Variable strategy toward carbasugars and relatives. 5. Focus on preparation of chiral nonracemic medium-sized carbocycles

The feasibility of sequential vinylogous aldol (intermolecular)/silylative aldol (intramolecular) addition reactions involving furan- and pyrrole-based dienoxysilanes, 6 and 12, in the synthesis of carbasugar frameworks is illustrated by the preparation of the scantily investigated carbaseptanose and carbaoctanose representatives of this class of compounds. The target compounds, 1, 2, 3, ent-2, ent-3, and 4, were obtained from readily available carbohydrate precursors (5 and 19) in yields of 21-30% over 8-12 steps. The irreversible silylative ring-closing aldolisation of γ-substituted dihydro-5H-furan-2-one and pyrrolidin-2-one aldehydes (9, 16, ent- 16, and 22) driven by the TBSOTf/ Pri2EtN Lewis acid-Lewis base couple was shown to be a practical, diastereoselective maneuver to forge the densely functionalized, medium-sized core carbocycles.