Other Ring-Forming Reactions

The oxidation of thioacyl amidines and related compounds is a versatile ring-forming reaction which can furnish a range of 3- and 5-substituted 1,2,4-thiadiazoles (see Section 5.08.9.4). Other ring-forming reactions that can give specific types of derivatives are discussed in the following sections. 

Adding substituents to other ring-forming reactions makes them go faster too in the next two examples the products are oxetanes and pyrrolidines. 

Reaction efficiency can also be enhanced by the use of tandem reactions, one-pot combinations of the Dotz reaction with other ring-forming reactions, including Diels-Alder, and nucleophilic aromatic addition reactions [33]. For example, alkynyl groups attached to Fischer... 

This chapter follows the organization used in the past. A summary of the electronic properties leads into reports of electrocyclic chemistry. Recent reports of studies of HDS processes and catalysts are then summarized. Thiophene ring substitution reactions, ring-forming reactions, the formation of ring-annelated derivatives, and the use of thiophene molecules as intermediates are then reported. Applications of thiophene and its derivatives in polymers and in other small molecules of interest are highlighted. Finally, the few examples of selenophenes and tellurophenes reported in the past year are noted. 

Potassium triethylborohydride, 260 Sodium borohydride, 21 Ring-forming reactions 2,2 -Dihydroxy-1,1 -binaphthyl, 113 Other asymmetric reactions Camphor-10-sulfonic acid, 62 Di-jjL-chlorobis(l,5-cyclooctadiene)di-rhodium-2,3-0-Isopropylidene-2,3-dihydroxy-1,4-bis(diphenyl-phosphine)butane, 153... 

By ring forming reactions /rans-Bromotetracarbonyl(methyl-methylidyne)tungsten, 49 Calcium acetate, 60 Chromium carbene complexes, 82 Ethyl (Z)-3-bromoacrylate, 130 Methyllithium, 188 By ether cleavage Diphosphorus tetraiodide, 127 Other methods... 

Besides the ring-forming reactions already described, trimethylsilyl azide (14) gives an access to a great variety of products. As an important property the excellent stability of 14 must be noted228,229> which makes it more advantageous to employ TMSA (14) instead of other azides. 

There are nonetheless just a few isolated examples of three- or four-membered ring forming anionic cyclisations curiously it was in fact a three- and a four-membered ring forming reaction which provided the first two pieces of evidence that organolithiums do indeed cyclise onto unactivated double bonds. In 1960, Wittig107 showed that n-BuLi not only added to one of the double bonds of norbornadiene but that the major product of the reaction arose from organolithium cyclisation of the intermediate 201 onto the other double bond. This odd reaction is presumably made possible by the strain already inherent in the bicyclic system. After protonation, the major product was 202. 

There are a number of methods for the direct synthesis of chloro-and bromocyclophosphazenes and their alkyl and aryl derivatives. Other derivatives have invariably been prepared by replacement reactions of the halogenocyclophosphazene precursors. In this section, ring-forming reactions will be discussed and the following section will deal with the substitution reactions. 

The Robinson annulation is a ring-forming reaction that combines a Michael reaction with an intramolecular aldol reaction. Like the other reactions in Chapter 24, it involves enolates and it forms carbon-carbon bonds. The two starting materials for a Robinson annulation are an a,P-unsaturated carbonyl compound and an enolate. 

Valuable preparative applications are rare and especially in ring-forming reactions other stereochemical factors such as ring strain and conformational effects will determine the stereochemistry. However the use of metal compounds to influence regio- and stereo-selectivity continues to excite interest. For C—C bond formation the coupling of allylic acetates with organocuprates offers broad synthetic potential as it generally seems to proceed with double bond shift. ... 

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