Trimethylsilyloxy-l,3-butadiene

Cyclic enones, lactones, and lactams have also been used often as dienophiles in Diels-Alder reactions with (1), again giving mainly the endo adduct, e.g. (12) reacted with (1) to give (13) as the major product in 74% yield (eq 3). As mentioned earlier, the initial adducts are often oxidized (either directly or after hydrolysis of the silyl ether) with Jones reagent to the corresponding enone, e.g. (14) to give (15) (eq 4). This corresponds to the annulation of a cyclohexenone unit onto an existing enone or unsaturated lactone unit and has been used often in synthesis, e.g. in 

There are some exceptions to the preference for endo stereochemistry, especially with unsaturated sulfones. A curious and useful reversal of the stereochemical preference has been reported, namely Diels-Alder cycloaddition of the pyrazolecar-boxylate (18) with (1) followed by photochemical elimination 

Stereocontrol with acyclic dienophiles can also be high, e.g. (21) giving only (22) (eq 8), although the relative stereochemistry of the adjacent allylic center can cause nearly stereorandom addition as well. The versatility of the initial adducts has been evidenced most clearly in the synthesis of anthraqumones and their derivatives. The adducts of (1) with various quinones and substituted quinones have been transformed into simple aromatics, phenols, e.g. (23) gave (24) (eq 9), cyclohexenones, e.g. (25) gave (26) (eq 10), and allylic alcohols, all in excellent 3delds. 

The reaction can be carried out with excellent enantiocon-trol (generally 80% or better) using juglone (27) and a catalyst 

A list of General Abbreviations appears on the front Endpapers 

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Ohfune and coworkers78 used Diels-Alder reactions between 2-trimethylsilyloxy-l,3-butadiene (63) and acrylate esters 64 to synthesize constrained L-glutamates which they intended to use for the determination of the conformational requirements of glutamate receptors. The reactions between 63 and acrylate esters 64a and 64b did not proceed. Changing the ethyl and methyl ester moieties into more electron-deficient ester moieties, however, led to formation of Diels-Alder adducts, the yields being moderate to good. In nearly all cases, the cycloadducts were obtained as single diastereomers, which is indicative of a complete facial selectivity (equation 22, Table 1). Other dienes, e.g. cyclopentadiene and isoprene, also showed a markedly enhanced reactivity toward acrylate 64g in comparison with acrylate 64a. 

TRIMETHYLSILYLOXY-l,3-BUTADIENE AS A REACTIVE DIENE DIETHYL traits-4-TRIMETHYLSILYLOXY-4-CYCLOHEXENE-l,2-DICARBOXYLATE... 

A. 2-Trimethylsilyloxy-l,3-butadiene (1). An oven-dried 500-ml., three-necked, round-bottomed flask is fitted with two oven-dried addition funnels, a glass stopper, and magnetic stirrer, and placed in a 80-90° oil bath. Under an inert atmosphere, methyl vinyl ketone (25.0 g., 0.357 mole) in 25 ml. of dimethylformamide and chlorotrimethylsilane (43.4 g., 0.400 mole) in 25 ml. of dimethylformamide are added over 30 minutes to a magnetically stirred solution of triethylamine (40.5 g., 0.400 mole) in 200 ml. of dimethylformamide (Note 1). The reaction gradually darkens from colorless to yellow or dark brown, and supports a white precipitate of triethylamine hydrochloride. The reaction is set up to run overnight, or ca. 14 hours. 

The first reference to 2-trimethylsilyloxy-l,3-butadiene (1) was a report2 of its reaction with tetracyanoethylene by Cazeau and Frainnet without mention of any experimental details. Later, Conia3 reported its synthesis in 50% yield with only a reference made to the usual House procedure4 for silyl enol ethers. The diene 1 has also been prepared using lithium diisopropylamide as base and chlorotrimethylsilane in tetrahydrofuran-ether (1 1) in yields up to 65%, but on a smaller scale.s... 

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. 

Butadienes with alkyl substituents in the 2-position favor the formation of the so-called para-products (Figure 15.25, X = H) in their reactions with acceptor-substituted dienophiles. The so-called mefa-product is formed in smaller amounts. This regioselectivity increases if the dienophile carries two geminal acceptors (Figure 15.25, X = CN). 2-Phenyl-1,3-butadiene exhibits a higher para -selectivity in its reactions with every unsymmetrical dienophile than any 2-alkyl-1,3-butadiene does. This is even more true for 2-methoxy- 1,3-butadiene and 2-(trimethylsilyloxy)-l,3-butadiene. Equation 15.2, which describes the stabilization of the transition states of Diels-Alder reactions in terms of the frontier orbitals, also explains the para "/"meta "-orientation. The numerators of both fractions assume different values depending on the orientation, while the denominators are independent of the orientation. 

