Dimethyl ketal, formation with

The direct formation of a dimethyl ketal by reaction of the ketone with methanol is particularly sensitive to steric effects. Only cyclohexanones react under these conditions.In the steroid series only saturated 3-ketones form dimethyl ketals with methanol and acid although partial reaction of a 2-ketone has been observed in the presence of homogenous rhodium catalyst. ... 

Dimethyl ketals and enol ethers are stable to the conditions of oxime formation (hydroxylamine acetate or hydroxylamine hydrochloride-pyridine). Thioketals and hemithioketals are cleaved to the parent ketones by cadmium carbonate and mercuric chloride. Desulfurization of thioketals with Raney nickel leads to the corresponding methylene compounds, while thioenol ethers give the corresponding olefin. In contrast, desulfurization of hemithioketals regenerates the parent ketone. ... 

Acetalization or ketalization with silylated glycols or 1,3-propanediols and the formation of thioketals by use of silylated 1,2-ethylenedithiols and silylated 2-mer-captoethylamines have already been discussed in Sections 5.1.1 and 5.1.5. For cyclizations of ketones such as cyclohexanone or of benzaldehyde dimethyl acetal 121 with co-silyl oxyallyltrimethylsilanes 640 to form unsaturated spiro ethers 642 and substituted tetrahydrofurans such as 647, see also Section 5.1.4. (cf. also the reaction of 654 to give 655 in Section 5.2) Likewise, Sila-Pummerer cyclizations have been discussed in Chapter 8 (Schemes 8.17-8.20). 

For instance, 2-methylpropene reacted with acetic acid at 18°C in the presence of Al-bentonite to form the ester product (75). Ion-exchanged bentonites are also efficient catalysts for formation of ketals from aldehydes or ketones. Cyclohexanone reacted with methanol in the presence of Al-bentonite at room temperature to give 33% yield of dimethyl ketal after 30 min of reaction time. On addition of the same clay to the mixture of cyclohexanone and trimethyl orthoformate at room-temperature, the exothermic reaction caused the liquid to boil and resulted in an almost quantitative yield of the dimethyl ketal in 5 min. When Na- instead of Al-bentonite is used, the same reaction did not take place (75). Solomon and Hawthorne (37) suggest that elimination reactions may have been involved in the geochemical transformation of lipid and other organic sediments into petroleum deposits. 

While normal Knorr conditions involve initial N-C bond formation, the overall reaction can also be accomplished under conditions which begin with C-C bond formation. Condensation of the dimethyl ketal of acetamidoacetone with silyl enol ethers give adducts which cyclize to pyrroles (Scheme 49) <88S38l). 

Many important natural products are (formerly) derived by chain elongation at position 5 of pentoses, or at position 6 of hexoses. Uronic acids, which are easily prepared, can be converted into the 4 radical 90 by chemistry based on the thiohydroxamate 6.77 We postulated that, if the hindrance on the a-side of the molecule was great enough, the carbon-carbon bond formed by reaction of 90 with a suitable radicophilic olefin would be the natural / -bond. In fact, even a dimethyl-ketal as in 90 (B = natural base or protected derivative thereof) was sufficient to direct the bond formation very largely to the desired face.77 The diacetone ketal of glucuronic acid 91 upon conversion to its iV-hydroxy-2-thiopyridone derivative 92 and then photolysis in the usual way in the presence of methyl acrylate gave the expected derivative 93 as a mixture of... 

Formation of intermediate chloro derivative 112 was confirmed by its hydrolysis to benzophenone and alcoholysis to corresponding dimethyl ketal. Reaction of CBT with diazomethane resulting in formation of 1- and 2-chloromethylbenzotriazoles also confirms the proposed reaction scheme. 

Wasserman and Saito have used diphenyl sulfide to intercept dioxetanes formed in the sensitized photooxidations of certain unsaturated systems. Thus reaction of cw-dimethoxystilbene (1) with singlet oxygen yields only the diox-etane (2) and the product of cleavage, methyl benzoate. However, addition of diphenyl sulfide to the reaction results in formation of benzil dimethyl ketal... 

The reactions studied have been the first examples of solvolysis of 2-norbornyl aryl sulphonates without exclusive formation of exo products. But the comparison of solvolyses between compounds 113,114 and 63 is complicated by different hybridization of the C atom and by the possible formation of an intermediate hemiketal with 113 and 114. These complications are avoided by acetolyzing the isomeric dimethyl ketals 118 and 119 having the sp -hybridized C atom The solvolysis of compound... 

Epoxy-5/ -cholestan-3-one allowed to react 6 hrs. at room temp, with methanol in dry ether dimethyl ketal. Y ca. 10%.—Ketal formation protects the 4,5-epoxy-3-oxo moiety against attack by LiAlH4. F. e. s. D. J. Gollins and J. J. Hobbs, Ghem. Ind. 1964, 1063. 

The cycloaddition of allal derivative 24 with dichloroketene followed by treatment of NaOMe in methanol afforded 1,2-dideoxy-P-D-altropyranoside 25 and dimethyl ketal 26 in 36 and 29% yields respectively. " In contrast to NaOMe, chlorocyclopropanated sugar 27 was also isolated in the case of NaOEt in ethanol. It has been suggested that the formation of ethyl ester 25a and chlorocyclopropanated sugar 27 proceeds through the same intermediate 28. Favorskii type reaction pathway has been ruled out due to ring strain in chlorocyclopropanone 28b. Alternatively, benzilic acid rearrangement has been proposed as more probable reaction pathway through common intermediate for 25,25a and 27. 

