Ingrid 't Hart

63 Synthesis of globo-series glycolipids 3 The successful synthesis of globo-series glycosylsphingosines 1a , 2a and 3a encouraged us to synthesize the intact glycosylceramides. To this end, common amide coupling with a fa�y acid was chosen. All coupling reac�ons were performed by shaking the respec�ve glycosylsphingosine ( 1a - 3a ) with palmi�c acid (C16:0) and EDC/HOBt in DMF (Scheme 4A-C). Full conversions were seen on TLC (CHCl 3 /MeOH/H 2 O) and by MALDI-TOF for all compounds ( 1b-3b ). Again material was lost on the C18 cartridge for compounds 1b and 2b . Gb5-Cer, on the other hand, was purified by C4 SPE (MeOH/H 2 O) purifica�on and this resulted in a moderate yield ( 3b , 29%). All products ( 1b-3b ) were characterized by 1D and 2D NMR and HRMS. Scheme 4. ( A ) Chemical synthesis of Gb3Cer 1b from Gb3Sph 1a ; ( B ) Gb4Cer 2b from Gb4Sph 2a ; ( C ) and Gb5Cer 3b from Gb5Sph 3a . Reagents and condi�ons: (a) palmi�c acid, EDC, HOBt, Et 3 N, DMF. Isolated yields a�er C18* or C4** SPE purifica�ons. Conclusions Several synthe�c methods have been explored for the synthesis of globo-series glycosphingolipids. Gb5 was synthesized before and was used as a star�ng point for the synthesis of Gb5-Sph. Successful chemical glycosyla�on condi�ons were found to form β-lactosylsphingosine in the absence of neighbouring group par�cipa�on. However, further glycosyla�ons on lactosylsphingosine resulted in mixtures of products and lower yields, which might be due to side-reac�ons with the alkene moiety or poor acceptor reac�vity. Glycosyla�on of a Gb5 trichloroace�midate donor and sphingosine acceptor provided the desired protected Gb5-Sph as a separable α/β mixture in absence of neighbouring group par�cipa�on. A pentasaccharide donor with a C-2 benzoyl ester was also prepared, in order to overcome the forma�on of an anomeric mixture. However, due to low reac�vi�es of both the donor and acceptor only trace amounts of product were formed. Conven�onal benzyl ether cleavage by hydrogena�on is not suitable in the presence of the unsaturated sphingosine. However, our a�empts to cleave benzyl ethers of Gb3-Sph by Birch reduc�on failed, which might be due to solubility issues. Next, an chemoenzyma�c approach was explored star�ng by the chemical synthesis O HO OH O AcHN O HO OH O HO O O OH HO OH O O OH HO OH HN OH (CH 2 ) 12 CH 3 O (CH 2 ) 14 CH 3 O O HO OH HO HO O HO OH O AcHN O HO OH O HO O O OH HO OH O O OH HO OH NH 2 OH (CH 2 ) 12 CH 3 O O HO OH HO HO a O HO OH HO AcHN O HO OH O HO O O OH HO OH O O OH HO OH HN OH (CH 2 ) 12 CH 3 O (CH 2 ) 14 CH 3 O O HO OH HO AcHN O HO OH O HO O O OH HO OH O O OH HO OH NH 2 OH (CH 2 ) 12 CH 3 O a O HO OH HO HO O O OH HO OH O O OH HO OH HN OH (CH 2 ) 12 CH 3 O (CH 2 ) 14 CH 3 O O HO OH HO HO O O OH HO OH O O OH HO OH NH 2 OH (CH 2 ) 12 CH 3 O a 1a (Gb3-Sph) 1b (Gb3-Cer), 3%* 2b (Gb4-Cer), 10%* 3b (Gb5-Cer), 29%** 2a (Gb4-Sph) 3a (Gb5-Sph) A B C