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