(i) an unsubstituted aminomethylene group; or

(ii) an aminomethylene group, wherein the amino portion of the aminomethylene group is protected with a protecting group selected from alkyl, alkenyl, alkylsilylalkoxyalkyl, alkoxyalkyl, cyanoalkyl, alkoxyarylalkyl, cycloalkyl, nitroaryl, arylalkyl, and hydroxyarylalkyl; or

(iii) an N-acylated aminomethylene group, wherein the acyl portion of the N-acylated aminomethylene group is aroyl, arylaroyl, haloaroyl, nitroaroyl, or an acyl group of the formula —CO—R^a where R^a is selected from alkyl, alkoxy, alkenyl, arylalkyl, arylalkenyl, aminoacid, and heterocyclyl, each of which is optionally substituted with halo, cyano, nitro, carboxyalkyl, alkoxy, aryl, aryloxy, heterocyclyl, heterocyclyloxy, alkyl, amino or substituted amino; and

R¹⁸ is an OH group protected in the form of an alkyl ether, an alkoxyalkyl ether, an aryloxyalkyl ether, an alkoxyalkoxyalkyl ether, an alkylsilylalkoxyalkyl ether, an alkylthioalkyl ether, an arylthioalkyl ether, an azidoalkyl ether, a cyanoalkyl ether, a chloroalkyl ether, a heterocyclic ether, a cycloalkylalky ether, an alkenyl ether, a cycloalkyl ether, an alkylarylalkyl ether, an alkoxyarylalkyl ether, a nitroarylalkyl ether, a haloarylalkyl ether, an alkylaminocarbonylarylalkyl ether, an alkylsulfinylarylalkyl ether, or an alkylsilyl ether.