Boron Trifluoride Etherate Diels-Alder reactions

Bis(l,5-cyclooctadiene)nickel(0), 35 By Diels-Alder reaction Boron trifluoride etherate, 43... 

Kunieda and colleagues155 used a similar kind of l,3-oxazolidin-2-one (234) and studied the diethylaluminum chloride and boron trifluoride etherate catalyzed Diels-Alder reactions of its A-acryloyl and A-crotonyl derivatives with cyclopentadiene. The yields were high (80-100%), the reactions being almost completely endo selective. The diastere-omeric excesses obtained ranged from 71% to more than 99%. 

In this article a differentiation of concerted and two-step cycloadditions will not be made although this point will be briefly discussed in the theoretical part. However, products which could be formally formed by a hetero Diels-Alder reaction, but for which a different mechanism has been proven will not be included. Thus, quite recently it has been shown that the formation of an oxazine by reaction of N-sulfinyl-p-toluenesulfonamide and an excess of propanal in the presence of boron trifluoride etherate does not involve a hetero Diels-Alder reaction [37]. 

Imidazole-containing compounds have been utilized as reagents for various synthetic transformations. A convenient access to substituted allyl enol carbonates was established through the reaction of ketone enolates with the complex of allyl l//-imidazole-l-carboxylates 74 and boron trifluoride etherate <07JOC9372>. Relatively mild and highly efficient Cul-catalyzed /V-arylation procedures for imidazoles with aryl and heteroaryl bromides or chlorides have been developed in the presence of ligands 75 and cesium carbonate <07JOC2737>. a, -Unsaturated 2-acyl imidazoles 76 are an alternative and practical class of dienophiles for the DNA-based catalytic asymmetric Diels-Alder reaction in... 

Diel -AldeT catalyst. Kelly and Montury have examined the effect of three Lewis acids on the Diels-Alder reaction of perf-hydroxylated naphthoquinones with l-methoxy-3-methyl-l,3-butadiene magnesium iodide, boron trifluoride etherate, and boron triacetate. All three increase regioselectivity. BF, etherate and B(OAc)3... 

Recently, Sisko and Weinreb have developed a new, in situ procedure for effecting A -sulfonyl imine Diels-Alder reactions. The method consists of treating an aldehyde with an A -sulfinyl sulfon-amide/boron trifluoride etherate in the presence of a diene (Scheme 3). Under these conditions the IV-sul-finyl sulfonamide reacts with the aldehyde to produce an /V-sulfonyl imine. This reaction, developed by Kresze, has been used previously for preparing imines from simple, non-enolizable aldehydes (c/. 1,2), but had not been applied to more complex aldehydes. As can be seen from the scheme, the cycload-... 

As a rule, cycloadditions with acyclic -acyl imino dienophiles are useful synthetic reactions which demonstrate excellent regio- and stereo-selectivity. A commonly used source of IV-acyl immonium dienophiles are bis-urethanes such as (13) (equation S), which upon treatment with a Lewis acid and a diene afford Diels-Alder adducts. In this case, (13) reacts with boron trifluoride etherate to afford immonium ion (14) which adds to diene (15), yielding tetrahydfopyridine (16). ... 

Erraan and Stonehave devised a new method for the construction of the tricyclic skeleton associated with such sesquiterpenes as a-patchoulene (438) and cyperene (439). The route they employed was the formation of the acid-ester (440 R = OH) by a Diels-Alder reaction of 3-ethoxycarbonyl-2,4-dimethyl-penta-1,3-diene with acrylic acid. The corresponding diazo-ketone (440 R = CHN2), on treatment with boron trifluoride etherate, gave the keto-esters (441) and (442) directly. This direct acid-catalysed cyclization proved to be more efficient than the conventional copper-catalysed thermal decomposition of (440 ... 

The synthesis of a soluble nonacenetriquinone based on the well-known Diels-Alder reaction of 1,3-diarylisoben-zofurans was developed by L.L. Miller and co-workers. The preparation of the 1,3-diarylisobenzofuran commenced with the Paai-Knorr furan synthesis. The substrate was an aromatic 1,4-diketone, which was treated with excess neat boron trifluoride etherate for almost two days to afford the desired 2,5-diarylfuran in almost quantitative yield. Interestingly, this cyclization could not be achieved efficiently by using the more traditional acid catalysts such as H2SO4 or PPA. 

The first total synthesis of ( )-lycoridine margetine (33), a member of the non-basic alkaloids which possess antimitotic activity has been achieved (Scheme 3). The homoallylic alcohol (26) served as a latent diene in a Diels-Alder reaction with ethyl acrylate to give a diastereomeric mixture of adducts which upon base equilibration and hydrolysis provided the tmns-acid (27). Modified Curtius reaction on (27) afforded the corresponding isocyanate which was cyclized to the lactam (28) in 89% yield by a new method using boron trifluoride etherate. Compound (28) was converted into an N-acetyl derivative which upon basic hydrolysis gave the acid (29). Treatment of (29) with NBS followed by reflux in pyridine solution gave the lactone... 

Weinreb and Sisko have reported the first examples of Diels-Alder reactions of Af-tosyl imines derived from enolizable aldehydes [12]. The imines were generated in situ from the aldehyde, N-sulfinyl-p-toluenesulfonamide and boron trifluoride etherate. Two examples of these cycloadditions are shown in Egs. (50) and (51). It was also possible to effect the cycloaddition intramolecularly [Eq. 

Similar structures can be realised in low yield from the Friedel-Crafts reaction of 1,4-dimethoxybenzene with substituted phthalic anhydrides (R = Me, Et, i-Pr), which were obtained by the appropriate Diels-Alder synthesis. The acylations were effected in the presence of boron trifluoride etherate as catalyst, followed by demethylation of the product (ref.55). 

Boron trifluoride is a highly moisture-sensitive gas (31). It is utilized in esterification, ether formation, Friedel-Crafts alkylation and acylation, and Lewis acid-catalyzed Diels-Alder reactions. A more widely used, easy-to-handle and convenient liquid source of BF3 is boron trifluoride etherate [BF3-0(C2H5)2] (32). Its main usage as catalyst is in the direct esterification of all types of acids, rearrangements, aldol condensation, and Lewis acid-catalyzed Diels-Alder reactions. It is the most frequently used acid in epoxide ring opening and rearrangement (33). 

The synthesis commences with alkylation of oxindole 120 with spiroaziri-dinium triflate 109, providing the 3,3-disubstituted 121 in 53% yield (cf. Scheme 2.17). Treatment of 121 with boron trifluoride etherate at 100°C in toluene initiates the tandem retro Diels-Alder/intramolecular aza Diels-Alder process, leading to spiro-tetracyclic oxindoles 122 and 123 (1.5/1) in 61% yield. Addition of 2-lithio-l,l-diethoxy-2-propene to oxindole 122 provides carbinolamine 124 (95%). Exposure of 124 to p-toluenesulfonic acid in acetone-water followed by treatment with excess triethylamine in acetonitrile at 80°C effects the biomimetic transformation to adduct 126, which possesses the pentacyclic carbon framework of pseudotabersonine. This unique two-step one-pot transformation generates the inherently unstable dihydropyridine portion of dehydrosecodine 125, which participates in an intramolecular reverse electron-demand Diels-Alder reaction, providing 126 in 50% yield. The total synthesis is completed by transformation of the formyl group into the requisite carbomethoxy unit followed by N-benzyl deprotection (Scheme 2.19). 

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