Cyclohexylidenes are slightly harder to cleave than acetonides  [c.65]

The C. 100-C. 101 diol group, protected as an acetonide, was stable to the Wit-tig reaction used to form the cis double bond at C.98-C.99, and to all the conditions used in the buildup of segment C.99-C. 115 to fully protected palytoxin carboxylic acid (Figure 1,1).  [c.7]

Pyridinium tosylate, -PrOH, 80-85% yield. An acetonide was not affected by these conditions.  [c.38]

CYCLIC ACETALS AND KETALS 123 7. Acetonide (Isopropylidene Ketal) (Chart 3)  [c.123]

Acetonide formation is the most commonly used protection for 1,2- and 1,3-diols. The acetonide has been used extensively in carbohydrate chemistiy to mask selectively the hydroxyls of the many different sugars. In preparing acetonides of triols, the 1,2-derivative is generally favored over the 1,3-derivative, but the extent to which the 1,2-acetonide is favored is dependent on stmcture. Note that the 1,2-selectivity for the ketal from 3-pentanone is better than that from acetone.  [c.123]

In cases where two 1,2-acetonides are possible, the thermodynamically favored one prevails. Secondaiy alcohols have a greater tendency to form cyclic acetals than do primaiy alcohols,but an acetonide from a primaiy alcohol is preferred over an acetonide from two trans, secondaiy alcohols.  [c.123]

In the case below, isomerized to ii, producing a trans derivative, but acetonide iii fails to isomerize to the internal derivative because the less favorable cis product would be formed.  [c.124]

Cyclop entylidene are slightly easier to cleave than acetonides  [c.65]

Nonanedione, another 1,3-difunctional target molecule, may be obtained from the reaction of hexanoyl chloride with acetonide anion (disconnection 1). The 2,4-dioxo substitution pattern, however, is already present in inexpensive, symmetrical acetylacetone (2,4-pentanedione). Disconnection 2 would therefore offer a tempting alternative. A problem arises because of the acidity of protons at C-3 of acetylacetone. This, however, would probably not be a serious obstacle if one produces the dianion with strong base, since the strongly basic terminal carbanion would be a much more reactive nucleophile than the central one (K.G. Hampton, 1973 see p. 9f.).  [c.204]

In all cases examined the ( )-isomers of the allylic alcohols reacted satisfactorily in the asymmetric epoxidation step, whereas the epoxidations of the (Z)-isomers were intolerably slow or nonstereoselective. The eryfhro-isomers obtained from the ( )-allylic alcohols may, however, be epimerized in 95% yield to the more stable tlireo-isomers by treatment of the acetonides with potassium carbonate (6a). The competitive -elimination is suppressed by the acetonide protecting group because it maintains orthogonality between the enolate 7i-system and the 8-alkoxy group (cf the Baldwin rules, p. 316).  [c.265]

Unique chemistry is associated with the cyclopentenone all five carbon atoms can be functionalized, and the endo-methyl groups of the acetonide assure clean stereoselective addition of the alkenylcopper reagent from the convex side. The use of the acetonide group to control enolate regioselectivity and to mask alcohols should be generally applicable.  [c.277]

The 9 — 15 fragment was prepared by a similar route. Once again Sharpless kinetic resolution method was applied, but in the opposite sense, i.e., at 29% conversion a mixture of the racemic olefin educt with the virtually pure epoxide stereoisomer was obtained. On acid-catalysed epoxide opening and lactonization the stereocentre C-12 was inverted, and the pure dihydroxy lactone was isolated. This was methylated, protected as the acetonide, reduced to the lactol, protected by Wittig olefination and silylation, and finally ozonolysed to give the desired aldehyde.  [c.322]

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  [c.100]

Several additional protective groups were used in the coupling of the eight different segments. A tetrahydropyranyl (THP) group was used to protect the hydroxyl group at C.8 in segment C.8-C.22 and a r-butyldiphenylsilyl (TBDPS) group for the hydroxyl group at C.37 in segment C.23-C.37. The TBDPS group at C.37 was later removed by Bu4N F /THF in the presence of nine p-methox-yphenylmethyl (MPM) groups. After the coupling of segment C.8-C.37 with segment C.38-C.51, the C.8 THP ether was hydrolyzed with pyridinium p-toluene-sulfonate (PPTS) in methanol-ether, 42°, in the presence of the bicyclic ketal at C.28-C.33 and the cyclic ketal at C.43-C.47. (As noted above, the resistance of this ketal to these acidic conditions was due to the electron-withdrawing effect of the benzoate at C.46.) A cyclic acetonide (a 1,3-dioxane) at C.49-C.51 was also removed by this step and had to be reformed (acetone/PPTS) prior to the coupling of segment C.8-C.51 with segment C.1-C.7. After coupling of these segments to form segment C.l-C.51, the new hydroxyl group at C.8 was protected as an acetate, and the acetonide at C.49-C.51 was again removed without alteration of the  [c.6]

The synthesis of segment C.77-C.115 from segments C,77-C.84 and C.85-C.115 involved the liberation of an aldehyde at C.85 from its protected form as a dithioacetal, RCH(SEt)2, by mild oxidative deblocking (l2/NaHC03, acetone, water) and the use of the p-methoxyphenyldiphenylmethyl (MMTr) group to protect the hydroxyl group at C.77. The C.77 MMTr ether was subsequently converted to a primary alcohol (PPTS/MeOH-CH2Cl2, rt) without affecting the 19 t-butyldimethylsilyl (TBS) ethers or the cydic acetonide at C.lOO-C.lOl.  [c.7]

Me3SiBr, CH2CI2, 0°, 8-9 h, 80-97% yield.This reagent also cleaves the acetonide, THP, trityl, and r-BuMe2Si groups. Esters, methyl and benzyl ethers, r-butyldiphenylsilyl ethers, and amides are reported to be stable.  [c.19]

NCSBu2Sn)20 1%, THF, H20. Acetonides and TMS ethers are also cleaved under these conditions, but TBDMS, MTM, and MOM groups are stable. This catalyst has also been used to effect transesterifications.  [c.32]

Dichlorodicyanoquinone (DDQ), CH2CI2, H2O, 40 min, it, 84-93% yield.This method does not cleave simple benzyl ethers. This method was found effective in the presence of a boronate. The following groups are stable to these conditions ketones, epoxides, alkenes, acetonides, to-sylates, MOM ethers, THP ethers, acetates, benzyloxymethyl (BOM) ethers, and TBDMS ethers.  [c.54]

TsOH, DMF, Mc2C(OMe)2, 24 h. This method has become one of the most popular methods for the preparation of acetonides. It generally gives high yields and is compatible with acid-sensitive protective groups such as the TBDMS group.  [c.124]

See pages that mention the term Acetonides : [c.64]    [c.64]    [c.277]    [c.321]    [c.323]    [c.426]    [c.94]    [c.94]    [c.94]    [c.97]    [c.101]    [c.104]    [c.104]    [c.27]    [c.244]    [c.494]    [c.439]    [c.446]    [c.5]    [c.7]    [c.7]    [c.14]    [c.18]    [c.21]    [c.44]    [c.116]    [c.116]    [c.121]   
Protective groups in organic synthesis (1991) -- [ c.0 ]

Protective groups in organic synthesis (1999) -- [ c.0 ]