oxy, C1???C6 alkoxycarbonyl, and benzyloxycarbonyl; or, (ii) is H and R2 is -x-(CH2)p???W wherein x is as defined above and p is as defined below and W is H, N-maleimide or V as defined below, or (iii) -y-C and R2 is -x-(CH2)p???W; or (iv) R1 and R2 together are a single bond;