, efforts to improve the synthetic efficiency have been notable, as illustrated with the use of microwave technology. As shown below, a synthesis of highly functionalized pyridine 14 from 3-siloxy-l-aza-1,3-butadiene 15 (conveniently prepared from p-keto oxime 16) and electron-deficient acetylenes utilizes microwave irradiation to reduce reaction times and improve yields <06T5454>. 

Cycloaddition of 1-Dimethylamino-3-siloxy-1,3-butadiene with Methyl Acrylate Application to the Synthesis of 4-Hydroxyethyl-2-cyclohexen-1-one. 

Y. Huang and V. H. Rawal, Hetero Diels-Alder reactions of l-amino-3-siloxy-l,3-butadienes under strictly thermal conditions, Org. Lett., 2 (2000) 3321-3323. 

Cycloalkenones generally perform poorly as dienophiles in Diels-Alder reactions but their reactivity can be enhanced by Lewis acids [105]. SnCU is effective in promoting the Diels-Alder reaction between simple 1,3-butadienes, for example isoprene and piperylene, and cyclopentenone esters (Eq. 67) [106], Cycloaddition does not, however, occur in the presence of SnCU when the diene contains an oxygen-bearing substituent such as an alkoxy or siloxy group. For such compounds, as is generally true for the Diels-Alder reactions of cycloalkenones, other Lewis acids such as zinc chloride are more effective. 

Danishefsky and co-workers pioneered the use of chiral lanthanide complexes as catalysts in organic reactions. They found out that Eu(hfc)3, which is used as an NMR shift reagent, promoted hetero Diels-Alder reactions [30] of aldehydes with siloxydienes and induced enantiomeric enrichment (Sch. 1) [31]. Suitable substituents on the dienes were introduced to improve the extent of asymmetric induction. The best result was obtained in the reaction of benzaldehyde with l-methoxy-2-methyl-3-(trimethyl-siloxy)- , 3-butadiene using 1 mol % Eu(hfc)3 the enantiomerie excess was, however, moderate (58%). The authors maintained that the major advantage of lanthanide catalysis lay in the survival of otherwise labile systems used as adducts. 

Kozmin, S. A., Rawal, V. H. Preparation and Diels-Alder reactivity of 1-amino-3-siloxy-1,3- butadienes. J. Org. Chem. 1997, 62, 5252-5253. 

SILOXY-1,3-BUTADIENE WITH METHYL ACRYLATE 4-HYDROXYMETHYL-... 

BICYCLOPROPYLIDENE possesses unique reactivity toward a wide range of electrophiles and nucleophilic carbenes. (E)-I-DIMETH YLAMINO-3-tert-BUTYLDIMETHYL-SILOXY-1,3-BUTADIENE is a highly reactive diene for Diels-Alder reactions, as described in an accompanying procedure for the synthesis of 4-HYDROXYMETHYL-2-CYCLOHEXEN-1-ONE via the Diels-Alder reaction with methyl acrylate. Finally, this section concludes with the preparation of DIETHYL [(PHENYLSULFONYL)METHYL]PHOSPHONATE, a reagent that is very useful for synthesis of [Pg.285]

Rawal et al. have reported that under mild reaction conditions l-amino-3-siloxy-1,3-butadiene 105 a adds smoothly to a variety of dienophiles, for example electron-deficient alkynes and alkenes [290], aldehydes, aldimines [291], and ketones... 

In fhe nineteen-eighties Danishefsky [307] and Stoodley [308] reported asymmetric DA reactions of 3-siloxy-l,3-butadienes bearing a chiral alkoxy group at the 1-posi-tion. Recently, Rawal et al. described similar reactions using an oxazoUdinone or 2,5-diphenylpyrroUdine as a chiral auxiliary and their applications to the asymmetric synthesis of 2-cyclohexen-l-ones (Scheme 10.114) [309]. 

The magnesium bromide catalyzed cycloaddition of (7 )-menthyl glyoxalate (la) to 1-trimethyl-siloxy-1,3-butadiene (2) gives adducts in 14% yield and a d.r. 54,5 45.5 (for 3), (f )-8-Phenyl-menthyl glyoxalate (lb), under identical conditions, produces adducts with a d.r. 91 9 (for 3) but with only 5% yield32. 

Hetero-Diels-Alder reactions involving A -phenethyl-(2-pyridylmethylene)imine as the dienophile have been reported." A Cr complex 77 is useful for the synthesis of trisubstituted tetrahydro-4-pyranones, whereas a (salen)Cr(III) complex is involved in those reaction with l-amino-3-siloxy-l,3-butadienes." ... 

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