Cycloaddition reactions alternative generation procedures

An alternative route [53] that provided the azetidine-2,3-dione 93 in better yield was the cycloaddition reaction between 90 and the Schiff base. 75 promoted by phenyl dichlorophosphate reagent followed by oxidative hydrolysis of the in situ generated p-lactam 92. In this way the P-lactam 93 was obtained in 90% overall yield from 90. Conversion of 93 into the 3-acetyl-P-lactam 97 was achieved in three steps according to our procedure [54]. Namely, the a-keto-P-lactam 93 was treated with nitroethane in the presence of base. 

Sol 2. (i) Butadiene sulfone or 3-sulfolene, a solid, is a convenient substitute for gaseous buta-1,3-diene. Diels—Alder reaction between buta-1,3-diene and dienophiles with low reactivity usually requires prolonged heating above 100 °C. This makes a procedure rather dangerous, if neat buta-1,3-diene is used, and requires special equipment for work under elevated pressure. Alternatively, buta-1,3-diene can be generated in situ by thermal sulfur dioxide extrusion from sulfolene, in which case no buildup of buta-1,3-diene pressure could be expected as the liberated diene is consumed in the cycloaddition, and, therefore, the equilibrium of the reversible extrusion reaction acts as an internal safety valve. ... 

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