the diazirine, terminal alkyne, and chain length of 16 carbons, minimizing the digression from the parent PI5P structure. The synthe,c strategy for the development of the diazirine-func,onalized PI5P, 9, was adapted from our recent report of isotope enriched PI4P and PI5P deriva,ves. Briefly, using standard condi,ons, myo-inositol 10 was protected to form the orthoformate ester 11 (Scheme 1). The axial hydroxyl groups were then alkylated with PMBCl and sodium hydride to afford orthoformate 12 in a 30% yield. The orthoformate was removed under acidic condi,ons to form inositol 13 in an 80% yield. The 1-posi,on of inositol 13 was then selec,vely acetylated using an enzyme from Thermomyces lanuginosus, Lipozyme TL-IM®, to form a single enan,omer of compound 14 (>99% e.e., >99% yield). The 2-and 3-hydroxyl posi,ons were simultaneous protected as the cyclopentylidene acetal, using dimethoxycyclopentane 15 and PTSA⋅H2O, giving 16 in a 76% yield. The remaining free hydroxyl was then phosphitylated using o-xylylene N,N-diethylphosphoramidite and 1H-tetrazole, and the phosphite intermediate was oxidised using mCPBA, to give the acetylated-inositol phosphate ester core 18, in a 78% yield. Compound 18 was deprotected with potassium carbonate to give the free alcohol 19, ready for phosphoryla,on, in an 82% yield.