 Solution

The cyclic amino sugar equilibrates with its 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:This corresponds to ${\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{N}}_2{\mathrm{O}}_8$, 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.

Dehydrogenative aromatization gives B, retaining both hydroxylated side chains and releasing H${}_2$: the intermediate loses two hydrogen atoms to reach ${\mathrm{C}}_{12}{\mathrm{H}}_{20}{\mathrm{N}}_2{\mathrm{O}}_8$, 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 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.