Oxygen substituents, introduction

The electrochemical oxidation of cyclic and acyclic, V-monosubstitilted and ATY-disubstituted amides and carbamates in a nucleophilic solvent, known as the Ross-Eberson-Ny berg reaction, is a synthetically very useful, clean and efficient method for the introduction of a-oxygen substituents under mild reaction conditions6 1 0. 

Introduction of an oxygen substituent at C-6 of a Hantzsch-type dihydropyridine having a sulfinyl group at C-5 affects the reduction of ketones with respect to both reactivity and stereoselectivity <96CC2535>. 

In the spectra of isoflavones (64) and flavanones (25), in which rings A and B are not conjugated, band I frequently appears only as a shoulder on band II. Not surprisingly, introduction of oxygen substituents in ring B has little effect on the spectra, whereas substitution in ring A shifts band II to the red. 

The aldehydes 346 and 347 were prepared from the methoxyl derivatives by osmic acid sodium chlorate oxidation and deketalization followed by base-catalyzed condensation in an overall yield of 50%. Acetalization of 346 with /i-toluenesulfonic acid and ethylene glycol in refluxing benzene afforded the acetal 348. The latter has an active site at C-6 suitable for the introduction of oxygen substituents at this position. 

The Canadian group have also followed their synthesis of anhydrovinblastine (266) by studying the introduction of oxygen substituents to positions 15 and/or 20. Oxidation of anhydrovinblastine with t-butyl hydroperoxide afforded a moderate yield of leurosine following experience with other substrates in this reaction Kutney etal. formulate leurosine as the -epoxide, rather than the a-epoxide (258) favoured " by other workers. However, both formulations are at present tentative, and it will be intesting to note which one ultimately proves to be correct. 

Oxygenated Silanes. Controlled introduction of oxygen substituents onto silicon is an interesting special case, since several syntheses are possible. For mono-oxygenated compounds, one can simply react a chloro-(chloromethyl)silane with two equivalents of triazole and then displace the labile silicon-bound triazole with water or an alcohol. However, a better alternative in practice is to introduce first an alcohol and then triazole. In this case, the hydroxy compound can be prepared by hydrolysis of the silyl ether with aqueous acid. Scheme II outlines these options. 

For the removal of 7V-oxide oxygen with concomitant introduction of oxygen substituents at a different position, see Section 7.2.2.2.1.5.4. 

The conversion of phenylalanine, a C-6—C-3 precursor, to the C-6—C-1 unit of the Amaryllidaceae alkaloids requires the formal loss of two carbon atoms from the side chain of the amino acid as well as the introduction of at least two oxygenated substituents into the aromatic ring. The results shown in the latter part of Table III emphasize the specificity of the C-6—C-1 precursor. Benzaldehyde, -hydroxybenzal-dehyde, isovanillin, and protocatechuic acid are not incorporated to any appreciable extent into the alkaloids, while cinnamic, -hydroxy-cinnamic and caffeic acids, protocatechuic aldehyde, and 3-hydroxy-4-methoxy-i C-A-methyl-i4( ., enzylamine readily become part of the C-6—C-1 unit. 

Ether (Chapter 18 introduction) A compound that has two organic substituents bonded to the same oxygen atom, ROR . 

If the sulfur atom is oxidized to a sulfonyl moiety, then oxygen donation occurs to the phosphorus as observed in compounds 49a-49h (Fig. 10) [61, 64-66]. In most examples, the existence of an equilibrium between penta- and hexacoordinated phosphorus was demonstrated in solution. Introduction of electron withdrawing substituents on the ligands around the phosphorus increase its Lewis acidity and consequently enhance the proportion of P( VI) over P(V). 

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