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