Carbocations 3-lactam

The /rans-3 -benzylth io-3-ch Ioro- ( -1 actams 192, the appropriate (3-lactam car-bocation equivalents, were prepared by stereospecific chlorination of their corresponding frans-3-benzylthio-(3-lactams 191 using IV-chlorosuccinimide and catalytical amount of AIBN [125]. These (3-lactam carbocation equivalents 192 on treatment with propargyl alcohol or allyl alcohol in the presence of ZnCVSiCL were further transformed to suitable substrates, such as, m-3-benzylthio-3-(prop-2-ynyloxy/enyloxy)-(3-lactams 193 and 195 respectively [126]. 

The seleno-(3-lactam 207, required for this study, was prepared from 2-benzyl-selenoethanoic acid [130] 205 and appropriate imine 206 in the presence of phosphorus oxychloride as the condensing reagent and triethylamine as the base. This was further transformed to appropriate (3-lactam carbocation equivalent,... 

In 2000, a route to novel C-3 substituted 2-azetidinones (I, Fig. 3) has been reported involving a reaction of a (3-lactam carbocation equivalent with active aromatic nucleophiles in the presence of a Lewis acid [245]. 

Pyridone is O-alkylated more readily than normal amides, because the resulting products are aromatic. With soft electrophiles, however, clean N-alkylations can be performed (Scheme 1.7). The Mitsunobu reaction, on the other hand, leads either to mixtures of N- and O-alkylated products or to O-alkylation exclusively, probably because of the hard, carbocation-like character of the intermediate alkoxyphosphonium cations. Electrophilic rhodium carbene complexes also preferentially alkylate the oxygen atom of 2-pyridone or other lactams [20] (Scheme 1.7). 

There were very few reports of Schmidt reactions involving alkyl azides for almost 40 years after Boyer s papers appeared. Some papers described sequences that resembled the bona fide Schmidt reaction in that azides ultimately afforded lactams, but were thermally enacted and mechanistically distinct from the classical Schmidt reaction (see Section 7.5). hi the early 1990s, a series of disclosures finally established synthetically useful versions of the Schmidt reaction using various kinds of alkyl azides as the key substrates. The following discussion of this chemistry will concentrate first on those reactions in which the electrophilic partner is a carbonyl group or carbonyl equivalent. Later sections will describe processes in which an alkyl azide attacks a carbocation derived from an alkene, alcohol, or a related precursor. Finally, the ways in which these reactions have been used to construct natural products or other compounds of interest will conclude this chapter. 

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