Isomerization Boron trifluoride etherate

In particular, keeping cyclopropane (23b) in boron trifluoride etherate at 20°C leads to a quantitative isomerization to give the corresponding nitronate (24b). 

A convenient route to 2-alkylthio-4-alkyl-4-hydroxy-5,6-dihydro-4/7-l,3-thiazine derivatives 176 is the reaction of A-alkyldithiocarbamates with a,/3-unsaturated ketones in the presence of boron trifluoride etherate at 0°C (Scheme 15) <2002HAC377>. The predominant diastereomer displayed a m-relationship between the hydroxyl group and the C-4 substituent. Subsequent dehydration led to two isomeric products 177 and 178 with an equilibrium mixture resulting in a ratio of 94 6 in the case of 2-benzylthio-4-hydroxy-4-methyl-5,6-dihydro-4/7-l,3-thiazine. 2-Phenyl-4-alkyl-4-hydroxy-5,6-dihydro-4/7-l,3-thiazine derivatives are similarly prepared by reacting thio-benzamide with o ,/3-unsaturated ketones at room temperature <2002EJP307>. 

These reagents are also useful for the preparation of 1,2-diols. Upon exposure to Lewis acids such as boron trifluoride etherate (BFa-OEta), the a-alkoxy and oc-siloxyallyl stannanes undergo a stereospecific, intermolecular 1,3-isomerization to give y-alkoxy- and y-siloxy allylic stannanes.3. .7 When tert-butyldimethylsilyl trifluoromethanesulfonate is substituted for chloromethyl methyl ether in the above procedure, the isomeric -y-siloxy allylic stannane can be obtained directly with no loss of enantioselectivity.6 These stannanes can then be added to various aldehydes to give monoprotected 1,2-diols with high diastereoselectivity.8... 

Allyloxy-l,2,3,6-tetrahydropyridine 241 undergoes Claisen rearrangement in cymene to give 4-allyl-3-oxo-piper-idine 242. The boron trifluoride etherate-mediated Claisen rearrangement gives only the 2-allyl isomer 244 due to the initial isomerization of tetrahydropyridine 241 into 1,2,3,4-tetrahydropyridine 243 by the Lewis acid (Scheme 61) <1999S1814>. 

Terpene epoxides are very reactive compounds. Some products formed by isomerization of such epoxides are valuable raw materials for perfumes, synthetic flavourings and pharmaceuticals, and also provide useful intermediates in organic syntheses. The isomerization of isophorone oxide 23 (Eq. 15.2.9) was originally investigated by H.O. House and R.L. Wasson using boron trifluoride etherate as a homogeneous catalyst (24). 

Engman reported that the acetoxyselenenylation of alkenes could be better carried out with PhSeBr in acetic acid in the presence of acetic anhydride and KNO3 [40]. In the case of terminal olefins the addition proceeds with poor regio-control. However, when the chloroform solution of the two products was treated with catalytic amounts of boron trifluoride etherate, isomerization takes place and the anti-Markovnikov adduct was transformed into the Markovnikov product. The acetoxyselenenylation of alkenes can be cleanly effected also by oxidation of diphenyl diselenide with iodobenzene diacetate in acetonitrile [22]. 

Explain the following observations (a) hydrolysis of epoxy-cyclopentene (5) with dilute acid gave a mixture of 1,2-dihydroxy-cyclopent-3-ene (20%) and 1.4-dihydroxycyclopem-2-ene (80%) (b) treatment of the epoxide of 1,2-diphenylethene (stilbene) with boron trifluoride etherate gave an isomeric compound which formed a 2,4-dinitrophenylhydrazone and was oxidized to an acid, Ci Hi O,. 

As Peterson outlined in his preliminary communication of the method, either basic (KH, KOBu or NaH) or acidic conditions (acetic acid, sulfuric acid or boron trifluoride etherate) may be utilized to effect the elimination of the silylcarbinol. Alternatively the initial adduct may be treated in situ with thio-nyl or acetyl chloride. This procedure may be advantageous in cases where isomerization of the alkene is problematic, and is particularly useful in the synthesis of terminal alkenes. As discussed in Section 3.1.3.4.2, the Johnson group has successfully employed aqueous HF to effect the elimination and this method may also have advantages in situations complicated by base-catalyzed isomerization. ... 

