Mesylates hydroxy group activation

The earliest method developed for the preparation of nonracemic aziridine-2-car-boxylates was the cyclization of naturally occurring (3-hydroxy-a-amino acid derivatives (serine or threonine) [4]. The (3-hydroxy group is normally activated as a tosyl or mesyl group, which is ideal for an intramolecular SN2 displacement. The cyclization has been developed in both one-pot and stepwise fashion [4—9]. As an example, serine ester 3 (Scheme 3.2) was treated with tosyl chloride in the presence of triethylamine to afford aziridine-2-carboxylate 4 in 71% yield [9]. Cyclization of a-hydroxy- 3-amino esters to aziridine-2-carboxylates under similar conditions has also been described [10]. 

The strategy involved the activation of the hydroxy group as the mesylate in the first step and subsequent ring-closure with DBU as the base. Small-scale reactions... 

The presence of the soft selenium atom and the hard oxygen however, make, the reaction of p-hydroxy selenides site selective. These have in fact been transformed selectively to vinylselenides 7) or olefins4-9,11 12), by selective activation of the hydroxy group, inter alia, with thionyl chloride alone or with thionyl, mesyl and phosphoryl chloride, trifluoracetic anhydride, phosphorus triiodide or diphosphorus tetraiodide in the presence of triethyl amine (Schemes 8Ab 8Bb). The formation of olefins from p-hydroxyselenides is regio- and stereoselective and occurs by formal removal of the hydroxyl and selenenyl moiety in an anti fashion. 

Release of compounds from polymer-bound carbamates has not to be performed via nucleophilic attack but can also be induced by the addition of acids like, for example, TFA in various concentrations. The released target compounds are either primary or secondary amines. The synthesis described in Scheme 34 has been recently published by DoUe et al. [183]. Synthesis of piperazines 227 has been conducted on a or-methylbenzyl resin 223 that has been activated with 4-NPCF and that has been fmther reacted with diethanolamine 224. After mesylation of the two resulting terminal hydroxy groups, reaction with a primary amine 226 and base gave immobilized piper-... 

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