Ketals ketones

However, this does not explain the structural requirements for the oxygen function vicinal to the nitrite ester a-ketones, ketals and hydroxyls are cleaved, but a-acetoxyls are not. 

The reaction is remarkably specific hydroxyls (even at C-21), saturated as well as conjugated ketones, ketals and even hindered isolated double bonds e.g., are left substantially unattacked. Another advantage of this... 

Deep fluorinalion of alkanes, ethers, acid fmlides, esters, alkyl chlorides, most ketones, ketals, orthoesters, and combinations of these functional groups produces principally the perfluonnated analogues (Table 2) Chlorine substituents (or chloro groups) usually survive fluorination... 

Nal, CeCl3 7H20, CH3CN, rt, 0.5-21 h, 84-96% yield. Chemoselective cleavage of ketone derivatives is observed in the presence of aldehyde derivatives, and enone ketals are cleaved in the presence of simple ketone ketals. 

The following reaction provides an example of selective ketal formation. The product of the reaction is a single ketone-ketal with the formula CnHigOs. 

Since all of the steps in ketal formation are reversible, we expect the more stable ketal to form selectively. Obtain the energies of the alternative ketone-ketal s (ketone-ketal A and ketone-ketal B). Which one is more stable What factors do you think contribute to the stability of this ketal ... 

Figure 1. CD investigation of the ketone-ketal equilibrium for cholestan-3-one in acidified methanol. The Cotton effect due to free ketone increases with successive additions of water to the ketal. (From Ref. 18).

This figure also shows the potential for using CD as a tool for kinetic analysis. The Cotton effect at 289 nm is due to the concentration of free ketone present. The dimethyl ketal absorbs at much shorter wavelength. Repeated additions of small amounts of water shifts the ketone-ketal equilibrium toward the free ketone, which increases the value of the experimental molecular ellipticity. The formation of ketal also depends upon the structure of the alcohol, as well as on stereochemical factors. Thus cholestan-3-one gives 96% of dimethyl ketal, 84% of diethyl ketal and only 25% of the diisopropyl ketal. The proportion of ketal formed from the 3-keto-5 P- steroids is higher than in the case of their 5 a isomer. 

Reactions of cyclopropenone ketal with terminal alkenes afford 1,4-divinyl ketone ketals in good yields (Eq. 51) [88]. 

The Pauson Khand reaction is compatible with a wide variety of functionalities, such as ethers, alcohols, tertiary amines, thioethers, ketones, ketals, esters, tertiary amides, carbamates, and benzene, furan, and thiophen rings. Disubstituted alkynes, alkenes with bulky allylic substituents, and trisubstituted alkenes frequently afford reduced yields of products. Because of the reduced ability of sterically hindered alkenes to coordinate and undergo insertion, insertion of one or more molecules of alkyne occurs instead. 

Group 10 is a fairly small group covering secondary aliphatic saturated or unsaturated alcohols, ketones, ketals and esters. Important members are the typical dairy compounds diacetyl (Flavis 07.052, FEMA 2370) and acetoin (Flavis 07.051, FEMA 2370). 

Such successive half-reactions are not properly self-consistent, and a tally of such combinations could be made and used mechanically to discard certain sequences from a derived set if a criterion of true self-consistency is desired 5>. On the other hand many of the refunctionalization steps in the actual s mtheses surveyed 24,31) are of exactly this kind, i.e., derivatization of alcohols to mesylates, acetates, etc., ketone-ketal protective interconversions, ester-acid interconversions, etc. Thus, although the self-consistency criterion was applied for reasons of efficiency and does strongly reduce the tree, the fact that the original definition of Z is broad allows a generous amount of implicit refunctionalization in practice. Furthermore, it should be possible to tabulate which of the pairs of successive half-reactions require it and which do not so that its exclusion can be a matter of choice. 

Taking the lessons learned thus far, a revised sequence was developed for the construction of the key 6-azaindole 69. We proposed that starting from A -tosyl-protected Pictet-Spengler cyclization product 71 would be beneficial. It was envisioned that formation of the dimethyl ketal (71-72) would be followed by the loss of methanol to arrive at methyl enol ether 73. Since the previous aromatization attempts of the heterocycle had not occurred, the functional groups at C4 were expected to behave as standard ketone, ketal, and enol ether. Therefore, this pathway was expected to be much more controlled in comparison to the earlier attempts described in Scheme 14. 

Secondary alicyclic saturated and unsaturated alcohol/ketone/ ketal/ester (e.g., menthol, menthone, isomenthone, menthyl acetate)... 

Tian et al. [188a] described the synthesis of very useful protected homoallylic amines by a four-component reaction catalyzed by the inexpensive and environmentally friendly FeSO HjO. The reaction between carbonyl compounds (aldehydes, ketones, ketals, acetals) 200, CbzCl, HMDS, and allyltrimethylsilane afforded the corresponding homoallylic amines 201 in 43-87% yields (Scheme 3.73, Eq. 1) under mild conditions. This methodology was also applied to the synthesis of A-protected primary and branched amines 202 by replacing the allyltrimethylsilane for EtjSiH [188b], which gave rise to the desired products in 40-95% yields (Scheme 3.73, Eq. 2). 

Addition of alcohols to unsaturated carbon molecules is one of the most straightforward and environmentally friendly procedures to form C-O bonds. Several species are known to catalyse such transformations. Among them, gold complexes have proven to be extremely active to catalyse the addition of oxygen nucleophiles to all nes, in order to produce the corresponding ketone, ketal or vinylether species. Moreover, such reactions are among the most efficient gold-catalysed transformations reported to date. 

Extending this technique to the reduction of the dione, L, demonstrated an intramolecular selectivity attributable to conversion of the more reactive C-3 carbonyl to a monoketal intermediate (Gemal and Luche, 1979). This allows the preferential reduction at C-17, and also demonstrates the sharp contrast between aldehyde and ketone ketalization using lanthanide chlorides as catalysts. 

Zn-powder in glacial acetic acid, and stirred 3 hrs. ejco-methylene ketone ketal deriv. 

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