Stereoselective synthesis of the pyran subunit of portentol

An efficient strategy for the stereoselective synthesis of a fully elaborated pyran fragment of portentol was developed. The final route eventually proceeds through nine steps with an overall yield of 20% and key transformations include a Paterson <i>anti</i>-aldol reaction, selective lactol formation and a concise diastereoselective Corey-Winter elimination. The resulting pyran adopts a chair-type conformation with all methyl bearing centers in equatorial positions, facilitating a top-side attack on the olefin, which is crucial for the projected total synthesis of portentol.