Sugar enolones, XII. Peroxidation of pyranose‐derived enol esters: An efficacious synthesis of peracetylhexosuloses and their conversion into γ‐pyrones via 3,2‐enolones

Abstract

AbstractPyranose‐derived 1,2‐enol acetates of type 4 readily react with 3‐chloroperbenzoic acid in ether to an anomeric mixture of glycos‐2‐uloses 6, as is demonstrated by the conversions 7 → 11/12, 18 → 19/20 and 21 → 22. Structural and configurational assignments were based on NMR‐data, on the characterization of hydrogenation products (10 and 23), and on the independent formation of 11, 12, and 22 by RuO4‐oxidation of the respective partially acetylated pyranoses 8, 9, and 23. –Acid‐catalyzed acetylation converts the glycos‐2‐uloses into their 2,2‐diacetoxy derivatives (13, 24, and 35) with anomerization of β‐isomers, whereas cautious treatment with acetyl chloride/pyridine affords the peracetylated 2,3‐dehydropyranoses 15, 17, and 36 with retention of configuration at C‐1. Mildly basic conditions initiate the elaboration of the γ‐pyrone systems (42/43) via double elimination of acetic acid, the respective intermediates, 3,2‐enolones 37α, 37β, and 38, being readily isolable. The alternate triacetyl‐enediolone 41, allegedly40 an intermediate in the conversion 11 → 42 could be excluded as an intermediate on the basis of its synthesis from tetraacetyl‐glucose by oxidation and elimination of acetic acid (44 → 45 → 41).