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