Azetidinones thermolysis

Pauson-Khand cyclization of vic-enyne derivatives of /3-lactams gave good yields of fused tricyclic compounds. The 1,4-disubstituted 2-azetidinone 391 and cobalt octacarbonyl gave the alkyne-cobalt carbonyl complex, which on thermolysis gave the tricycle 392 in 95% yield (Equation 54). When the complexes of 393 with cobalt octacarbonyl were treated with TMANO, a lower yield (65%) of 394 was obtained (Equation 55). A single diastereoisomer was formed in each case <1996TL6901>. 

A tandem one-pot elimination-intramolecular Diels-Alder reaction occurs when the mesylate of 4-homoallylic azetidinone having a orc-alkene or alkyne substituent is heated in a sealed tube in the presence of an equimolecular quantity of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The method has been used to produce derivatives of oxace-pham. In a similar way, the 3,4-disubstituted azetidinone mesylate 473 afforded an 88% yield of 474. The method can be further elaborated through the introduction of a novel [3,3]-sigmatropic rearrangement of a-allenic mesylates thus, 475 yielded 476 on thermolysis C1999TL1015, 2000JOC3310, 2005EJ098>. 

Some reactions of munchnones occur via acylamino ketenes, the covalent valence tautomers of the betaines. The ketenes are intermediates in the thermolysis (see Scheme 22) and in the formation of azetidinones from imines (equation 69) they are thought to be involved in the aminolysis of the mesoionic compounds, which results in amides of a-acylamino acids, and in the formation of the benzodioxin (247) by the combined action of acetic anhydride and tetrachloro-o-benzoquinone on Af-benzoylalanine (equation 70). 

There have also been a few examples of ring expansions involving azetines. One such instance, which follows a pericyclic mechanism, is the thermolysis of 4-cyano-l-t-octyl-3-t-octylamino-2-t-octyliminoazetine (160), producing the 4-amino-5-cyanoimidazole (Scheme 89). Such azetine species have been implicated as intermediates in the photolysis of enaminonitriles to imidazoles (Section 4.08.1.1.1 Scheme 17). In strongly basic medium the azetidinone (161), which possesses a lactam group, is ring expanded to the 4H-imidazolinone (163), probably via the anionic acylic species (162 Scheme 89) <80AHC(27)241>. 

Instead, cyclization occurred between a sulfenic acid at azetidinone C4 and an alkyne moiety at nitrogen [206]. The ( )-2-alkylidenepenams 374 were selectively obtained by thermolysis of appropriate 4-rert-butylthioazetidinone precursors, through the intermediacy of transient sulfenic acids 373. The high stereoselectivity of the reaction is suggestive of a pericyclic ring closure. Sulfoxide reduction of 374 was achieved with PBZ3 in DMF. 

The thermolysis of azetidinone ( -lactam) disulphides (89) yields the iso-thiazolin-3-ones (90), amongst other products these, in turn, can be converted (in high yield) into 3-alkyl-(or aryl-)aminoisothiazoles (91) by reaction with methyl fluorosulphonate followed by ammonia, as shown in Scheme 6. 

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