Hydrazines hydride elimination

The 2,5-dibromo-2,5-dideoxy-D-xylono-1,4-lactone (6) (Scheme 2) can also be selectively reduced to give 5-bromo-2,5-dideoxy-D-fhreo-pentonolactone (7) by reaction with either aqueous hydrazine as mentioned above, or by consumption of 1 mole of hydrogen [ 14]. Further reaction with hydrogen and Pd/C leads to 3-hydroxypentanoic acid by an initial reductive elimination [15]. Reduction of the primary bromine in these cases can be obtained by means of tributyltin hydride [25] to give 8. 

The synthetic proposal in the same scheme suggests the two-step transformation of phthalanhydride to aldehyde TM 7.6b, first by standard hydride reduction and then by partial oxidation. Condensation with hydrazine passes over aldimine as a stable intermediate, and then the cyclization requires heating and continuous water elimination. Activation of the heterocyclic ring by chlorination of the enolic form is required before substitution by hydrazine to TM 7.6. 

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