s a group represented by the formula: ???Y1???Y2???Y3???Ra wherein Y1, Y3, and Ra are as defined (1); Y2 is ???O???, ???NRb???C(???O) or ???C(???O)???NRb???; Rb is hydrogen or optionally substituted alkyl; 4) R1 is a group represented by the formula: ???Y1???Y2???Y3???Ra wherein Y1, Y2, Y3, and Rb are as defined (1); Ra is optionally substituted carbocyclic group, optionally substituted heterocycl