Syntheses of (+)-Preussin and Asperidine B

Abstract

Asperidine B (1), a new piperidin-3-ol, was isolated from the soil-derived fungus Aspergillus sclerotiorum PSU-RSPG178 by Rukachaisirikul and co-workers in 2018. Asperidine B exhibited mild 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) inhibitory activity and was noncytotoxic to noncancerous Vero cells.In addition, asperidine B showed highly potent inhibitory activity for cystic fibrosis transmembrane conductance regulator (CFTR)-mediated chloride secretion with an IC50 value of 0.96 ?M and reactive oxygen species (ROS) inhibitory activity. However, the originally proposed piperidine structure of asperidine B was misassigned and later revised to (+)-preussin (2), a previously reported 2,6-disubstituted pyrrolidin-3-ol, through a racemic synthesis. Compounds 1 and 2 are structural isomers, differing only by the number of methylene groups in the ring and on the side chain. This work involved developing a synthesis of 1 that could also be applicable to the synthesis of 2 via a chiron approach to provide materials for further evaluation of biological activity. The synthesis of both 1 and 2 began with preparation of key lactol 4 starting from D-isoascorbic acid (3). Putative asperidine B (1) was synthesized using two synthetic routes. The first synthetic route began with two-carbon homologation for carbon moiety of the piperidine ring via Wittig olefination of lactol 4 to yield 5 in 2 steps. Subjection of 5 to TBS protection, reduction and oxidation afforded aldehyde 6. The lipophilic side chain was installed through acetylide addition of precursor 6 with 1-octyne to provide propargyl alcohol 7 which was transformed to azidoketone 8 via epoxide formation and azide displacement as key steps. Reductive amination of 8 under hydrogenation furnished desmethyl analogue of asperidine B (11) which was subjected to N-methylation to provide 1 in a total of 23 steps and 1.84% overall yield. The second synthetic route was performed in 14 steps and 8.0% overall yield which was shorter than the first synthetic route by 9 steps. The key reactions in the second synthetic route closely resembled those in the first synthesis with the only difference involving the use of epoxy ?-lactone 9 as a requisite intermediate which reduced unnecessary protection-deprotection steps. Moreover, the synthesis of (+)-preussin (2) was inspired by the synthetic strategy of 1, which could be successfully accomplished in 15 steps and 9.6% overall yield from lactol 4. The key reactions were identical to the synthesis of 1 except for one-carbon homologation to construction of the pyrrolidine core, acetylide opening of lactam and hydride-mediated reductive amination. In addition, synthetic compound 1 and its desmethyl analogue 11 were evaluated for antioxidant, antidiabetic, and cholesterol-lowering activities. Both piperidines 1 and 11 at 6 �g/mL showed potent antioxidative effects, lowering ROS in HepG2 cells by 63% and 40%, respectively. Preliminary investigation of mechanism of antioxidative effect was performed by evaluating the expression of antioxidant genes including catalase (CAT), glutathione peroxidase (GPx) and superoxide dismutase (SOD). The desmethyl analogue 11 induced both GPx and SOD gene expression, while 1 significantly induced only SOD expression. In addition, 1 and 11 exhibited antidiabetic activity via ?-glucosidase inhibition with IC50 values of 0.143 and 0.292 mg/mL, respectively, compared to that of acarbose (IC50 = 0.183 mg/mL), an antidiabetic drug, whereas both 1 and 11 inhibited ?-amylase with lower potency (IC50 = 1.284 and 0.737 mg/mL) which showed much less inhibition than acarbose (IC50 = 0.183 mg/mL). Furthermore, piperidines 1 and 11 significantly inhibited intestinal cholesterol absorption, with a 5-fold higher potency than the selective cholesterol absorption drug ezetimibe, as demonstrated by fluorescent-cholesterol transport assay.asperidine B (1) ???????????? piperidin-3-ol ?????????????????? Aspergillus sclerotiorum PSU-RSPG178 ??? Rukachaisirikul ?????????? ?.?. 2018 asperidine B ??????????????????????? 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) ?????????????????????????????????????????? ????????? asperidine B ???????????????????????????????????????? cystic fibrosis transmembrane conductance regulator ??????? IC50 ??????? 0.96 ?M ??????????????????????????????????????????? ????????????????????????????????????? asperidine B ??????????????????????????????????????????????????????????????????? (+)-preussin (2) ???????????????? pyrrolidin-3-ol ??????????????????????????????????? ?????? 1 ??? 2 ?????????????????????????????????????????????????? methylene ???????????????????? ???????????????????????????????????????????????????? 1 ???????????????????????????????????????? 2 ????????? chiron approach ??????????????????????????????????????????????????????????? ??????????????????? 1 ??? 2 ????????????????? lactol 4 ??? D-isoascorbic acid (3) ???????????????? 1 ?????????????????? ????????????????????????????????????????????????????????????????????? Wittig olefination ?????? 4 ?? 2 ????????????? 5 ????????????????????????????????? TBS ??????????????????????????????????????????????????????????? 6 ????????? 6 ???????????????????????????????????????????????????? acetylide addition ??? 1-octyne ??? propargyl alcohol 7 ??????????????? azidoketone 8 ????????????????????? ????????????????? epoxide ????????????????????????? azide ???????????? 8 ?????????????????????????????????? reductive amination ??????????????????????????????????? 11 ?????????????????? methylation ???? asperidine B (1) ???????????????????????? 23 ????????????????????????????????????? 1.84 ???????????????????????????????????? 1 ??????? 14 ??????????????????????????????? 9 ?????????????????????????????????????????????????????????????????????????? 8.0 ??????????????????????????????????????????????????????????? epoxy ?-lactone 9 ?????????????????????????????????????????????????????????????????????????????????????? ????????????????????????????????????????????? 1 ??????????????????????????? (+)-preussin (2) ??????????? 15 ?????????? lactol 4 ??????????????????????????????? 9.6 ???????????????????????????????????????????????? 1 ????????????????????????????????????????????????????????? ?????????????????? lactam ?????????????????????????? hydride-mediated reductive amination ????????????????????????????????????????????? ??????????? ??????????????????? 1 ??? 11 ??????? ?????????? 1 ??? 11 ??? 6 �g/mL ???????????????????????????????????? ?????????????????????????????????????????????? HepG2 ?? 63% ??? 40% ???????? ????????????????????????????????????????????????????????????????????????????????????????????????????? catalase (CAT) glutathione peroxidase (GPx) ??? superoxide dismutase (SOD) ????????????? 11 ??????????????????????????????????? GPx ??? SOD ??????????? 1 ??????????????????????????????????????? SOD ???????????? 1 ??? 11 ???????????????????? ???????????????????????????? ?-amylase ??? ?-glucosidase ???????????? 1 ??? 11 ???????????????? ?-glucosidase ?????????? IC50 ??????? 0.143 ??? 0.292 mg/mL ???????? ?????????????????????????????? acarbose (IC50 = 0.183 mg/mL) ???????????????????????????????????????????? ?-amylase ??????? IC50 ??????? 1.284 ??? 0.737 mg/mL ??????????????????????????????? acarbose ?????????? 1 ??? 11 ????????????????????????????????????????????????????????? ?????? fluorescent-cholesterol transport assay ????????????????????????????????????????????? 1 ??? 11 ?????? ezetimibe ??????????????????????????????????????????? 5 ????