Alkoxy butadienes

Alkoxy allylic tins, transformations, 9, 353 Alkoxybiphenyl isonitriles, liquid crystals, 12, 277 2-Alkoxy-1,3-butadienes, germanium-substituted, 3, 708 Alkoxycarbenes... 

Perichs and coworkers studied the endo selective reactions of 1-alkoxy-1,3-butadienes and 1-alkoxy-1,3-octadienes with maleic anhydride. They found that the trani-2-phenyl-cyclohexan-l-ol and 3-( Ao-(neopentyloxy)isobornan-l-ol based chiral dienes induced the highest facial selectivities. The relative transition state energies for the formation of the different diastereomers were calculated using semi-empirical methods (AMI). 

Alkoxy-1,3-butadienes containing a carbohydrate moiety are excellent dienes for the Diels-Alder reaction conducted in water as a solvent. It has been observed that the reaction is much faster in water and is highly eflrfo-selective, but unfortunately the diastereoselectivity of these reactions is rather low16,17 (see Table 7). 

In an effort to cause unreactive aldehydes to react in the hetero Diels-Alder reaction Kubler introduced the use of the activated, electron-rich 1-alkoxy-1,3-butadiene.21 This diene shows increased reactivity and high regioselectivity but only with activated aldehydes. In 1971 Scheeren showed that the potent diene l,l-dimethoxy-3-silyloxy-1,3-butadiene also reacts with aldehydes.22 The high reactivity of... 

Other monomers that copolymerize with alkyl vinyl ethers are vinyl ketones [47], acrolein diacetate [48], acrylamide [49], alkoxy 1,3-butadienes [50], butadiene [51], chloroprene [52], chlorotrifluoroethylene [53], tri-and tetrafluoroethylene [54], cyclopentadiene [55], dimethylaminoethyl acrylate [56], fluoroacrylates [57], fluoroacrylamides [58], A-vinyl car-bazole [59,60], triallyl cyanurate [59,60], vinyl chloroacetate [61,62], N-vinyl lactams [63], A-vinyl succinimide [63], vinylidene cyanide [64, 65], and others. Copolymerization is especially suitable for monomers having electron-withdrawing groups. Solution, emulsion, and suspension techniques can be used. However, in aqueous systems the pH should be buffered at about pH 8 or above to prevent hydrolysis of the vinyl ether to acetaldehyde. Charge-transfer complexes have been suggested to form between vinyl ethers and maleic anhydride, and these participate in the copolymerization [66]. Examples of the free-radical polymerization of selected vinyl ethers are shown in Table IV. 

DMF dineopentyl acetal has been used as a reagent for dehydrative decarboxylation in order to avoid the possibility of competing 0-alkylation of the carboxyl group. This conversion of P-hydroxy carboxylic acids to alkenes by reaction with a DMF acetal involves an anti elimination, and it is thus complementary to the syn elimination of these hydroxy acids via the p-lactone. These reactions have been used to obtain both )- and (Z)-1 -alkoxy-1,3-butadienes (eq 6). For additional examples of alkenes obtained from p-hydroxy acids with DMF acetals, see Scheeren et al. ... 

This strategy has been applied to the synthesis of the Cis Cecropia juvenile hormone (JH) (163 Scheme 13) using the ketal (160) rather than an alkoxy butadiene. Surprisingly, an a,p-unsaturated ester is obtained as an -3 1 mixture from which the undesired ( )-isomer can be separated by fractional distillation. The iterated allylic alcohol (163) is transformed into its isomeric allylic cohol which is subsequently reduced to an alkene and epoxidized to realize the target (163). The scheme also lends itself to... 

Nitrostyrene (15a) can react as a dienophile in the Diels-Alder reaction with 2-alkoxy butadienes producing cyclic enol ethers (Scheme 9.23). By using an excess of nitrostyrene a domino reaction should take place with the in situ-generated enol ether. j -Nitrostyrene (15a) may react subsequently as a dienophile in the Diels-Alder reaction with a 2-alkoxy butadiene, as a heterodiene in the inverse Diels-Alder reaction of alkoxy cyclohexene which is formed primarily, and as a di-substituted dipolarophile in the 1,3-dipolar cycioaddition of the nitronate formed in the inverse Diels-Alder reaction. 2-Methoxy-l,3-butadiene (61) was selected for the Diels-Alder reaction, since it reacted in a completely regioselective manner with nitroalkenes. 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

A further extension of the MIMIRC reaction is seen in the synthesis of enantiomerically pure cyclohexanones. A successful diastereoselective MIMIRC reaction with 2-(rer/-butyldimethylsi-lyloxy)-4-phenyl-l,3-butadiene and an optically pure (Z)-y-alkoxy-substituted enone was performed using catalytic amounts (5 mol%) of triphenylmethyl perchlorate at — 78 ,C 360,408 (for a further example see Section 1.5.2.4.4.1.). 

Scheme 15 The HOMO polarization of butadiene by the 2-alkoxy group...

A combination of a Diels-Alder and a Fisher carbene-cyclopentannulation is described as the last example in this subgroup. Thus, Barluenga and coworkers used a [4+2] cycloaddition of 2-amino-l,3-butadienes 4-115 with a Fischer alkoxy-arylalky-nylcarbene complex 4-116 this is followed by a cyclopenta-annulation reaction with the aromatic ring in 4-116 to give 4-117 (Scheme 4.25) [36]. An extension of this domino process is the reaction of 4-118 with 2equiv. of the alkynyl carbene 4-119 containing an additional C-C-double bond (Table 4.2) [37]. The final product 4-120, which was obtained in high yield, is formed by a second [4+2] cycloaddition of the primarily obtained cyclopenta-annulated intermediate. 

Cycloaddition of 2-alkoxy-l,3-butadienes, H2C=C(OAlk)CH=CH2, and nitrile oxides to give isoxazolines 51 proceeds with the participation of only one of the conjugated C=C bonds. With benzonitrile oxide, only the vinyl group in alkoxydienes participates in cycloaddition reactions while in the case of phenyl-glyoxylonitrile oxide both double bonds react (222). Nitrile oxides RC=NO react with iron complexed trienes 52. The reaction proceeds with good yield and diastereoselectivity ( 90/10) to give isoxazolines 53 (223). 

The application of in situ-generated (alkoxy)palladium(II) species (Scheme 14.23) can be extended to reactions of a-carbonates with organoboron compounds. Crosscouplings of allenes 108 with aryl (or alkenyl) boron acids or their esters catalyzed by a palladium(O) complex afforded the 2-aryl(alkenyl)-l,3-butadienes 109 in excellent yields (Scheme 14.24) [53], The coupling reactions of 9-BBN-derived intermediates such as ester 111 can be accelerated by applying K3P04 as additive (Eq. 14.15). 

Cyclohexyl based chiral auxiliaries have been widely employed in asymmetric syntheses170. Barluenga and coworkers171 reported the first chiral 2-alkoxy-l,3-butadienes of... 

Binaphthol catalyst 417 proved effective in the cycloadditions of 1-alkoxy-l,3-butadienes with methacrolein and 1,4-naphthoquinone257. More recently, it was found that the use of molecular sieves was essential for the in situ preparation of the catalyst, but also that this had dramatic effects on the enantioselectivity258. In the presence of molecular sieves, the cycloaddition of juglone (342) with 1-acetoxy-l,3-butadiene was catalyzed by 10 mol% of 417 to give cycloadduct 343 with only 9% ee. In the absence of molecular sieves, the enantiomeric excess increased to 76-96% (equation 124). 

Alkyl alkoxy silanes have been found to be very effective in reducing alkali-aggregate expansion [11] (Fig. 6.4). Of the silanes used in the study, hexyl trimethyl siloxane and decyl trimethoxyl silane were found to be more effective in decreasing the expansion than the others. In the same study, Ohama et al. [11] investigated the effect of sodium silicofluoride, alkyl alkoxy silane, lithium carbonate, lithium fluoride, styrene-butadiene rubber latex and lithium hydroxide on compressive strength and the expansion of mortar containing cement with 2% equivalent Na20. The reduction of the level of expansion shown in Fig. 6.4 with the siloxanes was attributed to... 

