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