A Substrate‐Based Approach to Skeletal Diversity from Dicobalt Hexacarbonyl (C1)‐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 BF₃^.Et₂O. 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.