Dimethyl oxidation scheme

A very useful procedure for introducing a cyano group into a pyridazine ring is the Reissert-type reaction of the A/-oxide with cyanide ion in the presence of an acyl halide or dimethyl sulfate. The cyano group is introduced into the a-position with respect to the A-oxide function of the starting compound. The yields are, however, generally poor. In this way, 6-cyanopyridazines (111) can be obtained from the corresponding pyridazine 1-oxides (Scheme 33). 

P-coupling occurs in the formation of azophosphonic esters [ArN2PO(OCH3)2] from diazonium salts and dimethyl phosphite [HPO(OCH3)2] (Suckfull and Hau-brich, 1958). P-coupled intermediates are formed in the reaction between diazonium salts and tertiary phosphines, studied by Horner and Stohr (1953), and by Horner and Hoffmann (1956). The P-azo compound is hydrolyzed to triphenylphosphine oxide, but if a second equivalent of the tertiary phosphine is available, phenyl-hydrazine is finally obtained along with the phosphine oxide (Scheme 6-26 Horner and Hoffmann, 1958). It is likely that an aryldiazene (ArN = NH) is an intermediate in the hydrolysis step of the P-azo compounds. 

A similar condensation of 177b with chloromethyl phosphonic dichloride gave 2-chloromethyl-l,3,2-oxazaphospholidine-2-oxide 182 which was converted into the cyano derivative 183 by reaction with potassium cyanide in anhydrous dimethyl-sulfoxide (Scheme 51) [82],... 

More recently, this method has been extended to preparation of a variety of disulfonium dications from both acyclic and cyclic bis-sulfides, including very labile dications not observed when other methods were used.78 Thus, simple acyclic S-S dications were prepared by an intermolecular reaction of a monosulfide, a monosulfoxide and triflic anhydride.79 In the first step, reaction of triflic anhydride with dimethylsulfoxide generates a highly electrophilic80 complex 50 (dimethyl sulfide ditriflate).81 The latter reacts with dimethyl sulfide to give labile tetramethyldisulfonium dication 51 identified by NMR spectroscopy.79 In a similar manner, bis-(tetramethylene)disulfonium dication 52 is obtained from tetrahydrothiophene and its S-oxide (Scheme 17). 

Catalytic properties of external chiral additives such as (2S,3/ )-4-dimethyl-amino-l,2-diphenyl-3- methyl-2-butoxide (A 16) (574, 575) and 2-magnesium-3-zinc salts of dialkyl (f ,f )-tartrate (A17) were employed in the highly stereoselective addition of organozinc reagents to derivatives of 3,4-dihydro-isoquinoline-A-oxide (Scheme 2.147) (576). 

Similar reactivity is known for divalent sulfur nucleophiles (75JA3850). For 3,3-dimethyl-1,2-dioxetane, initial attack at carbon by azide ion has been postulated to explain the formation of acetone and nitrogen and imine TV-oxide (Scheme 40) (71TL749,72JA1747). 

Our synthesis started with ethyl 5-methyl-4-isoxazole carboxylate (50), prepared from ethyl acetoacetate and DMF dimethyl acetal (Scheme 5.4).14 Ester 50 was reduced with LiAlH4 and the resulting alcohol was oxidized to afford aldehyde 51. Enone 52 was obtained from aldehyde 51 using conditions developed by McCurry and Singh.15 The next step was the aromatization of the cyclohexane ring of 52 to produce the aromatic "A" ring of the monomer. Treatment of enone 52 with iodine in the presence of sodium ethoxide produced phenol 53.16... 

The marked activation of the iV-oxide function on the chlorine atom of 2-chloropyrazine 4-oxide and 2-ehloro-3,6-dimethylpyrazine 4-oxide is also demonstrated by the milder conditions under which these compounds react with ammonia and amines compared with chloro-pyrazine and 2-chloro-3,6-dimethyl pyrazine, respectively. Although 2-chloropyrazine 4-oxide undergoes the expected displacement reaction with ammonia on heating at 115°-120° for 2.5 hours, reaction at 140° for 16 hours gives 2,3-diaminopyrazine, possibly as a result of an addition-elimination reaction on the initially formed 2-amino-pyrazine 4-oxide (Scheme 47).417... 

