A Substrate‐Based Approach to Skeletal Diversity from Dicobalt Hexacarbonyl ( C 1)‐Alkynyl Glycals by Exploiting Its Combined Ferrier–Nicholas Reactivity

Novel substrates that combine dicobalt hexacarbonyl propargyl (Nicholas) and pyranose‐derived allylic (Ferrier) cations have been generated by treatment of hexacarbonyldicobalt ( C ‐1)‐alkynyl glycals with BF 3 . Et 2 O. The study of these cations has resulted in the discovery of novel reaction pathways that have shown to be associated to the nature of O ‐6 substituent in the starting alkynyl glycals. Accordingly, compounds resulting from ring expansion (oxepanes), ring contraction (tetrahydrofurans), or branched pyranoses, by incorporation of nucleophiles, can be obtained from 6‐ O ‐benzyl, 6‐hydroxy, or 6‐ O ‐silyl derivatives, respectively. The use of a 6‐ O ‐allyl alkynyl glycal led to a suitable funtionalized oxepane able to experience an intramolecular Pauson–Khand cyclization leading to a single tricyclic derivative.