The Lewis acid-catalysed orientation reversal in the reaction between substituted cyclohexa-1,3-dienes and 2,6-dimethyl-l,4-benzoquinone ° has been employed in an interesting synthesis of quassin (218). ° Thus, reaction at room temperature of the diene (215) with the above quinone in the presence of an equivalent quantity of Bp3,OEt2 gave the adduct (216) which was converted by several subsequent steps into (218). In the absence of the catalyst the alternative adduct (217) was obtained. Periodic acid oxidation of substituted o-cresols ° and of 2-methoxyphenols in methanol solution affords intermediate o-quinol methyl ethers or o-quinone dimethyl ketals which dimerize to give dienediones with structures related to those of (216) and (217). Another report concerns the formation of a Diels-Alder dimer upon hypochlorite oxidation of 2,2 -methylenebis(4-methyl-6-t-butyl)phenol. ... 

The formation of cyclic acetals and ketals from alkane diols other than ethylene glycol has been investigated in some detail [30, 30a, 30b]. It was found that with ketones ease of ketal formation is in the order 2,2-dimethylpropane-l,3-diol > ethylene glycol > propane-1, 3-diol, and that stability to acid is greatest with the resulting 5,5-dimethyl-l,3-dioxans. In the case of the diketone (1) it has been foimd that mono-ketalization at the 1-keto-group was more selective with... 

An alternative method of hydroboration is to use diisopinocampheylborane (12) (Scheme 4). This reaction is particularly useful for sterically hindered alkenes. Diisopinocampheylborane (12) is prepared from borane-dimethyl sulfide and (+)-pinene.[23-24] Treatment of 4-meth-ylenecyclohexanone ethylene ketal with diisopinocampheylborane (12) gives the borane 13.[25] Further treatment with 2 equivalents of an aldehyde results in the elimination of pinene and the formation of a new dialkyl boronate, e.g. treatment of 13 with acetaldehyde gives the diethyl cyclohexylmethylboronate 14J261 The dialkyl boronates thus produced can be transesterified with pinanediol to give 15[26] or with other cyclic diols. 

The effect of reaction time on the major components of the reaction of cystine and DMHF in water is shown in Table IV. It is noteworthy that amounts of 2,4-hexanedione, 3,5-dimethyl-l,2,4-trithiolanes and thiophenones were found at a maximum after one hour. It was also found that the amount of 2-acetylthiazole increased with time and that acetol acetate decreased with time as expected. In the glycerol medium, the effect of reaction time on the major components is shown in Table V. Apparently, the 1,3-dioxo-lane, which is a ketal formed from glycerol and acetone, decreased over time. Also, long reaction time favors the formation of cyclic compounds, including 2,5-dimethyl-2-hydroxy-3(2H)-thiophene, cyclo-pentenones and 4,5-dimethyl-l,2-dithiolenone. 

This section deals only with solvents whose reduction products are insoluble in the presence of lithium ions. The list includes open chain ethers such as diethyl ether, dimethoxy ethane, and other polyethers of the glyme family cyclic ethers such as THF, 2Me-THF, and 1,4-dioxane cyclic ketals such as 1,3-dioxolane and 1,3-dioxane, esters such as y-butyrolactone and methyl formate and alkyl carbonates such as PC, EC, DMC, and ethylmethyl carbonate. This list excludes the esters, ethyl and methyl acetates, and diethyl carbonate, whose reduction products are soluble in them (in spite of the presence of Li ions). Solutions of solvents such as acetonitrile and dimethyl formamide are also not included in this section for the same reasons. Figure 6 presents typical steady state voltammo-grams obtained with gold, platinum, and silver electrodes in Li salt solutions in which solvent reduction products are formed and precipitate at potentials above that of lithium metal deposition. These voltammograms are typical of the above-mentioned solvent groups and are characterized by the following features ... 

The diols (97) from asymmetric dil droxylation are easily converted to cyclic sii e esters (98) and thence to cyclic sulfate esters (99).This two-step process, reaction of the diol (97) with thionyl chloride followed by ruthenium tetroxide catalyzed oxidation, can be done in one pot if desired and transforms the relatively unreactive diol into an epoxide mimic, ue. the 1,2-cyclic sulfate (99), which is an excellent electrophile. A survey of reactions shows that cyclic sulfates can be opened by hydride, azide, fluoride, thiocyanide, carboxylate and nitrate ions. Benzylmagnesium chloride and thie anion of dimethyl malonate can also be used to open the cyclic sulfates. Opening by a nucleophile leads to formation of an intermediate 3-sidfate aiuon (100) which is easily hydrolyzed to a -hydroxy compound (101). Conditions for cat ytic acid hydrolysis have been developed that allow for selective removal of the sulfate ester in the presence of other acid sensitive groups such as acetals, ketals and silyl ethers. 

In a similar vein, 3 equiv. of trimethylsilyldimethylamine have been used at room temperature to form the enamine from cyclohexanone, conditions under which the water is presumably consumed as hexa-methyldisiloxane (equation 5). This method has been extended to the formation of IVA -dimethyl-enamines from unsaturated aldehydes. Alternatively, enamines have also been obtained by converting the carbonyl compound to a ketal before reaction with the amine. The first process is endothermic but can be readily driven to completion and provides the driving force for formation of the enamine. 

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