Ketals cyclic

H2Sil2, CDCI3, —42°, 1-10 min, 100% yield. Aromatic ketals are cleaved faster than the corresponding aliphatic derivatives, and cyclic ketals are cleaved more slowly than the acyclic analogues, such as dimethyl ketals. Substituted ketals such as those derived from butane-2,3-diol, which react only slowly with Mc3SiI, can also be cleaved with H2Sil2. If the reaction is run at 22°, ketals and acetals are reduced to iodides in excellent yield. 

Ketones react with 1,2-ethanediol under acidic conditions to give cyclic ketals. 

In the mass spectrum (Figure 6) of 3-deoxy-l,2 5,6-di-0-isopropyli-dene-D-xt/Zo-hexofuranose (9) the fragmentations described above are found at m/e 229, 171, 143, 111, and 101. The fragments at m/e 143 and 101 arise by cleavage of C-4-C-5 with charge retention on C-4 and C-5, respectively (see Equations 17 and 18). Scheme 2 summarizes the losses of a methyl group, acetone from the second cyclic ketal function, and... 

Retrosynthetic disassembly of the tetrahydropyran ring in 14, a mixed cyclic ketal, provides ketone 15 as a plausible precursor. In the synthetic direction, the solvolytic cleavage of the ester functions in 15 would likely be attended by the formation of a cyclic hemi-ketal. On treatment with acidic methanol, this substance could then be converted to mixed ketal 14. 

Kelsey et al. reported that die cyclic ketal of 4,4,-dihydroxybenzophenone (DHBP) can polymerize with 4,4,-difluorobenzophenone in DMAc at 150°C (Scheme 6.16).85 The polymerization afforded soluble amorphous polyketal that was quantitatively converted to PEK. Because of relatively lower reaction temperature, the PEK had minimal defect structures and thus possesses higher crystallinity and higher Tg, and has better physical properties than its counterpart made under higher temperatures. 

C in CH2CI2, in the formation of enol ethers 404 in high yields [26] (Scheme 5.4). Likewise, silylated alcohols 13 and free 1,2- and 1,3-glycols react with ketones in the presence of TMSOTf 20 to cyclic ketals [27]. 

An alternate approach for biomolecule recover is to employ degradable surfactants [157]. A series of nonionic surfactants has been synthesized that contain the acidic pH-degradable cyclic ketal linkage [153]. These surfactants readily form w/o-MEs when at neutral pH or higher but, the surfactants readily degrade at moderately low pH (ca. 5), releasing the encapsulated aqueous phase and its constituents. Work is ongoing to develop these surfactants in w/o-ME protein extraction processes [153]. 

Ene-nitrile oxidoisoquinolinium betaine 131 was readily prepared from vinyl triflate aldehyde 79 (Scheme 1.14). Palladium-catalyzed cyanation of vinyl triflate 79 with Zn(CN)2 in DMF at 60 °C produced ene-nitrile aldehyde 129 in 85 % yield [54]. Using the previously developed Staudinger-aza-Wittig reduction sequence, aldehyde 129 was coupled with cyclic ketal azide 121 to afford a 79 % yield of amine 130. The cyclic ketal amine 130 was then treated with 9 1 mixture of CH2CI2/TFA to provide ene-nitrile oxidoisoquinolinium betaine 131 in 93 % yield. 

An expeditious route to the cyclic sulfamide HIV-1 protease inhibitors of type 145 and 146 (tetrahydro-l,2,7-thiadiazepine 1,1-dioxide derivatives) from 141 and 142 hinges on palladium-catalysed amidation reactions. These reactions of 144 and 143 were microwave promoted and provided, after removal of the cyclic ketal protecting group, moderate to good yields of (145, 57%) and (146, 66%) for example with R = NHCOCH2-2-naphthyl <06T4671>. 

Cyclic ketals (potential cosmetic ingredients) were obtained in excellent yields from a cineole ketone under the action of microwaves in solvent-free conditions or in toluene. The results reported compared very favorably with those obtained by use of conventional heating (Eq. (23) and Tab. 3.9) [76]. 

In an analogous approach explored by Dixneuf et al., a conjugated enynyl carbonate is converted into an oxolenone or a bicyclic lactone in significant yields via double carbonylation in the presence of methanol (Scheme 22) [128]. When a neighboring carbonyl group is present in the substrate, it can also participate in palladium-catalyzed cyclization-carbonylation. Indeed, 4-yn-l-ones lead to cyclic ketals that can be easily converted into 2-cyclopentenone carboxylates in an acidic medium (Scheme 22) [129]. 

When the reaction was applied to a chiral cyclic ketal instead, very low selectiv-ities were obtained. Introduction of chelating substituents into the ketal made improvement possible, though (Scheme 8.14) [23, 26]. 

In principle, a number of different types of acetal or ketal might be produced. In this section, we want to exemplify a small number of useful reactions in which two of the hydroxyl groups on the sugar are bound up by forming a cyclic acetal or ketal with a snitable aldehyde or ketone reagent. Aldehydes or ketones react with 1,2- or 1,3-diols under acidic conditions to form cyclic acetals or ketals. If the diol is itself cyclic, then the two hydroxyl groups need to be cA-oriented to allow the thermodynamically favourable fused-ring system to form (see Section 3.5.2). Thus, dx-cyclohexan-1,2-diol reacts with acetone to form a cyclic ketal, a 1,2-O-isopropylidene derivative usually termed, for convenience, an acetonide. 

Determined by NMR of diastereomeric cyclic ketals with (2 ,3R)-butanediol. 

The reagent of choice for the reduction of ketals to ethers is alone prepared in situ from lithium aluminum hydride and aluminum chloride in ether. At room temperature ethers are obtained in 61-92% yields [792, 934]. Cyclic ketals prepared from ketones and 1,2- or 1,3-diols afford on hydrogenolysis by alanes alkyl P- or y-hydroxyalkyl ethers in 83-92% yields [792]. 

Included are cyclic ketals and thioketals with five- or sbt-membered rings ... 

Non-Kolbe reactions are often favoured by skeletal reaiTangements which generate a more stable carbonium ion. Reaction of the cyclic ketal 22 is driven by formation of a carbonium ion stabilised by the oxygen substituent [114]. Reactions of nor-bomanecarboxylic acids are driven by the norbomane carbonium ion rearrangement [115, 116], Oxidation of adamant-1-ylacetic acid in methanol affords 1-methoxyhomoadamantane via a skeletal rearrangement [117],... 

H ->I Make the cyclic ketal with HSCH2CH2SH and hydrogenate over Raney nickel (frame 146). ... 

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