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

Abstract

Abstract Pyranose‐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 RuO 4 ‐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 , allegedly 40 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 ).