Isocyanate electrophilic scavengers

Scavenging acid chlorides, sulfonyl chlorides, isocyanates, and other electrophiles... 

Kaldor [49 i, 55] demonstrated the advantages of applying solid-supported scavengers to the preparation of parallel arrays in a multi-step fashion. In these studies he examined the clean-up of multiple amine alkylation and acylation reactions using a variety of immobilized electrophilic and nucleophilic scavenger reagents including an amine, isocyanate, aldehyde and acid chloride (Tab. 2.1). 

Williams and coworkers used a [G4]-PAMAM dendrimer as a scavenger in the reactions of an arylpiperazine with slight excesses of three different electrophiles (an isocyanate, a sulfonyl chloride, and a benzyl halide) [62]. After TLC had indicated that all amine substrate had reacted the dendritic scavenger was added. Solvent removal, adding of chloroform, and filtration of the unsoluble dendrimer afforded the products in high yields (87-99%) and purities (>95%, HPLC). 

Electrophilic attack on the C-3 urethane was achieved using chloro-sulfonyl isocyanate, which selectively transformed 510 to intermediate 514. Potassium iodide in acetone reacted with 514 to give allophanate (515), whereas isopropanol-collidine treatment of the same substance yielded 516. Phosphorus pentachloride reacted with protected cepha-mycin C (517) to form intermediate 518, which underwent solvolysis in methanol to 519 (25%). The chemoselective reaction of these electrophiles with urethane nitrogen stand in contrast to the selective acylation of amido nitrogen, which can be achieved with acid chloride and neutral acid scavengers (Karady et ai, 1972). 

---