2.4.2. Isomers that Generate the Deacetylated Aglycone at *m/z* 981.3

For other isomers, the loss of the deacetylated aglycone yields a large fragment at *m/z* 981.3. The alternative fragmentation patterns of the sugar residues for Lessoniosides C and E are described in Figure 3.

An interesting peculiarity of Lessonioside C and E is the presence of keto groups at both C-16 and C-22 positions, and 25β-*O*-Ac in the aglycone. This is the third example of the aglycone of sea cucumber glycosides bearing a ketone group at C-22 as Zhang *et al.* [32] and Liu *et al.* [33] stated the presence of a ketone group in Fuscocineroside A and Arguside D, respectively. The structure is characterized by the presence of an oligosaccharide chain composed of six units. The holostane-type aglycone features an endocyclic double bond at position C-9(11) and C-23 and a β-acetoxy group at C-25.
