The synthesis of a core arabinomannan (AM) oligosaccharide from Mycobacterium tuberculosis has been achieved using a convergent [6 + 6] glycosylation strategy and a defined set of building blocks. Dodecasaccharide 1, containing the key AM structural features of lipoarabinomannan (LAM), was obtained in excellent yield and selectivity from hexamannan 3 and hexaarabinan 5. This flexible synthetic strategy involves late-stage couplings and modifications, thus providing ready access to several different LAM fragments. The incorporation of a thiol linker at the reducing end of the oligosaccharide allows for the attachment of these compounds to microarrays and protein carriers.

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Keywords: Lewis bases / Phosphorus / Samarium / Radicals / Cyclization / Lithium Tripyrrolidinophosphoric acid triamide (TPPA) can replace carcinogenic HMPA as a Lewis basic additive in many reactions involving samarium ketyls. In most cases, yields and selectivities of cyclizations of (het)aryl, alkenyl, and alkynyl

Analogous applications of samarium diiodide as a promoter for ketyl cyclizations led to the discovery of several new and synthetically interesting reactions. Whereas samarium ketyl cyclizations with alkyne and allene units could be expected -though they have not been known before -the unplanned cyclization of the ketyl towards an aromatic ring uncovered a new mode of reaction. The resulting hexahydronaphthalene derivatives were formed in excellent diastereoselectivities and are synthetically very valuable building blocks. This new method was systematically investigated and extended to polycyclic compounds and those with heteroatoms in the spacer between the carbonyl group and the aromatic ring. The cyclizations involving indole and pyrrole derivatives are most promising for the stereoselective construction of alkaloid skeletons.

A surprising samarium diiodide-induced coupling reaction of carbonyl compounds with methoxyallene provided 4-hydroxy 1-enol ethers, which are versatile synthetic building blocks. In this coupling reaction, methoxyallene serves as an acrolein equivalent, which cannot directly be employed. [reaction: see text]