Dithioacetals from acetals

Bredereck and coworkers also studied the 1,6-ditrityl ether, and definite proof of its structure was afforded by an outstanding series of experiments. The amorphous material (m.p. 94r-96°) was treated with the ordinary acetylation mixture, and yielded 3,4,5-tri-O-acetyl-l,6-di-O-trityl-fce(o-D-fructose. Preparation of the oxime followed by further acetylation gave the crystalline acetate of the oxime [ m.p. 207-208° Ca ]D - -118° (in chloroform)]. < The oxime of l,6-di-t>-trityl-D-fructose affords the same product on acetylation. The keto form was proved by formation of the crystalline diethyl dithioacetal from the acetyl derivative. 

Hg(OAc)2, H2O, 80% AcOH, HSCH2CH2SH, 25°, 5-20 min H2S, 2 h, high yield. The removal of an 5-benzylthiomethyl protective group from a dithioacetal with mercuiy(II) acetate avoids certain side reactions that occur when an 5-benzyl thioether is cleaved with sodium/ammonia. The dithioacetal is stable to hydrogen bromide/acetic acid used to cleave benzyl carbamates. 

The a-oxoketene dithioacetals 6 (6.1-6.43) employed in this work as three carbon 1,3-biselectrophilic components have been drawn from various active methylene ketones and are described in the Table-1. The corresponding a-oxoketene N,S- and 0,S-acetals (17.1- 17.15) which are usually derived from a-oxoketene dithioacetals are described in Table-2. These examples are only a representative groups to demonstrate the general application of the new Jl-heteroaromatic annulation methodology... 

The a-oxoketene dithioacetal 6.40 is derived from indoxyl (l,2-dihydroindol-3-one), a heterocyclic carbonyl precursor, and its reaction with simple allyl anions will also yield the corresponding Jl-annulation product. Thus when 6.40 was reacted with allyl anions 65 the corresponding carbinol acetals 66 formed insitu underwent smooth BF3.Et20 assisted cyclization to afford the corresponding carbazoles 67 in high yields <99T11563>. 

Cyclic ketene acetals, which have utility as co-polymers with functional groups capable of cross-linking, etc., have been prepared by the elimination of HX from 2-halomethyl-l,3-dioxolanes. Milder conditions are used under phase-transfer conditions, compared with traditional procedures, which require a strong base and high temperatures. Solid liquid elimination reactions frequently use potassium f-butoxide [27], but acceptable yields have been achieved with potassium hydroxide and without loss of any chiral centres. The added dimension of sonication reduces reaction times and improves the yields [28, 29]. Microwave irradiation has also been used in the synthesis of methyleneacetals and dithioacetals [30] and yields are superior to those obtained with sonofication. 

Because of the polyfunctional nature of carbohydrates, protective-group strategy plays an important role in synthetic methodology involving this class of compounds. In the present Chapter, results are described from a study of the utility of N-trimethylsilyl- and N-tert-butyldimethylsilyl-phthalimide for the selective silylation of primary hydroxyl groups in carbohydrates. Also described, is a new, facile method for cleavage of acetals and dithioacetals in carbohydrate derivatives the method involves treatment of the derivatives with a dilute solution of iodine in methanol. 

The C-H bond-dissociation energies Z>c-h of the acetals 130 and 131, monothioacetal 132, and the dithioacetal 133 (Figure 7) were calculated from experimental reaction kinetic data <2005MI379>. The C-H dissociation energies of the 1,3-dithiane derivative 133 proved to be lowest, followed by the 1,3-oxathiane derivatives 132,... 

Acidic compounds of type R—XH, which are able to protonate thiocarbonyl ylides, also undergo 1,3-addition leading to products of S,S-, S,0-, or 5,A-acetal type (Scheme 5.20). Thiophenols and thiols add smoothly to thiocarbonyl ylides generated from 2,5-dihydro-l,3,4-thiadiazoles (36,38,86,98,99). Thiocamphor, which exists in solution in equilibrium with its enethiol form, undergoes a similar reaction with adamantanethione (5)-methylide (52) to give dithioacetal 53 (40) (Scheme 5.21). Formation of analogous products was observed with some thiocarbonyl functionalized NH-heterocycles (100). 

With due consideration of the explanations just presented for the observed, relative stabilities of cyclic acetals derived from polyols, in terms of their constitution and conformation, nearly all of the following observations on the selective hydrolysis of cyclic acetals of alditols and dialkyl dithioacetals may be readily understood. 

Less is known concerning the partial, acid hydrolysis of alditol polyacetals derived from ketones, compared to those derived from aldehydes. The acid hydrolysis of 1,2 3,4 5,6-tri-O-isopropylidene-D-mannitol to 3,4-O-isopropylidene-D-mannitol55,56 and of l,2 3,4-di-0-isopropylidene-L-rhamnitol to 3,4-O-isopropylidene-L-rhamnitol57 indicates an order of isopropylidene acetal stability of a-threo > a, and this order is supported by the partial hydrolysis of 2,3 4,5-di-O-isopro-pylidene derivatives of dialkyl dithioacetals of D-arabinose58 and D-xylose59 to 2,3-acetals. 

Acid hydrolysis of 2,4 3,5-di-0-isopropylidene-D-xylose diethyl dithioacetal to give63 the 2,4-acetal afforded a rare opportunity to establish that, in isopropylidene acetals, as with acetals derived from aldehydes, f3-erythro rings are more stable than /3 rings. 

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. 

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