Isomerization as a route to rare ketoses: the beneficial effect of microwave irradiation on Mo(VI)-catalyzed stereospecific rearrangement

Mo(VI)-catalyzed isomerization of easily accessible 2-C-(hydroxymethyl)-branched aldoses to rare ketoses was achieved in 3 min using microwave flash heating. This contribution highlights the remarkable advantages of Mo(VI) catalysis and the beneficial effects of microwave irradiation in carbohydrate synthesis. The transformation yielded the respective ketoses in good yields (46–86%). Furthermore, the potential of the Mo(VI)-catalyzed transformation was studied using a new branched 6-deoxy-aldose to prepare 7-deoxy-l-gluco-heptulose. The method is fast and useful for the preparation of rare ketoses under microwave conditions.

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6-Deoxy-2,3-O-isopropylidene-l-manno-pyranoseC9H16O5Ee = 100%[α]D = −11.0 (c 1.0, acetone)Source of chirality: 6-deoxy-l-mannose (l-rhamnose)Absolute configuration: (2S,3S,4S,5S)

6-Deoxy-2,3-O-isopropylidene-2-C-(hydroxymethyl)-l-manno-pyranoseC10H18O6Ee = 100%[α]D = −18.0 (c 1.0, acetone)Source of chirality: 6-deoxy-l-mannose (l-rhamnose)Absolute configuration: (2S,3S,4S,5S)

6-Deoxy-2-C-(hydroxymethyl)-l-mannoseC7H14O6Ee = 100%[α]D = −12.0 (c 1.0, H2O)Source of chirality: 6-deoxy-l-mannose (l-rhamnose)Absolute configuration: (2S,3S,4R,5S)

7-Deoxy-l-gluco-heptuloseC7H14O6Ee = 100%[α]D = −40.0 to −38.0 (c 1, H2O)Source of chirality: 6-deoxy-l-mannose (l-rhamnose)Absolute configuration: (1R,2R,3S,4R,5S)