2-methoxy-l,3-butadiene

Mikami et al. have reported that the chiral titanium reagent 12 derived from bi-naphthol and TiCl2(0-i-Pr)2 catalyzes the Diels-Adder reaction of a-bromoacrolein or methacrolein with isoprene or 1-methoxy-l,3-butadiene to afford the cycloadducts with high enantioselectivity [18] (Scheme 1.25). 

Because ketones are generally less reactive than aldehydes, cycloaddition reaction of ketones should be expected to be more difficult to achieve. This is well reflected in the few reported catalytic enantioselective cycloaddition reactions of ketones compared with the many successful examples on the enantioselective reaction of aldehydes. Before our investigations of catalytic enantioselective cycloaddition reactions of activated ketones [43] there was probably only one example reported of such a reaction by Jankowski et al. using the menthoxyaluminum catalyst 34 and the chiral lanthanide catalyst 16, where the highest enantiomeric excess of the cycloaddition product 33 was 15% for the reaction of ketomalonate 32 with 1-methoxy-l,3-butadiene 5e catalyzed by 34, as outlined in Scheme 4.26 [16]. 

Diels-Alder cyclization of ethyl pyruvate with 1-methoxy-l, 3-butadiene in the presence of A1C13 gave a 45% yield of 2//-pyran 149, apparently via a dihydropyran intermediate.195... 

T rimethylsily l)allylli thium. 5-LACTONES 1-Methoxy-l, 3-butadiene. 

To a mixture of 1-methoxy-l,3-butadiene (1 55.3 g, 0.66 mol) [43] and n-butyl glyoxylate (86.0 g, 0.66mol) [44], 0.1 g hydroquinone was added and the solution was refluxed at 100°C for 6 h under argon. Distillation at 88°-90°C/0.5 tonr gave 2 (R = Bu) (91.6 g, 65%) as a pale-yellow liquid with a faint odor. By gas chromatography this product is a mixture of cis and trans (ca. 7 3) isomers. Treatment of the isomer mixture in dichloro-methane solution with p-toluenesulfonic acid or zinc chloride for 2 h leads to an almost pure trans-isomer (ca. 95%). 

We have previously reported that the hetero Diels-Alder reactions of glyoxylates with 1-methoxy-l,3-butadienes proceed smoothly under catalysis by BINOL-Ti complex to give the cis product with high ee (Sch. 48) [128]. The hetero Diels-Alder products thus obtained can be transformed into monosaccharides [129], The hetero Diels-Alder product can, furthermore, readily be converted into the lactone portion of HMG-Co A inhibitors such as mevinolin or compactin [106] in few steps. 

The hetero-Diels Alder reaction of nonactivated carbonyl compounds requires activation before a successful cycloaddition reaction occurs. In general, three methods are available Lewis acid catalysis, elevated temperature and elevated pressure. In some cases, when elevation of temperature causes the destruction of reactants before the cycloaddition takes place or if one or both reactants are very susceptible to Lewis acid catalysis, the method of choice is high pressurel9. For the reasons mentioned above, the Diels Alder reaction of 2,3-O-isopropylidene-D-glycer-aldehyde (2) with 1-methoxy-l,3-butadiene (1) was performed under high pressure. Extensive studies on the influence of parameters, such as pressure, temperature and solvent, on the yields and diastereoselectivities of the [4 + 2] cycloaddition enabled the optimal conditions to be determined20. 

Introduction of the more bulky carbohydrate moiety increases the shielding on one of the faces of the formyl group. The D-galactose derived aldehyde 10 adds to 1-methoxy-l.3-butadiene (1) with complete diastereoselectivity. D-Ribose and D-glucose derived aldehydes 13 and 14 are less efficient, but the cycloadducts 15 and 17 are still produced with high diastereomeric ratios22. 

The [4 + 2] cycloaddition of 1-methoxy-l,3-butadiene (8) to (2/ )-/V-glyoxyloylbornane-10,2-sultam (7) leads to crystalline, easily separable adducts 9-12. In a noncatalyzed reaction (room temperature and high pressure) the diastereoselectivity obtained is moderate, but higher than that obtained when (-)-(/ )-menthol and (-)-(/ )-8-phenylmenthol arc used as chiral auxiliaries. The reaction is already catalyzed by very mild Lewis acids Eu(fod), yields adduct 9 with an excellent selectivity exceeding a d.r. 96 433. 

The high pressure, Eu(fod)3 mediated, [4 + 2] cycloaddition of, V,0-diprotected D-a/fo-threoni-nals 27 to 1-methoxy-l, 3-butadiene (9) proceeds at 1.1 x 107 Torr and 50°C. Adducts 28 and 29 are obtained with low (for 27a) and with moderate (for 27b) diastereoselectivity in favor of the (2/ )-isomer (adducts 29a, b). Analogous cycloaddition of the D-threoninals 30 gives adducts 31 and 32 with comparable diastereoselectivities. No change in the direction of chiral induction is observed5 3. 

The chiral europium complex F.u(hfc)3 has also been used as catalyst for the cycloaddition of diethyl mesoxalate (21a) and butyl glyoxylate (21b) to 1-methoxy-l,3-butadiene (20a) and... 

The synthetic potential of lanthanide catalysis has been recognized in high pressure chemistry mainly during the last decade although earlier work was concerned with the beneficial effect of the biactivation mode to overcome steric hindrance in hetero-Diels-Alder reactions [30]. In this way, alkyl pyruvates and aldehydes considered as dienophiles could react with 1-methoxy-l,3-butadiene yielding adducts which could serve as synthons in sugar synthesis [31]. 

Among the Lewis acids studied in asymmetric hetero Diels-Alder reactions of carbonyl compounds rare earth cations proved to be mild and efficient catalysts. This is demonstrated by the results observed in reactions of camphor sultam (63) derivatives with achiral 1-methoxy-l,3-butadiene. 

Draw the two disrotatory products in which acrylonitrile approaches 1-methoxy-l,3-butadiene from the top, using Figure 24.4 as a guide. 

Nitropyridine and 4-nitropyridine A-oxide have been shown to react with iso-prene, 1-methoxy-l,3-butadiene, and Danishefsky s diene to produce isoquinoline cycloadducts. One asymmetric and asynchronous transition state (TS) was detected between the reactants and the cycloadduct with isoprene and two TS were observed when 1-methoxy-l,3-butadiene and Danishefsky s diene were used. The Diels-Alder reaction of highly substituted dihydropyridines with e-deficient alkenes produced highly substituted isoquinuclidines with high stereo- and regio-selectivity. The Diels-Alder cycloaddition of cyclopentadiene to lithium ion encapsulated [60]fullerene proceeds at a higher rate than with that of empty [60]fullerene. ... 

The synthesis of methyl 4-deoxy-heptoslde stereoisomers by cls-hydroxylatlon of the dihydro-2H-pyran type [4+2]-cycloadducts from 2,3-0-isopropylldene-D-glyceraldehyde with 1-methoxy-l,3-butadiene... 

---