Example Synthesis of 2-Trimethylsilyloxy-l,3-Butadiene. Exploration of a Rising Ridge System by Canonical Analysis... 

The requisite silyloxycyclopropane starting materials were prepared by dienylation of the products of the reaction of methyl diazoacetate with 2-(trimethylsilyloxy)-l, 3-butadiene in the presence of a catalytic amount of bis(2,4-pentanedionato)copper([[)24. 

The photoreactions are carried out in 50-100 mL of 1,2-dimethoxyethane or cyclohexane solutions, flushed with Ar (15 min) prior to irradiation, with 0.1 M enone and 0.5 M 2-trimethylsilyloxy-l.3-butadiene reaction temp 8-10°C (water-cooled reaction vessel) reaction time 40-48 h. The reactions are run in a Rayonet reactor equipped with RPR 3000- (quartz vessel used) or —3500 A lamps (Pyrex vessel used) and monitored by GC and TLC. Purification and separation of the product mixtures are performed by flash chromatography (silica gel, 50-fold). 

A mixture of 89 mg (0.22 mmol) of (25 )-l,Tdiaryl-[A-(2-hydroxybenzylideneimino)]-3-phenylpropanol and 40 mg (0.20 mmol) of copper(II) acelale monohydrate in 2 111L of 1,2-dichloroethane is heated to 80 °C. Then a few drops of a mixture of 2.13 g (15.0 mmol) of 2-(trimethylsilyloxy)-l,3-butadiene and 1.00 g (10.0 mmol) of methyl diazoacetate in 8 mL of 1,2-dichlorocthane are added until nitrogen evolution starts (greenish-blue color turns to brownish-yellow). The mixture is cooled to 50 =C and. at this temperature, the mixture of olefin and diazo compound is slowly added to the activated catalyst within 4 h (ca. two drops/min). Further stirring for 15 min and evaporation of the solvent is followed by filtration of the residue through 20 g of neutral alumina (activity III) with pentane as eluant. After evaporation and removal of excess olefin at 80 C/0.8 Torr, the product is distilled yield 1.37 g(64%) bp 50°C/1 Torr [k] — 111.8 (c = 2.4, CHClj) d.r, (cisjtrans) 44 56 civ 76%ee, trans 57%ee (determined by H-NMR spectroscopy with shift reagent). 

The synthesis of Ilia starts from the unknown 4-nitrocyclohexanone, which was achieved by a Diels-Alder reaction between nitroethylene and 2-(trimethylsilyloxy)-l,3-butadiene, and following the sequence depicted in Scheme 34, vinyl bromide 112a was obtained. This amino-tethered ketone vinyl halide was treated with 0.2 equiv of Pd(PPh3)4 and 1.5 equiv of KOt-Bu at reflux temperature (THF) for 30 min to give the azatricyclic compound 111a in 54% yield. 

In a synthesis of the tricychc skeleton of FR901483 (45), Bonjoch and Sole reported a TMG (3) promoted conjugate addition reaction of nitroalkane to methyl acrylate. Reaction of methyl acrylate (40) and nitro acetal 41 [9,10], obtained from Diels-Alder reaction between nitroethylene and 2-(trimethylsilyloxy)-l,3-butadiene, gave nitro ester 42 in 71% yield. The ester 42 was further converted to the spiro compound 43, and a palladium promoted cyclization reaction led to the azatricyclic skeleton 44 (Scheme 7.7). 

DIELS-ALDER REACTIONS l,3-Bis(trimethylsilyIoxy)-l,3-butadiene. trans, rnms-1,4-Diacetoxybutadiene. 1,2-Dicyanocyclobulene. Ethyl /fans-1,3-butadiene-I-carbonate. Furane. 2-(2-MethoxyFallylidene-l,3-dithiane. 2-Methoxy-3-phenyIthiobuta-l,3-diene. l-Methoxy-3-trimcthylsilyloxy-l,3-butadiene. Sulfur dioxide. 1-Trimethylsily 1-1,3-butadiene. 2-Trimethylsilyloxy-l, 3-butadiene. 

Disilane trapping reagents behave in a conceptually similar fashion. Tsuji and coworkers found that a variety of simple dienes (143, e.g., butadiene, isoprene, 2-phenyl-l,3-butadiene, 2-trimethylsilyloxy-l,3-butadiene) undergo efficient Pd(dba)2-catalyzed linear dimerization disilane trapping with a variety of simple disilanes 144 to afford bis(allylsi-lane) products 145 (41-92% yield) (Scheme 47). Cyclic disilanes afford macrocyclic bis(allylsilane) products. ... 

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