Cyclization reactions sulfur compounds

Sulfur-containing acyclic and cyclic compounds have been prepared from allenyl sulfides in numerous transformations such as substitutions, additions, cydoaddi-tions and other cyclization reactions. Like the other donor-substituted allenes, allenyl sulfides are suitable substrates for regioselective lithiation and substitutions as exemplified in Scheme 8.86 [168, 169,175]. 

Reviews which cover heterocyclization of unsaturated sulfur compounds have been published recently.16 Cyclizations of sulfur systems can also be effected by radical chain mechanisms or by reactions in which the sulfur acts as an electrophile. Examples of nucleophilic sulfur functional groups used in heterocyclization reactions are shown in Figure 3. ... 

From these comments, it is evident that the crab-like cyclization reaction is a convenient method for preparation of a great variety of aza-crowns in good yields. Some of the polyaza products included /V-pivot lariat polyaza-crowns, polyaza-crowns and cyclams with one or two unsubstituted ring nitrogen atoms, peraza-cage compounds, and polyaza-crowns containing sulfur atoms. This new method could become the method of choice for preparation of the functionalized polyaza- and peraza-crowns. 

In 1982, Ueno reported the first uses of allylic sulfur compounds for carrying out the allylation of a carbon-centered radical [48], The reactions detailed are cycliza-tion reactions in which an aryl radical adds to an allylic sulfide to form either an indole or benzofuran product (Scheme 17). Tin, in the form of tributyltin hydride, was used to carry the radical chain. With low concentrations of tin hydride, yields of the desired products as high as 96% were observed. A more recent set of examples of cyclizations onto allylic sulfides was reported by Ward in 1991 [49]. 

Properties and Reactions of Ylidenemalononitriles, F. Freeman (1980). Dicyanomethylene compounds, their molecular structure and spectral properties, toxicity and analyses are reviewed. Various reactions like hydrolysis, oxidation and reduction, dimerizations, cyclizations, photochemistry and thermolysis are discussed. The review, which contains 380 references, also deals with unsaturated compounds, oxygen, nitrogen, phosphorous and sulfur compounds. 

Corresponding reactions involving cyclization through sulfur (Scheme 5 ) have also been described. Indole derivatives have been obtained by cyclization through nitrogen, but in these cases the precursors are often made from appropriate carbonyl compounds (Eq. 18), where carbonyl has served the purpose of facilitating carbanion formation for the first displacement of fluoride in Scheme 4. The nitrile function is also effective for this purpose (Eq. 19). 

Their substrates are 2-phenylpyridines, imines, benzo[/i]quinones, phosphines, oxygen compounds, sulfur compounds, etc., as described in the previous sections or in Fig. 5.11, Tables 5.5 and 5.6. These reactions are arylations, alkenylations, alkylations, acylations, cyclizations, hydrogenations, oxidations, hydrosilylations, dehydrogenations, etc. 

Wynberg and co-workers have prepared helicenes composed of fused thiophene and benzene units from the photocyclization of alkene precursors [134], It was shown that the effect of substituting thiophenes for benzene rings in these structures led to a blue shift of the absorption maxima. Compound 61 can be converted to the circulene structure 62 in two steps and the latter compound represents another interesting class of fused thiophene stmctures [135]. Trithiophenes with a benzene core (63) can be prepared from diacetylene-fimctionalized bithiophenes via a five-coordinate rhodium(I) intermediate by the addition of elemental sulfur [136]. A related structure (64) was reported by Pei et al. [137], along with the elaborate helicene 65. The compounds were constracted via oxidative (FeCb) cyclization reactions of 1,2-dithienyl benzene derivatives. 

The intermediacy of an aci-nitro compound has been proposed for the sulfuric acid cyclization of o-nitrophenylacetic acid to yield a mixture of 2,1-benzisoxazole and 2,1-benzisoxazole-3-carboxylic acid. The acid does not decarboxylate under the reaction conditions. The proposed aci-nitro intermediate cyclized to an A/ -hydroxy compound which decomposed to the products (Scheme 179) (70JCS(C)2660). 

Benzisothiazoles are best prepared by oxidative cyclization of o-aminothiobenz-amides (see Section 4.17.9.1.1), reaction of o-toluidines with thionyl chloride (see Section 4.17.9.2.1) or by sulfuration of 2,1-benzisoxazoles (see Section 4.17.10.2). 1,2-Benzisothiazoles can also be prepared from o-disubstituted benzene compounds, cyclodehydration of o-mercaptobenzaldoximes or oxidative cyclization of p-mercaptobenzylamines (see Section 4.17.9.1.1) being the most convenient. Both series of benzo compounds are readily substituted at the 5- and 7-positions by electrophilic reagents. 

Cyclization of the diazo compound (108) with a copper catalyst affords the clavulanic acid derivatives (110) and (111), possibly via rearrangement of the sulfur ylide (109) (80H(14)1999). Similar reactions have been reported in the recent literature (80H(14)1967, 81H(16)1305, 80TL31). 

In addition there are certain other methods for the preparation such compounds. Upon heating of the thionocarbonate 2 with a trivalent phosphorus compound e.g. trimethyl phosphite, a -elimination reaction takes place to yield the olefin 3. A nucleophilic addition of the phosphorus to sulfur leads to the zwitterionic species 6, which is likely to react to the phosphorus ylide 7 via cyclization and subsequent desulfurization. An alternative pathway for the formation of 7 via a 2-carbena-l,3-dioxolane 8 has been formulated. From the ylide 7 the olefin 3 is formed stereospecifically by a concerted 1,3-dipolar cycloreversion (see 1,3-dipolar cycloaddition), together with the unstable phosphorus compound 9, which decomposes into carbon dioxide and R3P. The latter is finally obtained as R3PS ... 

Ullmann condensation of the sodium salt of p-chlorothiophe-nol (31) with 2-iodobenzoic (32) acid gives 33. Cyclization by means of sulfuric acid affords the thioxanthone, 34. Reaction with the Grignard reagent from 3-dimethylaminopropyl chloride affords the tertiary carbinol (35). Dehydration by means of acetic anhydride affords chlorprothixene as a mixture of geometric isomers, 36. (Subsequent work showed the Z isomer-chlorine and amine on the same side—to be the more potent compound.) Chlorprothixene is said to cause less sedation than the phenothiazines. ... 

This class of aziridine-forming reaction includes the first reaction reported to afford aziridines. In 1888 Gabriel reported that aziridines could be prepared in a two-step process, by chlorination of ethanolamines with thionyl chloride, followed by alkali-induced cyclization [75]. Wenker subsequently reported that heating of 600 g of ethanolamine with more than 1 kg of 96 % sulfuric acid at high temperature produced P-aminoethyl sulphuric acid 282 g of it was distilled from aqueous base to give 23 g of aziridine itself, the first preparation of the parent compound in a pure condition [76]. Though there is no evidence to substantiate the hypothesis, the intermediate in these reactions is perhaps a cyclic sulfamidate (Scheme 4.51). 

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