Cycloadditions boron trifluoride etherate

Covalent fluondes of group 3 and group 5 elements (boron, tin, phosphorus, antimony, etc ) are widely used m organic synthesis as strong Lewis acids Boron trifluoride etherate is one of the most common reagents used to catalyze many organic reactions. A representative example is its recent application as a catalyst in the cycloadditions of 2-aza-l,3-dienes with different dienophiles [14] Boron trifluoride etherate and other fluonnated Lewis acids are effective activators of the... 

Kociehski s group assembled the 3-lactone segment utilizing a Lewis acid-catalyzed [2 + 2] cycloaddition strategy. In the presence of a catalytic amount of boron trifluoride etherate, the [2 + 2] cycloaddition between aldehyde 28 and trimethylsilylketene 29 took place rapidly and cleanly to give a mixture of diastereomers of P-lactone 30. After a delicate desilylation and a flash chromatography, the desired diastereomer 31 was obtained in 55% yield. 

A number of tosylhydrazones containing an alkene have been shown to undergo intramolecular cycloaddition in the presence of acid.103 96 Boron trifluoride etherate appears to be the acid of choice.103 Bridged, rather than fused, bicyclic pyrazolines are formed under these conditions. The mechanism almost certainly involves cationic intermediates. Thus, tosylhydrazone (192a) cyclized in 87% yield to the... 

Lewis acid-catalyzed [2+2] cycloaddition reactions of Ar-pivaloylaldiruincs 181 gave azetidines 182 and 183 (Scheme 38) <1995CL789>. Boron trifluoride etherate proved to be a better catalyst than zinc chloride, affording azetidines 182 and 183 in better yields. This transformation has been explained through the intermediacy of a /3-silyl cation. 

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]. 

The tetrazole analogue (341) of 7-oxocholest-5-en-3/3-yl acetate has been prepared 82 by reaction of the afi-unsaturated ketone with hydrazoic acid and boron trifluoride etherate hydrolysis of the 3)3-acetate and oxidation of the resultant alcohol furnished 7a-aza-B-homocholest-4-eno[7a,7-d]tetrazol-3-one. Cycloaddition of ethylenediamine to 2a-bromo-5a-cholestan-3-one has furnished a simple one-step route to 5 -cholestane[2,3-e]dihydro-2,3-pyrazine (342).183 Analogous cycloaddition has been achieved using 16a-bromoandrostan-17-ones. 

In consideration of conceivable strategies for the more direct construction of these derivatives, nitriles can be regarded as simple starting materials with which the 3+2 cycloaddition of acylcarbenes would, in a formal sense, provide the desired oxazoles. Oxazoles, in fact, have previously been obtained by the reaction of diazocarbonyl compounds with nitriles through the use of boron trifluoride etherate as a Lewis acid promoter. Other methods for attaining oxazoles involve thermal, photochemical, or metal-catalyzed conditions.12 Several recent studies have indicated that many types of rhodium-catalyzed reactions of diazocarbonyl compounds proceed via formation of electrophilic rhodium carbene complexes as key intermediates rather than free carbenes or other types of reactive intermediates.13 If this postulate holds for the reactions described here, then the mechanism outlined in Scheme 2 may be proposed, in which the carbene complex 3 and the adduct 4 are formed as intermediates.14... 

Ethyl aluminum dichloride mediates a formal [5 + 2] cycloaddition of complex (164) and (166) with enol silyl ethers to produce the highly strained seven-membered rings (165) and (167) respectively (Schemes 239 -240). Excellent stereoselectivity is observed in both cases. A related double alkylation affords complexed seven-membered rings via a formal [4 - - 3] cycloaddition. Incorporation of fluorine is observed using boron trifluoride etherate (Scheme 241). 

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). ... 

