Resumen de Synthesis of fluorinated analogs of krn7000
KRN7000 (α-GalCer) is a synthetic glycosphingolipid, that upon interaction with CD1d protein is capable of activating invariant natural killer T (iNKT) cells. NKT cells are a a subset of lymphocytes involved in the immune response for a large number of pathological conditions such as infections, autoimmunity and cancer. When activated by KRN7000, NKT cells produce a variety of anti-inflammatory (TH2) cytokines, such as interleukin-4 (IL-4), and pro-inflammatory (TH1) cytokines, such as interferon-γ (IFN-γ). These cytokines produce different biological response depending on their nature. Pro-inflammatory cytokines (TH1-type cytokines) activate cellular immune responses to fight against tumors, or viral/bacterial/parasitic infections (TH1 response). Instead, immunomodulatory cytokines (TH2-type cytokines) are responsible for the activation of lymphocytes and the production of immunoglobulins and antibodies, desired for the treatment of autoimmune diseases. KRN7000 stimulates both TH1 and TH2 responses undiscriminately while a biased response is desired because of the antagonic nature of both types of cytokines. For this reason, extensive research have been focused on the synthesis of new KRN7000 analogues capable for stimulating selective responses. In this regard, it is believed that TH1 response is certainly favoured by stabilization of the KRN7000-CD1d-NKT complex whereas moderate destabilization leads to TH2 response. In this sense, it has been recently demonstrated that perfluorinated chains produce stronger interactions with hydrophobic cavities of proteins than its hydrocarbon counterparts.
Within this context, this thesis aims the synthesis of different KRN7000 analogues bearing fluoroalkyl chains at ceramide moiety. We hypothesized that this structural modification would modulate the ligand-CD1d binding affinity, leading to biased cytokine responses.
Our contribution starts in Chapter 3 where KRN7000 analogues bearing highly fluorinated acyl chain and/or sphingoid base moieties were synthesized. Their preparation was accomplished following an adapted version of total synthesis developed by Panza et al., and the strategy of Yen and co-workers. The key steps are: 1- Initial -selective glycosylation. 2- Introduction of fluorinated acyl chain via N-acylation. 3- Introduction of fluorinated sphingoid base fragment via CM reaction.
The same synthetic pathway was again used, in chapter 4, for the synthesis of KRN7000 analogues bearing low fluorinated acyl chains. The major challenge of this chapter was the synthesis of long carboxylic acids bearing short fluoroalkyl moieties (C3F7 and CF3) at the end of chain (Scheme 2). The synthesis of such acids was achieved via initial formation of long -perfluoropropyl and trifluoromethyl alkyl bromides which then underwent Li2CuCl4 catalyzed coupling with -bromo carboxylic acids.
The research described in Chapter 5 aroused from an unexpected reactivity during Chapter 3. We observed that cyclic imines could be obtained from hydroxyalkene intermediates via one pot azidation-1,3-dipolar cycloaddition (IAOC) sequence. This phenomenon was seen as an opportunity to get access to the synthesis of conformationally rigid KRN7000 analogues.
Finally, the topic studied in Chapter 6 was in line with a previous methodology, developed by our group, for the synthesis of 2-deoxy-b-glycosides. The last step of the strategy dealed with 1,2-trans-stereoselective glycosilation controled by a bulky iodine atom at C-2 position of thioglycosyl donors. Although this methodology proceeded with exellent diastereoselectivities with simple alcohols, more "challenging" acceptors required higher reaction temperatures which ultimately decreased the b-stereoselectivity. Therefore our aim was to explore the feasibility of using 2-deoxy-2-iodo-pyranosyl sulfoxides instead of the corresponding sulfides as convenient electrophiles that permit activation at very low temperature, ensuring a precise kinetic control for a complete 1,2-trans stereoselective glycosylation promoted by the neighboring bulky iodine. This methodology enabled the synthesis of two interesting cardiac glycosides with excellent stereoselectivity.
