Boron trifluoride dioxolane

Unsubstituted 20-ketones undergo exchange dioxolanation nearly with the same ease as saturated 3-ketones although preferential ketalization at C-3 can be achieved under these conditions. " 20,20-Cycloethylenedioxy derivatives are readily prepared by acid-catalyzed reaction with ethylene glycol. The presence of a 12-ketone inhibits formation of 20-ketals. Selective removal of 20-ketals in the presence of a 3-ketal is effected with boron trifluoride at room temperature. Hemithioketals and thioketals " are obtained by conventional procedures. However, the 20-thioketal does not form under mild conditions (dilution technique). ... 

Most of the synthetic approaches toward this ring system utilize N-amino pyridinium salts functionalized at the a-position with a carbonyl group. Thus, the amination of 2-(l,3-dioxolan-2-yl)pyridine with tosyl-hydroxylamine gave 78, whose reaction with urea in the presence of boron trifluoride-acetic acid gave 79, which gave the thermally unstable... 

Cyclic oligomers with x - 2-9 are found to be present in poly(1,3-dioxolane) samples prepared by monomer-polymer-equilibrations using boron trifluoride diethyl etherate as catalyst. The molecular cyclization equilibrium constants 7fx are measured and the values are in agreement with those calculated by the Jacobson-Stockmayer theory, using an RIS model to describe the statistical conformations of the corresponding chains and assuming that the chains obey Gaussian statistics. 

Treatment of isobutylene oxide with methanol (Eq. 653) in the presence of a trace of boron trifluoride is reported to give only 2-methoxy-2-tnethyl-1 -propanol, along with polymerisation products and wcbutyraldehyde (isolated ae its dioxolane derivative).1 31 Ti in perhaps surprising that no l-methoxy 2-i etbyl-2-propand was formed at all under these conditions. 

Dioxolanes are inert to LAH reduction under normal conditions but in the presence of a Lewis acid reduction occurs to yield hydroxy ethers. When aluminum chloride is employed, the reaction requires at least three equivalents for complete reduction. Early reports on the use of boron trifluoride as the Lewis acid stated that it was ineffective. A reevaluation, however, showed that the order of mixing of the reagents was crucial. Best... 

Epoxyglycerides react with ketones in the presence of boron trifluoride to give 1,3-dioxolane derivatives. In this form they were analysed by GC on a column packed with 3% of OV-1 with temperature programming at 4°C/min from 260° C [649]. Cyclo-pentanone was recommended as the most suitable reagent, and acetone, methyl ethyl ketone, methyl isobutyl ketone and others can also be used. 

The five membered cydic 1,3-dioxolane (CHjOCHjCHjO) can be polymerised by a variety of catalysts including sulphuric acid (P7), perchloric acid (98), phosphorus pentachloride (PP) and alkyl aluminium compounds with water as a co-catalyst (100). The effect of the catalyst boron trifluoride diethyl etherate on the polymerisation of 1,3-dioxolane has also been studied and it has been found that equilibrium between monomeric 1,3-dioxolane and poly(l, 3-dioxolane) is set up in both the undiluted polymer and in solution (101-104). Controverf has arisen as to whether the equilibrium is between cyclic monomer and cyclic polymer (98) or between cyclic monomer and chain polymer (104). 

Meerwein was the first to succeed in obtaining dioxolanylium ions of type 2, sufficiently stabilized as salts with non-polarizable anions that they could be isolated crystalline. The compounds can be prepared by splitting out of an anion from cyclic ortho esters or acetals wherein the required ring-system is already present. The ortho ester 1 reacts with antimony pentachloride or boron trifluoride, with splitting out of OR, to give 2. Acetals (3) from aldehydes can be converted, by hydride abstraction with triphenylmethyl or triethyl-oxonium fluorohorate, into salts (2) this reaction proceeds well only with acetals of the 1,3-dioxolane type (3) that have little steric hindrance. With acetals of the 1,3-dioxane type, formed from aldehydes, the reaction of hydride abstraction is not, as a rule, possible. In all such reactions, the anion involved is either SbClg or BF4 . 

