A-Carbonyl substituents

In ketones having propyl or longer alkyl groups as a carbonyl substituent, intramolecular hydrogen abstraction can be followed by either cleavage of the bond between the a and P carbon atoms or by formation of a cyclobutanol ... 

There are at least two different conformers of 3-hydroxyfuran 18a,b (Scheme 8). The energy difference is calculated to be 0.4 kcal/mol. According to DFT calculations the keto tautomer 19 is 12 kcal/mol more stable than the 3-hydroxyfuran 18b (99UP1). If a carbonyl substituent is present... 

Recently a definitive study of several isoxazol-5-ones using infrared and ultraviolet spectroscopy (Table I) has shown that the balance between the various tautomers is a delicate one and that all three of the structural types can predominate depending upon the nature of the substituents and the conditions of the experiment. However, the hydroxy form is only found when it is stabilized by chelation (i.e., a carbonyl substituent in the 4-position). The other compounds exist in the CH form in nonpolar media increasing polarity of the solvent stabilizes increasing amounts of the more polar NH forms. 

Some enamine alkylation reactions are shown in Scheme 1.10. Entries 1 and 2 are typical alkylations using reactive halides. In Entries 3 and 4, the halides are secondary with a-carbonyl substituents. Entry 5 involves an unactivated primary bromide and the yield is modest. The reaction in Entry 6 involves introduction of two groups. This... 

Some examples of IMDA reactions are given in Scheme 6.5. In Entry 1 the dienophilic portion bears a carbonyl substituent and cycloaddition occurs easily. Two stereoisomeric products are formed, but both have the cis ring fusion, which is the stereochemistry expected for an endo TS, with the major diastereomer being formed from the TS with an equatorial isopropyl group. 

A destabilizing dipole-dipole interaction between the electron-deficient a-thiocarbonyl or a-carbonyl substituent and the double bond at R-[17],... 

In addition to the characteristic CH stretching VCD, a number of molecules that are strocturally similar to 3-methylcyclohexanone exhibit characteristic ROA features in the skeletal region below 700 cm. Figure 6 (11). In particular, a bisignate couplet near 500 cm is observed in six-membered ring compounds with a carbonyl substituent [(-F)-pulegone(73), (-F)-camphor (74), (-t-)-3-brom-ocamphor (74), (+)-nopinone (75)] and a broad low frequency couplet is observed in ketones with a 3-methyl substituent (75). The ROA of 3-methylcyclohexanone exhibits a third characteristic couplet near 400 cm. In all cases the sense of the observed couplets correlates with the absolute configuration of the most stable chair conformation. 

Indolo[3,2-3]carbazoles bearing a carbonyl substituent at the 6-position have been synthesized directly by an intramolecular cyclization approach from dichloroacetyl substituted precursors <2004EJO2593>. However, due to a facile decarbonylation under the reaction conditions it was found to be more expedient to employ esters, such as 165, produced by acylation with oxalyl chloride (Equation (79) Table 4). 

This method of synthesis is not general since a carbonyl substituent at the 3-position of indolizines is usually deactivated toward nucleophilic attack.17,18... 

Tris(trimethylsilyl)silyl radical is relatively stable and can therefore serve as a radical leaving group. This reaction has been extended to the radical-initiated allylation of organic halides202 203. Thus, thermolyses of bromides a to a carbonyl substituent 144 or of simple iodides with allyltris(silyl)silane in the presence of a radical initiator gives the corresponding allylation products (equation 112). 

The presence of a carbonyl substituent in the -position of the hetero-cycle is essential for the cleavage of five-membered rings. The compounds of this type which have been investigated most thoroughly are 3-acylindoles.287, 384-388 At 160-170° 3-acetylindole and hydrazine hydrate give 3-(o-aminophenyl)-5-methylpyrazole, the structure of which was proved by deamination. Other 3-acylindoles, their hydrazones, and azines, react analogously.385 For the mechanism of the reaction see Alberti.384,391 The reaction requires a fourfold excess of hydrazine hydrate,387 preferably a polar solvent,388 and about... 