On thermolysis, perezone (192) undergoes a [4+2] cycloaddition to produce equal amounts of the two pipitzols (193) and (194) i.e. there is no stereochemical induction by the chiral centre in (192). If, however, the reaction is carried at 0 °C in the presence of boron trifluoride etherate the cyclization becomes highly stereoselective producing largely a-pipitzol (193) and it has been shown that a stepwise mechanism (195)—K196)->(193) is operative in this case.108... 

The photochemical cycloaddition of 4-methyl-l,2,4-triazoline-3,5-dione to the dibenzo-cyclooctatetraene 289 yields 3.5% of the cycloadduct 290, together with 36% of 291, the product of a di-jr-methane rearrangement (equation 153. Anthrasteroids 293 (R = H, Ac or COPh) are produced in an oxidative rearrangement when the phenyltriazoline-dione adduct of 292 is treated with boron trifluoride etherate. The 5,7-diene system of ergosteryl acetate 294 can be protected by cycloadduct formation, allowing selective hydrogenation of the 22,23-double bond " ... 

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. 

A-1,3,5-Thiadiazines are prepared readily by (4-1-2) cycloaddition of A-thioacylimines with carbonitriles or cyanamides (see Section 6.18.10.2.2.iii). Analogous cycloadditions with alkoxy-thiocarbonyl isocyanates produce 2//-l,3,5-thiadiazine-4(3//)-thiones (see Section 6.18.10.2.2.iii). The boron-trifluoride etherate-catalyzed condensation of an aldehyde (or dialkyl ketone), a carbonitrile and a thioamide is also a versatile route to 4//-l,3,5-thiadiazines (see Section 6.18.10.3.2.2). 

The cyclotrimerization of selenoaldehydes is reversible <82ZAAC(488)99>. Thus, when aldehydes react with bis(trimethylsilyl)selenide in the presence of boron trifluoride etherate, triselenanes (120) are produced. These can be used as sources of selenoaldehydes, which undergo cycloaddition with... 

In efforts to increase the rate of simple 2-aza-1,3-butadiene participation in [4 + 2] cycloadditions, Mariano and co-workers investigated the related boron trifluoride etherate-catalyzed intermolecular cycloaddition reactions of (1 ,3 )- and ( E,3Z)-phenyl-2-aza-1,3-pentadiene with a... 

Only the E,3E stereoisomer of the 2-azadiene was found to be reactive under the reaction conditions. Consequently, each of the piperidine products was found to possess the cis 2-phenyl, 5-methyl relative stereochemistry. Moreover, the cycloaddition products derived from reactions of enol ethers possessed the all-cis 2-phenyl-4-alkoxy-5-methyl stereochemistry necessarily derived from exclusive endo cycloaddition. Dienophile geometry is maintained during the course of the [4 + 2] cycloaddition, and no products derived from a potential stepwise, addition-cyclization reaction were detected. Representative neutral and electron-deficient dienophiles failed to undergo cycloaddition. A related boron trifluoride etherate-catalyzed [4 + 2] cycloaddition of simple 2-aza-l,3-butadienes with carbonyl compounds provides 5,6-dihydro-2//-l,3-ox-azines and appears to proceed preferentially through an endo [4 + 2] transition state although evidence supporting a stepwise, addition-cycli-zation was occasionally detected.630... 

Disubstituted 17/-imidazolines 84 were synthesized from aziridine 82 and nitriles 83 in the presence of boron trifluoride etherate or triethyloxonium tetrafluoroborate via a [3+2] cycloaddition reaction <04TL1137>. Ritter reaction of enantiopure 2-(l-aminoalkyl)azidirines 85 with various nitriles afforded enantiopure tetrasubstituted imidazolines 86 <04OL4499>. 

C-C-C + N-N-N if). Several cases of intramolecular cycloadditions of azido groups to allyl ions and photochemi-cally generated zwitterions have been reported. Treatment of the indoles 255 and 257 with boron trifluoride etherate results in the formation of triazines 256 and 258 (Equations 112 and 113), and, after heterolysis, the cyclopentanol 259 furnishes the spiroanellated dihydro-1,2,3-triazine 260 (Equation 114) <1994JOG2682>. 

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