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