Imine compounds four-membered rings

Dichloroneopentylsilene is formed in situ by reaction of trichlorovinylsilane with LirBu [1], The [2+2] cycloaddition to imines yields Si-dichloro functionalized 2-silaazetidines in a preparative scale [2], When aldimines are used as trapping agents for the silene, the resulting SiN-four membered ring compounds are isolated as syn/anti-isomers (syn/anti 2/1). 

Several of the reactions described in Section 6.16.2.4.6.1 are two-step reactions. After the initial [2+1] addition of the silylene to the multiple bond, a second insertion reaction of a silylene into a reactive Si-X bond of the cyclopropane derivative takes place yielding four-membered ring compounds. Examples are the reactions with alkynes, nitriles, imines, and ketones. 

The thermal [2-1-2] cycloaddition of cumulenes with alkenes, imines or carbonyl compounds is one of the most useful methods of four-membered ring formation. The cycloaddition of ketenes with alkenes to give cyclobutanones represents a reaction of general importance. According to Woodward and Hoffmann, these reactions proceed via a [ttIs+ttIi,] pathway [24]. Dihaloketenes are more reactive than simple ketenes and readily react with electron-rich olefins [25]. 

Dihydropyrrole is a pseudo-four-membered ring molecule. Its structural relationship to trimethylene imine is the same as that of 2,5-dihydrofuran to trimethylene oxide. Again, both the parent and N-d compounds were studied. Figure 4.22 shows the far infrared spectrum of the light compound reported by Carreira and Lord89 The low-frequency Raman spectrum was reported later by Carreira et al.9°). Transitions with both Av = 1 and Av = 2 were observed. Table 4.14 summarizes the... 

The thermal [2 + 2J cycloaddition of cumulenes with alkenes, imines, and carbonyl compounds is one of the most useful routes to four-membered ring compounds. Ketenes and keteniminium salts add to alkenes to give cyclobutanones (Houben-Weyl Vol. IV/4 pp 174 205) 41 al-lenes add to alkenes to give methylenecyclobutanes (Houben-Weyl Vol. IV/4 pp 151-173), ketenes add to carbonyl compounds to give //-lactones (Houben-Weyl Vol. VI/2 pp 520-527), ketenes add to imines to give /1-lactams, and isocyanates add to alkenes to give //-lactams (Houben-Weyl Vol. V/lb p 1098). 

Cycloaddition reactions enable easy access to four-membered rings such as yS-lactams, oxetenes, cyclobutanones, and heterocyclic compounds. Only two examples, reaction of ketenes or j5-formyl enamides with imines or alkynes, respectively, have been investigated under the action of microwaves [3j], however. These limitations could be because of the facile polymerization of ketenes. 

In [2+2] cycloaddition reactions of carbon cumulenes, often only one four-membered ring compound is obtained. This reaction is of considerable importance in the synthesis of 8-lactams from ketenes and C=N double bond containing substrates. The j8-lactam structure is present in a variety of antibiotics. Also, j8-thiolactams are obtained from thioketenes and imines. 

In four-membered heterocycles, the ring strain is less than in the corresponding three-membered compounds and is approximately equal to that found in cyclobutane. Nevertheless, ring-opening reactions forming acyclic products predominate. At the same time, analogy with the reactivity of the corresponding aliphatic compounds (ethers, thioethers, secondary and tertiary amines, imines) becomes more evident. 

Over the last 12 years direct formation of a four-membered /8-lactam ring via [2 + 2 jcyclo addition of ketenes to imines and of isocyanates to olefins dominated over other methods. Because of the well-defined transition states, both cycloadditions usually offer an excellent stereoselectivity. In the case of cycloaddition of ketenes or their equivalents to imines, the cycloadducts were used not only for the synthesis of /8-lactam antibiotics, but also as intermediates for the preparation of other biologically active compounds. 

Owing to the importance of p-lactam antibiotics, the [2+2]cycloadditions of ketenes to imines and isocyanates to olefins play a special role because both reactions lead to direct formation of a four-membered azetidinone-2 ring. The first one can be performed using a large variety of ketenes and ketene equivalents, whereas the second one leads to useful compounds only in the case of chlorosulfonyl isocyanate and vinyl acetates, vinyl silyl ethers, and dienes. One example of the addition of an isocyanate to a vinyl ether has been reported (Scheme 1). ... 

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