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