Application of the Barbier‐type indium‐mediated allylation method to suitable substrates offers access to carbohydrates bearing a terminal olefin moiety. The C–C bond forming reaction generates a defined stereochemistry of the new chiral center and tolerates a wide variety of starting aldehydes thus allowing modifications in the carbohydrate backbone. Further transformations of the alkene moiety via an environmentally benign and subtle controlled protocol using potassium permanganate gives rise to the structural motif of 3‐deoxy‐2‐uloses in good yields. The final part of the reaction sequence focuses on the deprotection of the acetyl groups essential for the success of the oxidation step. The acidic and labile 3‐deoxy position of the target molecule is prone to elimination applying standard deacetylation conditions and therefore demands derivatisation of the molecule. The introduction of a thioketal moiety using microwave conditions shows promising results and subsequent standard transformations are applicable leading to the desired products.
A two-step reaction sequence starting with the indium-mediated allenylation of aldehydes with 4-bromo-2-butyn-1-ols and subsequent ozonolysis of the resulting allenylic product was developed to generate a variety of dihydroxyacetone derivatives. The regioselectivity of the indium-promoted C-C bond-forming reaction can be manipulated through hydroxy protecting groups on 4-bromo-2-butyn-1-ol, yielding either
We utilized the indium-mediated allylation reaction for the synthesis of carbohydrate structures containing the 3-deoxy-2-ulose motif, a barely investigated compound class. The stereoselective outcome can be controlled by the presence or absence of a chelating group in α-position to the carbonyl function. By introduction of an UV-active allyl building block, we enabled epimer separation by HPLC towards the synthesis of 3-deoxy-d-glycero-d-galacto-2-nonulose, the carboxylreduced analogue of widely distributed 3-deoxy-d-glycero-d-galacto-nonulosonic acid (Kdn). Ozonolysis of the introduced 2-C-methylidenepropan-1-ol motif provided the desired 3-deoxy-2-uloses.
The cover picture shows the application of an indium‐mediated allenylation reaction of aldehydes using 4‐bromo‐2‐butyn‐1‐ols to generate α‐hydroxyacetyl anion equivalents. This carbon chain elongation method allows the straightforward synthesis of various substituted dihydroxyacetone fragments, as well as pentuloses and derivatives thereof. Details are discussed in the article by W. Schmid et al. on p. 1645 ff. The background shows the lovely coast of Mayo, Ireland. The authors express their gratitude to Prof. Karl Grubmayr on the occasion of his 60th birthday.
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