Sugar enolones: synthesis, reactions of preparative interest, and γ-pyrone formation

Abstract

--An account on pyranoid sugar enolones is given with respect to their preparation, their use as educts for the synthesis of a variety of 2-deoxy and 4-deoxy sugars functionalised via the enol or carbonyl function, and their conversion into y-pyrones, allowing an evaluation of the biosynthetic pathways for kojic acid and maltol from carbohydrate precursors.

Environmentally friendly approaches to the synthesis of new antibiotics from sugars

“…This strategy for 3-enopyranos-2-ulose is advantageous to the previous reported approaches starting from 2-acyloxyglycal esters, which make use of less environmentally convenient methodologies based on chlorine-containing reagents. One of them involves chlorination of the glycal to give 1,2-dichlorohexoses which, upon hydrolysis and subsequent hydrochloric acid formation, afford pyran-2-uloses, easily converted into the corresponding enones by elimination [33,34]. Another method consists of the peroxidation of 2-acetyloxyglycals with 3-chloroperbenzoic acid, providing the monohydrates of the corresponding pyranos-2-uloses, which acetylation occurs with double elimination of acetic acid to give 3-enopyranos-2-uloses [33,34].…”

Carbohydrates: Occurrence, Structures and Chemistry

“…Endowed with high crystallinity and shelf stability, the hydroxyglycal esters are of considerable preparative interest not only for the generation of a plethora of other unsaturated compounds, e.g., pyranoid enones [116] and enolones [109,117], but also as precursors for the highly versatile 2-oxoglycosyl ('ulosyl') bromides, produced in high yields simply by exposure to N-bromosuccimide or bromine in the presence of ethanol [118,119]. The utility of these ulosyl bromides as glycosyl donors in the straightforward synthesis of b-D-mannosides has been amply demonstrated [65][66][67][118][119][120].…”

Novel, Highly Stereoselective Rearrangements of Chiral Dihydropyranones

“…Dihydropyranones are important chiral ~y n t h o n s~~l since the enolone grouping is flanked on each side by chiral centers. A similar synthetic potential attends enediolones (3) which hitherto have only been obtained sporadically and in multistep syntheses[41. We here wish to report on new, facile procedures for the preparation of (3) and on the very ready rearrangements of type (1) + (2) and (3) + (2), the latter being remarkable in so far as it comprises a highly stereoselective chirality transfer from one side of the ring to the other.…”

“…Pyranoid enediolones (3) are not only accessible from peracylated hexos-3-uloses by an expe~tedly[~] facile elimination of the anomeric substituent, but also from the corresponding hexos-2-uloses via a 2+3 shift of the carbonyl group prior to elimination. Thus, the preparatively most convenient method of synthesis is determined by the accessibility of the educt.…”

“…+ enediolone conversion may also be achieved by treatment with HBr/acetic acid (to the I-bromide) and subsequent treatment with pyridine. On BF3 catalysis, enediolones (3) add methanol at the anomeric carbon preferentially from the a-side, e. g. (3a) --t (13). Mildly basic conditions, however, such as methanol/potassium carbonate in DMSO, initiate more profound transformations with the elaboration of 5-methyldihydropyranones, as illustrated by the conversions (3a) + (Za), (3d) + (2a), and (3c) + (Zb), which proceed with high stereoselectivity[91.…”