Attachment as Esters

Both aliphatic alcohols and phenols have been immobilized as esters of support-bound carboxylic acids. The esterification can be achieved by treatment of resin-bound acids with alcohols and a carbodiimide, under Mitsunobu conditions, or by acylation of alcohols with support-bound acyl halides (see Section 13.4). 

Entry Loaded resin Cleavage conditions Product, yield (purity) Ref. 

Polystyrene-bound aliphatic sulfonic esters and O-aryl sulfamates can be cleaved by treatment with potassium carbonate or other nucleophiles, whereby the corresponding alcohols are released into solution (Entries 11 and 12, Table 3.34). O-Alkyl sulfamates, on the other hand, do not react with nucleophiles and cannot be used as linkers for alcohols [245], 

Alcohols can also be generated by hydride reduction of esters of support-bound alcohols or thiols [396,655,656], or by reduction of resin-bound imides (Table 3.35). Similarly, the reaction of esters of support-bound alcohols or thiols with Grignard or 

---

Structurally, the pyrrolizidine alkaloids are composed of a necine base (two fused five-membered rings with a nitrogen at one of the vertices) and one or more branched carboxylic acids attached as esters to one or two of the necine hydroxyl groups. The esterified branched mono-and di-carboxylic acids are unique to the pyrrolizidine alkaloids. The alkaloids are represented by three major structural groups saturated bases (i.e. platyphylline [29]), 1,2-unsaturated necines (i.e. senecionine [30]) and seco necines (i.e. (otosenine [31]). The N-oxides of these alkaloids frequently co-occur with the free alkaloids (Roitman and Panter, 1995). 

---