Acid-labile acetal

Murthy N, Thng YX, Schuck S, Xu MC, Frechet JMJ (2002) A novel strategy for encapsulation and release of proteins hydrogels and microgels with acid-labile acetal cross-linkers. J Am Chem Soc 124 12398-12399... 

A recent innovation in in-situ microencapsulation is the development of acid-triggered release of pesticide from the microcapsules [12]. Diols and aldehydes are reacted to form an acid labile acetal moiety. The acetal is then reacted with isocyanate to create a prepolymer. The prepolymer is a polyisocyanate cmitaining the acid labile moiety and suitable for in-situ shellwall polymerization. The prepolymer is dissolved into a pesticide, emulsified into water, and shellwall formed in-situ. Under alkaline or neutral pH conditions in a container, the insecticide is safely contained in the microcapsules. Acid could be added to the spray tank to rapidly release capsule contents prior to application. Alternate shellwall chemistry for in-situ microencapsulation utilizes etherified urea-formaldehyde prepolymers in the oil phase that are self-condensed with acid catalyst to produce encapsulating aminoplast shellwalls [13]. This process does not have the problem of continuing CO2 evolution. Water-soluble urea-formaldehyde and melamine-formaldehyde prepolymers can be selected to microencapsulate water or aqueous solutions [14]. 

Figure 5.3. Formation of a carrier-bound acid-labile acetal.

In model studies directed towards the synthesis of Xestospongine, a marine natural product, the appropriate heterocyclic backbone (an oxaquinolizidine ring system) is easily constructed using tosylate 439 (Scheme 62). Thus alkylation of trifluoroaetonide 446 with 439 results in the formation of 447 in moderate yield. Removal of the trifluoroacetyl group under basic conditions followed by removal of the two acid-labile acetal groups produces the desired product 449 directly as a single isomer [8]. 

Common organic functionality such as Lewis basic phenol 3, acid-labile acetal 4, highly reactive aryl friflate 6, and depsipeptide 5 are well-tolerated under the mild reaction conditions (Figure 2). In all cases the products are isolated as one regioisomer and olefin isomer after column purification. 

Cationic polymerization of vinyl ethers is extensively used for preparing various biodegradable polymers containing acid-labile acetal linkages with potential... 

Liu, R., Zhang, Y., Zhao, X., Agarwal, A., Mueller, L.J., and Feng, P. (2010) pH-responsive nanogated ensemble based on gold-capped mesoporous silica through an acid-labile acetal linker. 

---