Lead-sulfur rings

It has been shown that keratin [9008-18-8] and not ceUulose-type fibers, are dyed. It is speculated that a lead—sulfur—keratin complex is formed. The color penetrates the hair fiber to a limited extent, forming a ring around the outside edge and imparting a lifeless appearance. Once developed, the color cannot be removed. The shades are limited yeUows or light browns. Appealing mainly to men, the products are often called color restorers because of the... 

Transannular interactions lead to ring closures and reductions to adamantane compounds when dienes of the bicyclo[3.3.1]nonane family are treated with Brpnsted acids and triethylsilane. Compounds 48-51 form reaction mixtures containing various amounts of products 52-54 (R = OH, O2CCF3, Cl) under such conditions.243 The best yields of hydrocarbon 52 occur when the dienes are treated with a 25% excess of sulfuric acid and a 50% excess of triethylsilane in dichloromethane at 20°.243 The stereospecific nature of these transannular reductions is demonstrated by the observation that the enantiomeric purity of the chiral diene 55 is retained in the chiral hydrocarbon product 56 (Eq. 98).243 Dienes of... 

Reactions in which the ring is cleaved or completely fragmented are fairly common in these systems. Ring contraction to form five-membered rings is also common especially where sulfur is extruded, and these reactions are considered separately, as are reactions which involve desulfurization or reduction. Reactions which lead to ring opening but are initiated by nucleophilic attack at ring carbons have already been discussed in Section 2.28.3.3.3. 

As would be expected, nucleophilic attack on the derivatives (84) leads to ring-opening reactions with alcoholic sodium hydroxide to give the selenadiazole (85) and with amines to give compounds (86) (66BP1610). There appears to be no tendency to recyclize, as in the case of the corresponding oxygen and sulfur heterocycles (Scheme 4). 

Thiocarbonyl derivatives of 1,3-dioxolanes and 1,3-oxathiolanes are readily isomerized to the 2-carbonyl compounds as shown in Scheme 20. Alkylation of the sulfur atom with alkyl halides usually leads to ring-opened products (Scheme 21) (69JOC3011). Most of the other chemistry of the sulfur derivatives has focused on desulfurization and subsequent generation of alkenes. The reaction is shown in equation (20) and proceeds with cis elimination via carbene intermediate (see Section 4.30.2.2.5) and is usually carried out with a phosphine (73JA7161) or a zero-valent nickel complex (73TL2667). 

Intramolecular displacements of sulfur substitutents lead to ring formation, e.g., Scheme 73 <1999T4109>. Similar reactions occur with acyl radicals, for example, with the CONH2 radical from formamide. 

These results show that, in the hydrolysis of 15 with sulfurous acid, the 5-amino group of 18 is completely protonated in the strongly acidic solution accordingly, the acyclic compound 25 is obtained. However, in aqueous solution, or in the presence of amines, the 5-amino group undergoes a nucleophilic reaction which leads, by ring closure, to 26. 

Methyllithium attacked the sulfur atom of naphthothiete 205 to give 231 and oligomers 232 and 233 via 8-mercaptomethyl-l-lithionaphthalene. Treatment with lithium aluminum hydride followed by methylation gave 234. Treatment of thiete 215 with triphenylphosphine leads to ring-expanded products possibly via nucleophilic attack by phosphorus on sulfur. ... 

Organometallic chemistry has become an important player in the rings of sulfur. The existence of an isolobal relationship between S and Cp Ti leads to the prediction that it should be possible to substitute the latter for the former in sulfur rings. The formal replacement of one sulfur atom in by Cp Ti gives CpjTiSj [or replacing two sulfur atoms in by two CpjTi units gives l,5-(Cp2Ti)2S6]. In practice one takes these... 

Many reactions, which lead to ring cleavage, often followed by rearrangement and recyclization, are now explained by an initial nucleophilic attack at the sulfur atom rather than at C-3 or C-5. Thus, the reaction of carbanions with a variety of isothiozolium or 1,2-benzisothiazolium salts... 

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