Aldehydes dithiol acetals

Alkyl groups on l,2-dithiole-3-thiones react fairly readily with carbonyl groups to form 5-vinyl-1,2-dithioles <87EUP256929, 92JAP(K)04I 17377). For example, 5-methyl-l,2-dithiole-3-thione (102) reacts with aldehydes or acetals to form 5-vinyl-l,2-dithiole-3-thiones (103) <82AHC(3I)63,92SUL227). Carbanion formation, reaction with carbon disulfide, and further elaboration converts the dithiole-... 

Dithioacetals, 1,3-dithianes or 13-dithiolanes are prepared by reaction of the corresponding carbonyl compound in the presence of an acid catalyst (cone. HQ, Lewis acids such as Znh, BFs EtaO, TMS-Cl, etc.) with a thiol or dithiol. Silica gel treated with thionyl chloride was found to be an effective as well as selective catalyst for thioacetalization of aldehydes. Thioacetalization can also be achieved using a (polystyryl)diphenylphosphine-4odine complex as a catalyst Conversion of aldehydes or acetals into 1,3-dithianes is achieved with the aid of organotin thioalkoxides and organotin triflates or with 2,2-di-methyl-2-sila-l,3-dithiane. Direct conversion of carboxylic acids to 1,3-dithianes can be carried out by reaction with 1,3,2-dithiabomenane-dimethyl sulfide and tin(II) chloride or 1,3,2-dithiaborolene with trichloromethyllithium followed by basic hydrolysis. 

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. 

Resin-bound diols, amino alcohols, and dithiols, which reversibly form cyclic acetals with aldehydes and ketones, have been successfully used as linkers for carbonyl compounds (Entries 5-11, Table 3.40). Acetal formation on insoluble supports can be achieved by azeotropic removal of water (C6H6, TsOH, reflux [720]), whereas dithio-acetals can be prepared by acid-catalysis alone (BF3 OEt2 or TMSC1 CHCI3,0 °C, 2 h [721]). /V-Acylaminals such as R-CFI(OMe)NFI-CO-Pol have been prepared by treatment of resin-bound amides H2NCO-Pol with aldehydes in the presence of HC(OMe)3 and TFA [722],... 

Keywords ketone, aldehyde, ethane-1,2-dithiol, neutral alumina surface, dithio-acetal, 1,3-dithiolane... 

Thioacetals. Aldehydes, ketones, or acetals, both cyclic and acyclic, can be converted into thioacetals by reaction with a thiol or dithiol and MgBr2 (2.1 equiv.) in ether at 25°. The difference in reactivity between acetals and ketones permits selective conversion of acetals into thioacetals without acetalization of a ketone. 

Dimethylthioformamide reacts at a methylene group in the 3- or 5-position of a 1,2-dithiolium cation. This reaction may be performed in acetic anhydride. The resulting Vilsmeier salt (32), treated with aqueous sodium hydroxide, gives an a-(l,2-dithiol-3-ylidene) aldehyde.49... 

Some of the problems associated with the synthesis of a-dicarbonyl starting materials have been alleviated by the use of propane-1,3-dithiol, which reacts with aldehydes to give cyclic thioacetals. With butyllithium the resulting stable dithiane anions (134) can be transformed into a-diketones or a-hydroxy ketones (Scheme 73). A further approach to such compounds is found in the reaction of a-ketonitrate esters with sodium acetate (Scheme 73), while aryl a-diketones are also available from a-ketoanils (prepared from the cyanide-ion-catalyzed transformation of aromatic aldimines) (70AHC(12)103). 

Although 1,2-dicarbonyl substrates (especially unsymmetrical benzils) are often difficult to make, there are a number of approaches which may be appropriate. Propane-1,3-dithiol reacts with aldehydes to give cyclic thioacetals (in 52-91% yields) which form stable dithiane anions when treated with butyllithium. Subsequent quenching with an acid chloride followed by mercury(ll) chloride treatment gives a 1,2-dicarbonyl species. Alternatively, substitution of an aldehyde for the acid chloride gives rise eventually to an a-hydroxycarbonyl derivative (Scheme 5.1.2) [16j. An alternative approach to a-ketoaldehydes (82-86% yields) reacts an a-ketonitrate ester with sodium acetate in DMSO [17]. Aryl a-diketones can be made from a-ketoanils, which are in turn made by cyanide ion-catalysed transformation of aromatic aldimines [18], and the range of unsymmetrical benzils has been increased by... 

Dithiolanes can also be regarded as cyclic dithioacetals or ketals. They are accordingly obtained by cyclocondensation of aldehydes or ketones with 1,2-dithiols in acetic acid with... 

Dithians and 1,4-Dithians.—Equimolar amounts of propane-1,3-dithiol and phosgene condensed to give a high yield of l,3-dithian-2-one by a procedure which is a considerable improvement on earlier methods.A 1,3-dithian is also obtained when an aldehyde is condensed with a 1,3-dimercaptoacetone (Scheme 10) 300 mechanism of the formation of 2,3-dihydro-1,4-dithians (323) from a-halogeno-acetals (Scheme 11) appears to involve the carbonium ion (322). ... 

C=0. It is therefore susceptible to nucleophilic attack at carbon. Methods have been developed by which this polarization is effectively reversed so that the carbon atom itself becomes the nucleophilic centre. Such an inversion is known as umpolung. An example is provided by the 1,3-dithiane system. An aldehyde may be converted into a cyclic dithioacetal by reaction with propane-1,3-dithiol in the presence of an acid. The two adjacent electronegative sulphur atoms make the C—H bond of this acetal rather acidic. Treatment with butyllithium therefore affords a lithio derivative in which the carbon atom is susceptible to electrophilic attack. The 1,3-dithiane system is reconverted into a carbonyl group by acid hydrolysis in the presence of mercury(II) ions, which complex with the dithiol. The RCO group in the original aldehyde is thus equivalent to R—C=0 (Fig. 3.10). 

Another example of polymer-supported A1 based Lewis acid is cross-linked polystyrene-supported aluminum triflate (79). Cross-linked polystyrene-supported AICI3 (72) was easily converted into (79) by treatment with triflic acid. This catalyst was applied to dithioacetalization of carbonyl compounds and transdithioacetal-ization of acetals (Scheme 19.19) [44]. From benzaldehyde the corresponding dithioacetal (84) was obtained in the presence of the polymeric catalyst (79) in 98% yield in 30 minutes. The same product was also obtained from the dimethyl acetal (85) in 94% yield. Chemoselectivity of the polymeric catalyst was also demonstrated in Scheme 19.19. Aldehydes reacted faster in the presence of ketone (34) to give the dithioacetals with (79). Aliphatic ketone (89) exclusively reacted with dithiol (81) in the presence of aromatic ketone (34). These chemoselectivities were higher than those obtained from the reactions using nonsupported Al(OTf)3. 

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