2.2.2- Trichloroethyl carbonate alcohol protection

Six protective groups for alcohols, which may be removed successively and selectively, have been listed by E.J. Corey (1972B). A hypothetical hexahydroxy compound with hydroxy groups 1 to 6 protected as (1) acetate, (2) 2,2,2-trichloroethyl carbonate, (3) benzyl ether, (4) dimethyl-t-butylsilyl ether, (5) 2-tetrahydropyranyl ether, and (6) methyl ether may be unmasked in that order by the reagents (1) KjCO, or NH, in CHjOH, (2) Zn in CHjOH or AcOH, (3) over Pd, (4) F", (5) wet acetic acid, and (6) BBrj. The groups may also be exposed to the same reagents in the order A 5, 2, 1, 3, 6. The (4-methoxyphenyl)methyl group (=MPM = p-methoxybenzyl, PMB) can be oxidized to a benzaldehyde derivative and thereby be removed at room temperature under neutral conditions (Y- Oikawa, 1982 R. Johansson, 1984 T. Fukuyama, 1985). 

An excerpt from a synthesis of the phytosiderophore Nicotianamine illustrates the formation and cleavage of the 2,2,2-trichloroethyl carbonate protecting group [Scheme 4.357].676 Reaction of alcohol 357,1 with 2,2,2-trichloroethyl chloroformate and a catalytic amount of DMAP in pyridine gave the 2,2>2-tri-chloroethyl carbonate 357 2 in 98% yield. After oxidative destruction of the two p-methoxyphenyl rings to carboxylic acids and esterification, the 2224ri-chloroethyl carbonate 3573 was cleaved with zinc and acetic acid in 97% yield. 

Hydration with boranes proved difficult due to extensive reductive cleavage of the epoxide (Scheme 45). The best results (54%) were achieved with borane in THF. The subsequent oxidation with basic hydrogen peroxide led to the primary alcohol as well as to saponification of the acetate. Selective protection of the primary alcohol 397 as trichloroethyl carbonate was followed by oxidation with RuCl3/NaI04, thereby producing not only the y-lactone but also the cyclic ketone. 

The following three examples deal with modifications on the preformed taxol molecule. 10-Deoxytaxol 871 can be prepared from taxol 777 in four steps. Dehydration of the alcohol function at C-10 requires protection of the hydroxy functions at C-2 and C-7, which is accomplished using Troc-Cl, thereby affording the corresponding 2,2,2-trichloroethyl carbonate 870 in 46% yield [630]. 

When base-catalyzed hydrolysis is inappropriate, alcohols can be protected as trichloroethyl carbonate esters. These are prepared by reaction of the alcohol with trichloroethyl chloroformate in the presence of pyridine. This protecting group is removed reductively by zinc. ... 

---