Archaea forms one domain in a phylogenetic tree of biological evolution along with eukaryotes and eubacteria. N-Glycosylations are one of the most abundant posttranslational modiﬁcations in archaea as well as eukaryotes. Dolichol is used as a carrier lipid of N-glycans in their biosynthesis. To investigate the transglycosylation process in hyperthermophilic archaea, Thermococcus kodakarensis, we herein synthesized dolichol 8.
The first important point is that the scalable synthesis of key intermediate 3, since dolichol 8 consists of repeating structure of Z-isoprene unit. Thus, we investigated Z-selective Wittig reaction using ketone 2 and unstable ylide generated from 1 under kinetically controlled conditions. When the solution of ketone 2 was dropwisely added to the ylide solution at -78 ˚C, 3 was obtained in high yield (table 1, entry 1). However, Z-selectivity was decreased in larger scale owing to the insufficient removal of the reaction heat. We then applied microflow system to achieve the precise reaction temperature control (table 1, entry 2). After mixing the solution of ylide and the solution of 2 in CometX-01 micromixer at -78 ˚C, the resultant solution was stirred for additional 30 mins to give 3 with high Z selectivity (E/Z = 22/1). Scalable synthesis of 3 was then achieved under the continuous flow conditions. The coupling between 4 and 5, both of which were synthesized from 3, was next investigated. When 5 was added to the solution of lithiated 4 with n-BuLi, reproducibility was low owing to the instability of the lithiated 4 (table 2, entry 1). We then applied the Barbier-like protocol to trap generated anion immediately (table 2, entry 2). t-BuOK was added to a mixture of 4 and 5 in DMF at -50 ˚C to afford 6 in high yield (87%) with good reproducibility. After coupling of 6 with 7, containing a saturated moiety, cleavage of the protective group followed by removal of the PhSO2 groups to give dolichol 8.