A compound having the formula in racemic form, enantiomerically enriched form or enantiomerically pure form; wherein X is a radical precursor; R.sup.5 is a C.sub.1-10 alkyl group, an alkenyl group, an alkynyl group, or a benzyl group; R.sup.6 is an alkyl group, --Si(R.sup.8R.sup.9R.sup.10) or --(R.sup.7)Si(R.sup.8R.sup.9R.sup.10), wherein R.sup.7 is an alkylene group, an alkenylene group, or an alkynylene group; and R.sup.8, R.sup.9 and R.sup.10 are independently a C.sub.1-10 alkyl group, a C.sub.2-10 alkenyl group, a C.sub.2-10 alkynyl group, an aryl group or a --(CH.sub.2).sub.NR.sup.11 group, wherein N is an integer within the range of 1 through 10 and R.sup.11 is a hydroxy group, an alkoxy group, an amino group, an alkylamino group, a dialkylamino group, a halogen atom, a cyano group, --SR.sup.c or a nitro group; and R.sup.13 is H, F or --CH.sub.3.