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