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