Carbon atom 2 is ortho plus meta to the substituents, and carbon atom 3 is meta plus para. Use of Table 4.12 (Section 4.7.4) gives 8 129.6 for peak 2 and 8 133.2 for peak 3. Measurements of peaks in Figure 4.6 give 8 128.5 for peak 2 and 8 131.2 for peak 3. Reasonable, but perhaps unexpected to some who recall from Chapter 3 that a carbonyl substituent deshields the ortho position of protons more so than the para position. Chart D.l in Chapter 3 confirms this impression, at least for a single carbonyl substituent. And the result of comparing ortho plus with meta plus para gives the expected results 8 8.0 for peak 2, and 8 7.60 for peak 3. 

They are much more stable, because the electron-withdrawing carbonyl group stabilizes the diazo dipole, and are very useful sources of carbenes carrying a carbonyl substituent. There are two main ways of making diazocarbonyl compounds ... 

Similar phenol couplings have been attempted in the laboratory with compounds in the benzyl isoquinoline series but the nitrogen atom interferes if it is at all basic. When it has a carbonyl substituent the reactions do work reasonably well, but the yields are poor. Nature is still much better at this reaction than we are. 

As shown by thioacidolysis of coniferyl alcohol, compounds 22 and 23 are formed from coniferyl alcohol end groups. They are recovered in larger amounts, relative to compound pair 11, from kraft lignin than from than from milled wood lignin. Compound 24 originates from Q.C-, alkyl aryl ether structures with a-carbonyl substituents via elimination of the Cy hydroxymethyl group (Lapierre 1986, Lapierre et al. 1987). 

On electrophilic substitution of pyrrole, thiophene and furan derivatives carrying a carbonyl substituent at position 2, the orientation... 

BBr3 has been widely used to cleave ethers because the reaction proceeds completely under mild conditions. In a special case, BBra has been used to cleave acetals that cannot be deprotected by usual acidic conditions [14]. Because alkyl aryl ethers are cleaved at the alkyl-oxygen bond to give ArOH and alkyl bromides, BBr3 has been most generally used for the demethylation of methyl aryl ethers [13,15]. The presence of a carbonyl substituent facilitates selective deprotection of polymeth-oxyaryl compounds (Eq. 8) [16]. 

The full DBA structure is displayed in Figure 7. The electron transfer process corresponds nominally to the reduction of Co by an electron localized on the bpy ligand attached (by a carbonyl substituent) to the (pro)4 bridge this leads to the low-spin (doublet) Co final state. The G. HK and /. values were corrected for systematic limitations of the solvent model (as described in detail in 36]), and G and H include contributions from quantal tunneling. 

Irradiation of pyrazine derivatives with an a-carbonyl substituent under an atmosphere of nitrogen has given hydroxypyrazines. Thus 2,5-dimethoxycarbonyl-... 

The irradiation of pyrazine derivatives with an a-carbonyl substituent under an atmosphere of nitrogen gave hydroxypyrazines and 1,4-dihydropyrazines (solvent adducts). Thus irradiation of 2,5-dimethoxycarbonyl-3,6-dimethylpyrazine in diethyl ether with a 450-W high-pressure mercury lamp gave two significant photoproducts, 2-hydroxy-5-methoxycarbonyl-3,6-dimethylpyrazine and l(r-ethoxy-ethyl)-2,5-dimethoxycarbonyl-3,6-dimethyl-l,4-dihydropyrazine (1064). The mechanism of the reaction has been investigated and discussed (1064). 

In these compounds, the phenolic OH group is situated next to a position with a carbonyl substituent, 0=C—R. As in other conjugated / -hydroxycarbonyl compounds, these molecules are characterized by the formation of a stable, intramolecular hydrogen bond, OH - 0=C—R, closing a six-membered chelate ring (see Figure 2). 

---