Reductive Etherifications and Acetal Reductions. Additional applications of triethylsilane in the reduction of C-0 bonds also continue to surface. The Kusanov-Pames dehydrative reduction of hemiacetals and acetals with trifluorosulfonic acid/EtsSiH has proven especially valuable. Under such conditions, 4,6-O-benzyli-dene acetal glucose derivatives can be asymmetrically deprotected to 6-0-benzyl-4-hydroxy derivatives (eq 28) and thioketone derivatives can be converted to syn-2,3-bisaryl (or heteroaryl) di-hydrobenzoxanthins with excellent stereo- and chemoselectivity (eq 29). Triethylsilane is also useful in a number of related acetal reductions, including those used for the formation of C-glycosides. For example, EtsSiH reductively opens 1,3-dioxolan-4-ones to 2-alkoxy carboxylic acids when catalyzed by HCU. Furthermore, functionalized tetrahydrofurans are generated in good yield from 1,2-0-isopropylidenefuranose derivatives with boron trifluoride etherate and EtsSiH (eq 30). These same conditions lead to 1,4- or 1,5-anhydroalditols when applied to methyl furanosides or pyranosides. ... 

Acetate groups are present at both ends of the polymer molecules as shown above7 This was confirmed by analytical evidence. The initiation of dioxolane polymerization by boron trifluoride-etherate is pictured differently ... 

In addition to the above, liquid copolymers form from 1,3-dioxolane with ethylene oxide, when boron trifluoride is used as the catalyst. Also, a rubbery copolymer forms from tetrahydrofuran and 3,3-diethoxycyclobutane with phosphorus pentafluoride catalyst. A3,3-bis(chloromethyl) oxacy-clobutane copolymerizes with tetrahydrofuran with boron fluoride or with ferric chloride catalysis. The product is also a rubbery material. ... 

Details of the procedures used in the preparation of commercial formaldehyde copolymers have not been fully disclosed. The principal monomer is trioxan and the second monomer is a cyclic ether such as ethylene oxide, 1,3-dioxolane or an oxetane ethylene oxide appears to be the preferred comonomer and is used at a level of about 2%. Boron trifluoride (or its etherate) is apparently the most satisfactory initiator, although many cationic initiators are effective anionic and free radical initiators are not effective. The reaction is carried out in bulk. The rapid solidification of the polymer requires a reactor fitted with a powerful stirrer to reduce particle size and permit adequate temperature control. The copolymer is then heated at 100°C with aqueous ammonia in this step, chain-ends are depolymerized to the copolymer units to give a thermally-stable product. The polymer is filtered off and dried prior to stabilizer incorporation, extrusion and granulation. 

The preparation of dioxolane-type diphenyhnethylene acetals 22 and 23, some of which are new compounds, by use of dichlorodiphenyknethane in pyridine has been lepcmed. The synthesis of pyruvate acetal-containing disaccharides by the trichkapacerimidate method is covered in Chapter 3, and n.m.r. studies on 4,6-pyruvate acetals of a- and P-glucosides, -mannosides, and -galactosides are referred to in Chapter 21. Attempts to prepare the 4,6-0-pyruvate acetals of methyl 2,3-di-O-acyl-D-glucopyranosides by use of methyl pyruvate in the presence of triflic acid, rather then boron trifluoride etherate (see Vol. 26, Chapter 6, Ref. 13) led instead to the formation of 3,6-anhydrofuranose derivatives (see also Chapter 5). 

Dioxolanes are also prepared by KlO-catalyzed reaction of l-chloro-2,3-epoxypropane (Epichlorohydrin) with aldehydes or ketones, in carbon tetrachloride at reflux (eq 3). In the reaction of acetone with the epichlorohydrin, the efficiency of catalysts varies in the order K10 (70%) > Tin(IV) Chloride (65%) > Boron Trifluoride (60%) = Hydrochloric Acid (60%) >Phosphorus(V) Oxide (57%). 

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