A New Alkoxyallene-Based [3+2] Approach to the Synthesis of Highly Substituted Cyclopentenones

Lithiated methoxyallene 1 and aldehydes 2 provided after base- or gold-catalyzed cyclization dihydrofurans 3 which were oxidatively cleaved giving α,β-unsaturated γ-ketoaldehydes 4 as key intermediates. These smoothly underwent intramolecular aldol addition to furnish highly substituted cyclopentene derivatives 5 in good yields. Due to their dense pattern of functional groups compounds 5 are versatile intermediates, suitable for subsequent elaborations. This was demonstrated by transformation of 5e into enol phosphate 8 and of 5c into tetracyclic nitrone cycloadduct 13.