prov:value
| - X1 is selected from the group consisting of RO???, RC(???O)O???, R2C???NO???, and R2NO??? wherein each R is independently selected from the group consisting of hydrogen, alkyl, alkenyl, arenyl, aryl, and aralkyl, wherein each R, other than hydrogen, contains from 1 to 18 carbon atoms and, optionally, contains at least one oxygen or sulfur atom;
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