Trifluoride, boron dithioacetals

The stoichiometric equivalents of halofluorides have been recently applied to transform alkylene dithioacetals into gcm-difluorides.70-71 Dithioacetals such as 1,3-dithiolanes and 1,3-dithianes arc readily obtained from the corresponding carbonyl compounds by the reaction with ethane-1,2-dithiol or propane-1,3-dithiol in the presence of the complexes boron trifluoride-bis(acetic acid) or boron trifluoride-diethyl ether. Using a two-step procedure, a range of aldehydes and ketones can be converted into gem-difluorides under mild conditions. 

The orf/to-formylation of 2-aminopyridines can be effected via the rearrangement of the azasulfonium salt prepared from a 2-aminopyridine, 1,3-dithiane, f-butyl hypochlorite and sodium methoxide (74CC685). The crude sulfilimine (815) was refluxed in f-butanol containing potassium f-butoxide to yield the dithioacetal (816). Hydrolysis of (816) with mercury(II) oxide/boron trifluoride etherate gave the aldehyde (817 Scheme 191). This method should be applicable to the formylation of other heterocyclic amines. 

The reaction of aldehydes or ketones with ethane-1,2-dithiol or propane-1,3-dithiol to form 1,3-dithiolanes or 1,3-dithianes is an important reaction, as these compounds under suitable conditions are acyl anion equivalents (see Section 5.9, p. 626). These cyclic dithioacetals have been less used as protective groups, though when required are formed in high yield in the presence of boron trifluoride-etherate.138... 

The reaction of aldehydes or ketones with thiols, usually with a Lewis acid catalyst, leads to dithioacetals or dithioketals. The most common catalyst used is probably boron trifluoride etherate (BF3 OEt2). Similarly reactions that use 1,2-ethanedithiol or 1,3-propanedithiolleadto 1,3-dithiolanes, such as 18 or l,3-dithianes. " Dithioa-cetals can also be prepared from aldehydes or ketones by treatment with thiols in the presence of TiCU, SiCU, LiBp4, AKOTfls, with a disulfide RSSR (R = alkyl or aryl), or with methylthiotrimethylsilane (MeSSiMe3). " ... 

Aldosulose derivatives having unprotected ketonic functions have been converted into similarly substituted ethylene dithioacetals by the action of 1,2-ethanedithiol in the presence of boron trifluoride. l,6-Anhydro-2,4-dideoxy-jS-D-glt/cero-hexopyranos-3-ulose70 (9)... 

Cyclopropanecarbaldehydes, 1,1-dialkoxy-l-cyclopropylalkanes and cyclopropyl ketones reacted readily with ethanedithiol " " " and propanedithioP to give the corresponding 1,3-dithiolanes and 1,3-dithianes, respectively, in excellent yield. Boron trifluoride diethyl ether complex, in the presence or absence of acetic acid, has generally been used as the catalyst, but hydrogen chloride has also been utilized.An example of this reaction is the formation of the dithioacetal of cyclopropanecarbaldehyde 3. ... 

The reaction of propargylic esters with aldehydes and hydrogen sulfide in the presence of boron trifluoride etherate leads to 1,3-dithiins (86) (Equation (56)) <71T5753>. Dieckmann cyclization of suitably substituted acyclic 1,3-dithioacetals gives rise to l,3-dithian-5-ones (87) (Equation (57)) <59LA(624)79, 63LA(661)84>. 

Early workers reacted the ketone with an excess of the thiol in the presence of an acid catalyst such as zinc chloride , hydrogen chloride or />-toluenesulphonic acid to prepare dithioacetals. The results were erratic and the yields often disappointing. The use of boron trifluoride etherate has led to consistently better results . This method is particularly effective when the thiol is used for the solvent of the ketone as the boron trifluoride etherate is added. Ethanedithiol and propanedithiol are usually the thiols of choice forming 1,3-dithiolanes and 1,3-dithianes respectively. For example, the 1,3-dithiolane of cholestane-3-one (equation 1) can be prepared in high yield by this method . Occasionally the choice... 

Other Lewis acids such as aluminum chloride, aluminum bromide, boron trifluoride etherate and ferric chloride converted these substrates into dithioacetals, frequently in less than an hour. 

Lithium tetrafluoroborate in wet acetonitrile has been described as an effective combination for the hydrolysis of acetals under mild weakly acidic conditions. Dithians were unaffected. Methods for the hydrolysis of thioacetals continue to appear. Reagents that have been described include a polystyryl-mercury(n) trifluoroacetate combination, which retains the metal on the resin, lead(iv) dioxide and boron trifluoride etherate, aqueous hydrochloric acid in dioxan containing dimethyl sulphoxide, methyl-bis(methylthio)sulphonium hexa-chloroantimonate, and iodoxybenzene, catalysed by toluene-p-sulphonic acid. Dithioacetals derived from ethane-1,2-dithiol may be cleaved with dimethyl sulphoxide in combination with either t-butyl or trimethylsilyl bromides and iodides. Regeneration of ketones from ethanediyl-S S -acetals via the lithium-di-isopropylamide-promoted fragmentation to the thioketone and subsequent hydrolytic work-up only gives satisfactory yields for aryl methylketone derivatives. Dithioacetal SS-dioxides are rapidly cleaved in hot methanolic hydrochloric acid solution. ... 

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