R6 represents {TAIL ~ hydrogen} or an amino protective group selected from {HEAD ~ the group} consisting of vinyl, allyl, tert 
R6 represents { TAIL ~ hydrogen } or an amino protective group selected from { HEAD ~ the group } consisting of vinyl , allyl , tert
OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH