Acetals preparation from dithioacetals

The syntheses of dithioacetals are generally straightforward [43]. Standard methods may be unselective for multifunctional molecules. Therefore, new procedures have been developed. It has thus been reported that 1,3-dithianes are readily synthesized by reaction of aldehydes, ketones or acetals with 2-stanna-l,3-dithianes under catalysis of organotin triflates [45]. These odourless reagents are prepared from dialkyldichlorotin and 1,3-propanedithiol. 

The C23-C26 segment 173 was prepared from 180, which was derived from ribose (O Scheme 20). After 0-benzylation of 180, the resulting benzyl ether was treated with MeMgCl and CuBr Me2S to afford 181. Thiol acetal formation followed by selective silyla-tion provided 182. The dithioacetal was cleaved and the resulting aldehyde was reduced with NaBH4 to afford a diol, which was subjected to direct epoxidation to provide 173. 

In order to confirm the structures of solanapyrones, chemical synthesis of these phytotoxins were attempted based on biogenetic consideration [55], The retro synthesis envisaged intramolecular Diels-Alder reaction of the achiral polyketide triene (a), a key intermediate, which is further divided into a pyrone moiety (b) and a diene moiety (c). The moieties a and b were prepared from dehydroacetic acid and hexadienyl acetate, respectively. Aldol condensation of the aldehyde (72) with the dithioacetal (73) gave a dienol, which was further converted to a triene (74). The intramolecular Diels-Alder reaction of 74 in toluene at 170-190 °C for 1 hr in a sealed tube yielded a mixture of the adducts (75) and (76) in a ratio of 1 2. This product ratio depends on the solvents, i.e. in water (1 7), and should be useful in differentiating between artificial and enzymatic reactions in biosynthetic studies. Removal of the thioacetal groups in 75 and 76 yielded solanapyrone A (67) and D (70) in a ratio of 3 5. Though solanapyrone D (70) had not been isolated from the natural resources at this stage, the structure and stereochemistry were confirmed by H NMR spectrum. 

The acyl migration of benzylic acetates was also applied for the synthesis of indenes (equation 87). Isomeric indenes were also obtained as minor products. The transformation was actually shown to proceed via allenes, which could be independently prepared by Ag(I)-catalyzed 1,3-acetate migration from the same starting materials. Propargylic sulfides and dithioacetals undergo similar transformations as propargylic carboxylates to give indene derivatives with Au(I) or Au(III) catalysts. 

Deacetalization and dethioacetalization. Cyclic acetals of ketones are cleaved by NO2 in the presence of silica gel (5 examples, 88-100%). Carbonyl compounds are similarly recovered from dithioacetals by treatment with nitrogen oxides, which are prepared from arsenous oxide with concentrated HNO,. 

Transacetalization, usually starting from the dimethyl acetal, is an important method for the preparation of amide acetals (465 Scheme 86) with long chain or secondary alkyl groups7 4 83t3235 Recently a method was described which allows the preparation of even the di-r-butyl acetal of DMF by transace-talization. To achieve good yields it is necessary to drive the equilibrium reaction to completion by distilling off the alcohol formed. Cyclic acetals (467) or dithioacetals (466) can be more easily synthesized by transacetalization. 24 28-3i-32,35... 

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. 

Dialkyl dithioacetal derivatives of ketoses, such as D-fiuctose and L-sorbose, me inaccessible directly from the parent sugars, the ketose undergoing extensive decomposition under the conditions employed for mercaptaladon of aldoses. Such derivatives can, however, be prepared by indirect methods. Acetylation of D-fiuctose [40] and L-soibose with acetic adiydride and zinc chloride [41] leads to good yields of acyclic pentaacetates in which foe ketose carbonyl is not involved in a cyclic acetal. Subsequent treatment of these acetylated derivatives with thiols affords foe acetylated dialkyl dithioacetals in satisfactory yields, and conventional deacetylation affords foe unprotected dialkyl dithioacetals [40,41]... 

As we have mentioned above, ketene N9S- and N,0-acetals are the intermediates in the preparation of 1,1-enediamines from ketene dithioacetals and ketene acetals, respectively, and they can be isolated from the reactions5,51,52,72. Further substitution of the methylthio or the alkoxy groups by amines yields 1,1-enediamines. 

Further adaptations of the boranes has led to reagents which reduce carboxylic acids but afford thio-acetals rather than the aldehyde itself.Thus, with the thioborane (3 equation 1), aliphatic acids give 80-87% yields of thioacetals but aromatic acids respond less well in giving significant quantities of sulfides as well. Carboxylic acid esters are inert to this reagent but give sulfides if a Lewis acid is included. Similarly, the 1,3,2-dithiaborinane-dimethyl sulfide adduct (4 equation 2) affords cyclic dithioacetals in 70-90% isolated yields in the presence of SnCb. Aliphatic acids react in about 6 h at room temperature but aromatic acids need about 20 h and yields are somewhat poorer. This area has been reviewed.From a practical viewpoint, it should be noted that the dithiaborinane (4) requires a week for its preparation. 

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