Hydroxy ketals, preparation

Isopropylideneglycerol, a five-membered cyclic hydroxy ketal from acetone and glycerol, is prepared in 90% yield by removing the liberated water by an azeotropic distillation. In another procedure, calcium carbide is added directly to the reaction mixture as a desiccant. Acetaldehyde and benzaldehyde, unlike acetone, react with glycerol to form a mixture of the five- and six-membered cyclic hydroxy acetals. Alkoxy acetals are made by the acetalization of a,/3-olefinic aldehydes in weakly acidic solutions however, the addition of alcohol to the double bond may not go to completion. ... 

I60C-Hydroxy Derivatives of Gorticoids and their Acetonides. The preparation of 16a-hydroxy-9a-fluoroprednisolone (48) from the 3,20-bisethylene ketal of hydrocortisone acetate (49) has been reported (73). The latter was dehydrated with thionyl chloride in pyridine to yield the 4,9(11),16-triene (50). The 16,17-unsaturated linkage was selectively hydroxylated with OsO /pyridine to yield the 16a,17a-diol (51), which was converted... 

Butylethylidene and 1-phenylethylidene ketals were prepared selectively from the C4-C6, 1,3-diol in glucose by an acid-catalyzed transketalization reaction [e.g., Me3CC(OMe)2CH3, TsOH/DMF, 24 h, 79% yield PhC(OMe)2Me, TsOH, DMF, 24 h, 90% yield, respectively]. They are cleaved by acidic hydrolysis AcOH, 20°, 90 min, 100% yield, and AcOH, 20°, 3 days, 100% yield, respectively. Ozonolysis of the /-butylmethylidene ketal affords hydroxy ester, albeit with poor regiocontrol, but a more sterically differentiated derivative may give better selectivity, as was observed with the ethylidene ketal. ... 

Preparation of 11 -Hydroxy-6a-Methylprogesterone A mixture of 2.68 g of 11-keto-6(3-methylprogesterone 3,20-bis-(ethylene ketal), 161 ml of tetrahydrofuran (previously distilled from lithium aluminum hydride), 1.34 g of lithium aluminum hydride and 14.5 ml of absolute ether was stirred and refluxed under nitrogen for 1.5 hours, then 27 ml of water was added cautiously, to decompose excess hydride. The resulting mixture was filtered and the filter cake was washed with 135 ml of ether. The combined filtrate and wash was shaken with 135 ml of water and separated. The aqueous layer was washed with four 55-ml portions of ether, then the organic layer and the washes were combined, washed once with water, and evaporated to dryness under diminished pressure leaving a tan residue. 

It should be mentioned that when a hexa(hydroxyl) initiator is used for the lipase catalyzed polymerization of s-CL, only one hydroxy function is active [100]. This leaves five remaining OH groups for polymerization of new or another monomers. Comb poly(e-CL)s have also been prepared [101, 102] starting from a copolymer of e-CL and 5-ethylene ketal-e-caprolactone as shown below ... 

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... 

In this original and imaginitive approach, a rapid assembly of the phenanthrene core of morphine, containing a novel non-aromatic A ring, was achieved in an intermolecular Diels-Alder reaction between quinone 173 (prepared from 3-methoxy-2-hydroxy benzaldehyde in 7 steps and an overall yield of 35%) and diene 174 (from 1,4-cyclohexanedione monoethylene ketal in 2 steps with an overall yield of 30%), Scheme 20. In one of several unsuccesful attempts to aromatize ring A, an unexpected tandem... 

S)-3/3-Dimethylamino-18,20-diacetoxy-5a-pregnane (1) was prepared from 20/3-hydroxy-5a-pregnan-3-one cyclic ethylene ketal (2) in six steps. The key steps were iodination of (2) with HOI and cyclization to ether (3). ... 

For many further syntheses, it is necessary to block the hydroxy functions of tartaric acid. This can be done by acid-catalyzed formation of cyclic acetals or ketals (1,3-dioxolanes) with carbonyl compounds, e.g., 43. Acetone, acetophenone, benzaldehyde, pivalaldchydc, and other simple carbonyls have been used for this purpose28 31,42-43. The protected esters of tartaric acid, used as starting materials for many purposes (Sections 2.3.2. and 2.5.3.), can be prepared in a one-pot procedure. 

Acetophenone is converted in this way into the diethylene ketal in 74% yield. Under the same conditions benzophenone is unreactive. The reagent appears particularly useful for preparation of acetals of aromatic ortho-hydroxy aldehydes. Ketalization of A -3-keto steroids is accompanied by migration of the double bond [(1) (2)]. The tetrathio orthocarbonate (3) can be used in the... 

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