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