Pr, or Bu),88 alkyl-substituted 1-alkoxy-l,3-butadienes (for example, 202) were also used.89-91... 

Attempts were made in order to obtain adducts 198 in enantiomeric form by cyclo-addition of 1-alkoxy-l,3-butadienes to optically active esters of glyoxylic acid96 the enantiomeric purities of the adducts were, however, poor.96 Optically active butyl 2-alkoxy-5,6-dihydro-2H-pyran-6-carboxylates (198, R1 = Bu) were obtained when the R group in 197 was a carbohydrate moiety. The diastereoisomers resulting from cyclo-addition were separated by chromatography (see Section VII and Ref. 350). 

Butadienes substituted with alkoxy groups in the 2-position, e.g., 2-ethoxy-1,3-butadiene,6 have been prepared from methyl vinyl ketone, but they required several conversions and a tedious spinning-band distillation to purify the product. This slight modification of the House procedure has been used to conveniently prepare 2-trimethylsilyloxy-l,3-butadiene from the readily available methyl vinyl ketone. This one-step procedure has provided large amounts of a new and reactive diene for Diels-Alder reactions, as illustrated in Table I. 

New possibilities in hetero-Diels-Alder condensation have been opened by the introduction of highly active l-methoxy-3-trimethylsilyloxy-, 4-benzoyIoxy-l-methoxy-3-trimethylsilyloxy-, and 2-acetoxy-l-alkoxy-3-trimethylsilyloxy-l,3-butadienes ( Danishefsky dienes, 5). These compounds readily react under atmospheric pressure, in the presence of Lewis acids, with normal aldehydes (e.g., acetaldehyde, benzaldehyde, furfural) to furnish 2,3-disubstituted or 2,3,5-trisubstituted derivatives of 2,3-dihydro-4H-pyran-4-one 7 capable of readily functionalizing to sugars (Scheme 5) [26]. This approach... 

Hetero-Diels-Alder reaction with inverted electron demand between a, 3-unsatu-rated carbonyl compounds (1-oxa-l,3-butadienes 11 Scheme 6) and enol ethers provides an access to 6-alkoxy-3,4-dihydro-2/f-pyrans 12 [31,32]. These heterocycles are also useful... 

Thus, strong shieldings are observed for fi carbons of enol ethers and alkynyl ethers, as shown for 1,1 -dimethoxyethene (54.7 ppm) and ethoxyethyne (23.4 ppm) in Table 4.26. In 1-alkoxy-l,3-butadienes, transmission of the electron releasing effect along the conjugated double bonds affects alternate carbons similarly, shielding the carbons in / and <5 position as illustrated for l-ethoxy-2-methyl-l,3-butadiene. 

This asymmetric end has the alkoxy group of alkyl vinylethers by cationic polymerization, phenyl group of styrene when either anionically or cationicaiiy polymerized, the vinyl group of butadiene under anionic catalysts to poly-1,2-butadiene, the ester and methyl of methylmethacrylate under anionic catalysis and the methyl of propylene by cationic catalysis. 

Diels-Alder reactions with aldehydes.2 This catalyst is superior to zinc chloride3 for promoting [4 + 2]cycloaddition of aldehydes with l-alkoxy-3-trimethylsilyloxy-1,3-butadienes to form 2,3-dihydro-4//-pyrane-4-ones. The catalyst prepared from bornyl alcohol is somewhat more effective than similar catalysts from simple alcohols, and may be of value for asymmetric induction. 

In addition to these commonplace substrates, only a few extraordinary educts have been used in iron-catalyzed DA reactions, such as the naphthoquinones investigated by Brimble and McEwen [75]. Whereas the application of FeCl3 and a chiral bisoxazoline ligand gave only a 25% yield and no chiral induction in the reaction of 2-acetyl-l,4-naphthoquinone with cyclopentadiene, the corresponding copper(II) triflate gave a 66% yield and moderate enantioselectivities (up to 50% ee). Another example was reported by Shibasaki s group in which the 2-alkoxy-l,3-butadiene 40... 

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