Synthesis of spiroketals by iridium-catalyzed double hydroalkoxylation

Abstract

A highly efficient approach to the synthesis of spiroketals involves the double cyclization of alkynyl diols using transition-metal catalysts. The iridium complex [Ir(PyP)(CO)2]BPh4 where PyP = 1-[(2-diphenylphosphino)ethyl]pyrazole is an effective catalyst for promoting the formation of spiroketals via this double hydroalkoxylation reaction. The complex promotes the formation of a series of spiroketal products from alkynyl diol starting materials such as 3-ethynylpentane-1,5-diol and 2-(4-hydroxybut-1-ynyl)benzyl alcohol. Stereoselective cyclization occurs for 3-ethynylpentane-1,5-diol, 3-ethynylhexane-1,6-diol. The cycloadditions occur in all but one case with quantitative conversion in under 24 h at 120 °C.