80 at least about 90 at least about 95 at least about 97 or at least about 99 by weight of the compounds provided herein or salt thereof methods for isolating compounds are routine in the art the phrase u201c pharmaceutically acceptable u201d is employed herein to refer to those compounds materials compositions and or dosage forms which are within the scope of sound medical judgment suitable for use in contact with the tissues of human beings and animals without excessive toxicity irritation allergic response or other problem or complication commensurate with a reasonable benefit risk ratio reactions can be monitored according to any suitable method known in the art for example product formation can be monitored by spectroscopic means such as nuclear magnetic resonance spectroscopy e g 1 h or 13 c infrared spectroscopy spectrophotometry e g uv visible mass spectrometry or by chromatographic methods such as high performance liquid chromatography hplc liquid chromatography mass spectroscopy lcms or thin layer chromatography tlc compounds can be purified by those skilled in the art by a variety of methods including high performance liquid chromatography hplc u201c preparative lc ms purification improved compound specific method optimization u201d k f blom et al j combi chem 6 6 874 2004 which is incorporated herein by reference in its entirety and normal phase silica chromatography the radiopharmaceutical compounds namely 18 f t807 c f xia et al alzheimers dement 2013 in press doi 10 1016 j jalz 2012 11 008 and d t chien et al j alzheimers dis 2013 34 457 68 and 18 f t808 w zhang et al j alzheimers dis 2012 31 601 12 display excellent potency and selectivity in vitro.