Enantiospecific Syntheses of 2S,3R,4R,5S‐Trihydroxypipecolic Acid, 2R,3R,4R,5S‐Trihydroxypipecolic Acid, 2S,4S,5S‐Dihydroxypipecolid Acid, and Bulgecinine from D‐Glucuronolactone.