Following a route previously developed for the synthesis of deacetamidocolchicine, (+)-colchicine has been synthesised from (136 R = R = OMe), which was hydrolysed by acid to (136 RR = 0) and this was cyclised by boron trifluoride etherate to the acid (137). Ring-expansion of the methyl ester of (137) with trifluoroacetic acid gave a mixture of (138) and the isomeric ag-unsaturated ester, both of which, on oxidation, gave the tropolone (139), hydrolysis and decarboxylation of which gave deacetairaido-isocolchicine. Hydrolysis of (139) and treatment with diphenyl-... 

The technique of cyclization of a 1,4,5,6-tetrahydropyridine to an indole nucleus is already familiar (191), and this method was extended to compound 570. Treatment of 570 with boron trifluoride-etherate gave the keto lactam 571 in 62% yield. Thioketalization and Raney nickel reduction gave a lactam (572) which, upon lithium aluminum hydride reduction, gave 2Q-iso-20-deethylaspermidine (573). However, although the initial keto lactam 571 possessed the required C/D cis stereochemistry, under the thioketalization conditions C-20 isomerization occurred to give the more stable C/D trans ring juncture. 

In the presence of boron trifluoride etherate, 39 (R=H and Ar==C6H5) reacts smoothly with silyl enol ethers (the achiral anion source) at low temperatures ( — 80 to — 30 °C) to provide a chromatographically separable mixture of diastereomeric acids from which either 40 or 41 can be obtained in good yield. Oxidative decarboxylation of the chiral auxiliary with freshly crystallized lead tetraacetate occurs without racemization of the newly formed chiral center to provide either 42 or 43 with 98% ee. Noteworthy is the fact that if pure cis isomers are used, such as 39, they undergo facile isomerization to a 65 35 cis trans mixture at the low reaction... 

The isomeric 4-ep/-statine (834) is also readily accessible from 825 by alkylation at C-5 with methallyltrimethylsilane in the presence of boron trifluoride etherate via an A -acylimi-nium ion intermediate. The resulting lactam 832 is produced as an 11 1 mixture of trans and cis isomers which is readily separable by crystallization. Reduction of the olefin and protective group manipulation furnishes 833, an intermediate that intersects with a previous synthesis of 834. 

Cycloheptanes.—One synthesis of karahanaenone (231) depends upon thermal rearrangement of a 2-methylene-5-vinyltetrahydrofuran, and the conditions for this type of reaction have been examined on a simpler model (232), which arises from the dihydrofuran (233) at 140—200 °C. The reaction to the cycloheptenone (234) occurs rapidly at active sites on a glass surface, but is arrested in tubes coated with sodium hydroxide. Higher temperatures and lower pressures give two other compounds (235) and (236). All these reactions involve the biradical (237), as does the conversion of the cyclopropane (238) into the cycloheptenone (234). Karahanaenone (231) has also been made by isomerization of terpinolene epoxide (239) with boron trifluoride etherate. Eucarvone (240 R = H) should not be... 

Hydrolysis of the trifluoroacetyl-derivative (153) produces a mixture of the pyrrolin-2-ones (154) and (155) the latter is unstable, and gradually isomerizes to the former." Whereas the 7V-benzyl- and N-phenyl-pyrrolin-3-ones (156 R = CH2Ph or Ph) exist in the keto form shown, the iV-methyl derivative forms an equilibrium mixture of keto (156 R = Me) and enol tautomers (157)." The reaction of the pyrrolinone (158) with p-nitrobenzaldehyde unexpectedly yields compound (159)." The product of the action of thiobenzoic acid on (160) is the spiro-pyrrolinone (161), contrary to a previous report." Treatment of p-hydroxyphenylacetic acid with di-isopropylcarbodi-imide gives the imino-pyrrolinone (162), which rearranges to the bridged spiro-compound (163) in the presence of boron trifluoride etherate." ... 

A new method of D-glucan analysis involves reductive cleavage of permethylated compounds with triethylsilane and trimethylsilyl triflate or boron trifluoride-etherate as catalyst, followed by acetylation, which yields a series of acetylated, methylated 1,5-anhydro-glucitols. A number of isomeric substituted 1,5-anhydro-glucitols were synthesized as references. /3-Glycosyl-... 

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