Dimethyl-3-hexenedioic acid (171) was also prepared by an intermolecular diacylation of the dibromide 170 with 2-lithio-l,3-dithiane (161), followed by deprotection and final oxidation (Scheme 48)214. 

Indolo-steroids have been obtained by -coupling of steroid dienamines with dia-zonium salts in dimethylformamide followed by Fischer-indole cyclization of the resulting hydrazone39 (Scheme 37). In methylene chloride the /Fcoupled product 75 was obtained, cyclization of which gave indazoles 74 and 762a. The oxidative role of dimethyl sulphoxide in the formation of 76 was attributed to nucleophilic attack by the solvent on 75 leading to intermediates 77 and 78, with elimination of dimethyl sulphide (Scheme 38). 

The diester 87 with the same tetracyclic skeleton as 83 had previously been prepared by Paquette et al. via a domino Diels-Alder reaction of 5,5 -bicyclo-pentadienyl 84 with dimethyl acetylenedicarboxylate (Scheme 20) [73]. The key precursor 84 was obtained by iodine-induced oxidative coupling of the copper cyclopentadienide derived from the sodium derivative. The diester 85 formed along with 86 was transformed into a bissilyl bisenol ether by reductive cleavage of the central bond in the succinate moiety with sodium in the presence of trimethylsilyl chloride. Subsequent hydrolysis of the bisenol ether - actually a bisketene acetal - gave the dienic tetraquinacenedicarboxylate 87. This compound served as the key intermediate in the first synthesis of dodecahedrane 88 [74]. 

A new metabolite, 4-aminopyridine-2,3-dicarboxylic acid, was recently isolated from C. acromelalga in a yield of 0.000056% based on the weight of frozen fruiting bodies (41)5). A lethal effect of this compound on mice has not been observed however, bioactivity is still expected because of the similarity of its structure to the physiologicaUy active pyridine-2,3-dicarboxylic acid. The structure of this compound was deduced from H-NMR, C-NMR, and mass spectra, and the position of the substituents was assumed from comparison with clitidine, which contains a 4-aminopyridine-3-carboxylic acid moiety. The structural proposal was confirmed by synthesis from 2,3-dimethyl-4-nitropyridine 1-oxide (Scheme 91) when this compound was subjected to successive reduction and oxidation it yielded a product identical to the one occurring naturally. 

The P-phenyl group of a phosphole can be directly displaced by reaction with alkyl lithium reagents in TMEDA. Both t-butyl <72T47i> and -butyl <720MR(4)171> have been placed on P by this method. With 3,4-dimethyl-1-phenylphosphole, the former reaction occurred in 70% yield, the latter in 31.5% (with some oxide) (Scheme 79). This method is of considerable value for the introduction of the t-butyl group on phosphorus, as this group cannot be used as the P-substituent in a phosphonous dihalide in the McCormack reaction because of steric restrictions. 

The original Pfittner-Moffatt procedure for alcohol oxidation by activated dimethyl sulfoxide utilized dicyclohexylcarbodiimide (DCC) and a source of protons such as polyphosphoric acid or pyridinium tri-fluoroacetate. The use of strong acids such as the common mineral acids must be avoided since, although acidic conditions are initially required, the reaction must readily become basic in the later stages of the process. Mechanistically it is reasonable to suggest that the activation follows the pattern whereby initial attack of the nucleophilic sulfinyl oxygen of dimethyl sulfoxide, with the protonated carbodiimide, forms a sulfonium isourea. This is followed by displacement of dicyclohexylurea by the alcohol to form an alkoxysulfonium salt. Base treatment of this salt forms an ylide, which collapses via the proven cyclic mechanism to the carbonyl compound and dimethyl sulfide (Scheme 4). 

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