Patent ID: 7153971

Claim:
A compound of the formula: wherein each of R 3 and R 4 is independently hydrogen or —OR c , where R c is a non-ring forming hydroxy protecting group, or R 3 and R 4 together with the carbon atoms to which they are attached form an optionally substituted heterocyclyl; each of R 5 and R 6 is independently hydrogen or alkyl; and one of X 1 and X 2 is O and the other is NR 7 , where R 7 is hydrogen, alkyl, aryl, —(R 8 ) n —C(═O)—R 9 , or nitrogen protecting group, where n is 0 or 1; R 8 is alkylene, and R 9 is hydroxy, alkyl, alkoxy, aryl, aryloxy, or —NR 3 R b , where each of R 2 and R b is independently hydrogen or alkyl.