= Solution
The cyclic amino sugar equilibrates with its \b[open-chain $\alpha$-amino <aldehyde>]. Two such units condense head-to-tail: each amino group adds to the other unit's <aldehyde>, followed by dehydration to the cyclic bis-imine/dihydropyrazine system. Formula accounting identifies the observed intermediate:
$$
\boxed{2(179)-2(18)=322.}
$$
This corresponds to $\mathrm{C_{12}H_{22}N_2O_8}$, so the accumulating intermediate has lost two waters but has not yet acquired the fully aromatic <pyrazine> oxidation state. The unchanged portions of both carbon chains become the substituents on opposite ring carbons.
\b[Dehydrogenative aromatization gives B], retaining both hydroxylated side chains and releasing H$_2$: the intermediate loses two hydrogen atoms to reach $\mathrm{C_{12}H_{20}N_2O_8}$, mass 320. A competing elimination of the hydroxyl group immediately adjacent to the ring in one side chain, with loss of a ring-adjacent proton, gives an exocyclic unsaturated intermediate. Proton transfers shift that unsaturation into the ring and leave a deoxygenated side-chain methylene. This \b[dehydrative aromatization gives A], formally $322-18=304$, rather than requiring a separate unexplained reducing agent. Both pathways gain aromatic stabilization, while only the dehydrogenative branch requires H$_2$ release. The acid/base proton-transfer steps can proceed through several tautomeric representations; mass balance distinguishes the two net branches.
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