SUBSTITUTION REACTION WITH ETALS

As simple glycals are substrates for C-glycosidations, so are 1-substituted glycals. An example of C-glycosidations on substituted glycals was reported by Nicolau, etal.,7 and is illustrated in Scheme 2.3.40. In this study, reactions with allyltrimethylsilane and methyl trimethylsilylacetylene were explored. Utilizing titanium tetrachloride as the catalyst, yields in excess of 75% were obtained and the products exhibited the stereochemistry shown. 

Stumpf etal reacted MA with cis, trans, // (2 5-l,5,9-cyclododecatriene 36 in the presence of phenothiazine or thiamine as an inhibitor. The product cis, trans, /raAi5-2,5,9-cyclododecatrienylsuccinic anhydride 37 was obtained in yields as high as 76%. MA could be substituted in this reaction with maleic... 

For less activated substrates, specific tests are desirable. These are available in some cases. Thus Amstutz etal. showed that the reaction of 3-, 6-, and 8-bromoquinolines with piperidine at about 200° produced normal substitution products. 

A general route to [l,2,4]triazolo[3,2-A][l,3]thiazines was discovered by Britsun etal. <2001ZOR1102, 2004KGS1256, 2004ZOR260>. In these publications, reaction of triazole thiones 332 with various substituted acrylic chlorides was reported to yield a series of ring-closed products 333 with the substituent of the reagent in the six-membered ring at the position adjacent to the sulfur atom. 

Studies by Heinze etal. on donor-substituted thiophenes or pyrroles [33] such as methylthio (= methylsulfonyl) or methoxy-substituted derivatives provide further clear evidence for this reaction pathway. They found, for instance, that 3-methylthiothiophene or 3-methoxythio-phene (2) undergo a fast coupling reaction. However, deposition processes or insoluble film formation could not be detected in usual experiments with these compounds, even at high concentrations. Similarly, the corresponding 3,3 -disubstituted bithiophenes (2a) do not polymerize, but the anodic oxidation of 4,4 -disubstituted bithiophenes (2c) produces excellent yields of conducting polymers. 

Halogenation of pyrimidine bases may be done with bromine or iodine. Bromina-tion occurs at the C-5 of cytosine, yielding a reactive derivative that can be used to couple diamine spacer molecules by nucleophilic substitution (Fig. 41) (Traincard et al., 1983 Sakamoto etal., 1987 Keller etal., 1988). Other pyrimidine derivatives also are reactive to bromine compounds at the C-5 position. Either an aqueous solution of bromine or the compound N- bromosuccinimide can be used for this reaction. The brominated derivatives can be used to couple amine-containing compounds to the pyrimidine ring structure (chapter 17, Section 2.1). 

Calix[ ]arenes are a family of macrocycles prepared by condensation reactions between n /v/ra-substituted phenols and n formaldehyde molecules under either base or acid catalysis. Different sizes of the macrocycles can be obtained (n = 4-20) (Stewart and Gutsche, 1999) depending on the exact experimental conditions, which were mastered in the 1960 s (Gutsche, 1998), but the most common receptors are those with n =4,6,8 (macrocycles with an odd number of phenol units are more difficult to synthesize). We use here the simplified nomenclature in which the number of phenolic units is indicated between square brackets and para substituents are listed first.4 Calixarenes, which can be easily derivatized both on the para positions of the phenolic units and on the hydroxyl groups, have been primarily developed for catalytic processes and as biomimics, but it was soon realized that they can also easily encapsulate metal ions and the first complexes with d-transition metal ions were isolated in the mid-1980 s (Olmstead et al., 1985). Jack Harrowfield characterized the first lanthanide complex with a calixarene in 1987, a bimetallic europium complex with p-terf-butylcalix[8]arene (Furphy etal., 1987). 

Bachrach etal. investigated the mechanism for the reaction of thiolate (HS ) with 1,2-dithietane 17 (Equation 2) by applying B3LYP/ang-ee-pVDZ and MP2/6-31+G calculations. Both these methods concur on the SN2-type mechanism for a nucleophilic substitution at the sulfur atom in 1,2-dithietane <2002JOC8983>. 

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