Efficient syntheses of a series of trehalose dimycolate (TDM)/trehalose dicorynomycolate (TDCM) analogues and their interleukin-6 level enhancement activity in mice sera

We found an IL-6 level-enhancing compound during our synthetic study of trehalose-6,6'-dimycolate (1, TDM, formerly called cord factor) analogues. TDM is a glycolipid distributed in the cell wall of Mycobacterium tuberculosis and shows significant antitumor activity based on an immunoadjuvant activity. However, due to its significant toxicity, TDM is not yet applicable for practical use. In 1993, Datta and Takayama reported the purification of trehalose-6,6'-dicorynomycolate (2c, TDCM) from Corynebacterium spp. We have previously reported the synthesis of four diastereomeric TDCMs and showed that the synthetic (2R,3R,2'R,3'R)-TDCM (2c, hereafter abbreviated RRRR-TDCM-C-14) is identical to natural TDCM; we also demonstrated that 2c and SSSS-TDCM-C-14 (3c) showed significant antitumor activity as well as inhibitory activity in experimental lung metastasis based on the immunoadjuvant activity. Furthermore, we found that the significant lethal toxicity in mice by TDM (1) was no longer observed with the shorter-chain analogues of TDCMs. Therefore, we have elucidated that the 2,3-antistereochemistry (RR or SS) of the fatty acid residue is promising for biological activities. The chain length of the fatty acid residue should also be important for the biological activity, and thus, we designed a general synthetic procedure for trehalose diesters with 2,3-antistereochemistry and a series of chain lengths by using Noyori's asymmetric reduction of beta,beta-ketoesters followed by antiselective alkylation according to Frater to give beta,beta-hydroxy alcohols as the key steps. Thus, we prepared trehalose diesters (TDCM) 2a-d, 3a-d, and 4a-d as well as monoesters (TMCM) 5a-d and 6a-d. Immunological activities of TDCMs and TMCMs were evaluated by determining IL-6 level enhancement in mouse serum, and we found that RRRR-TDCM-C-14 (2c) and RRSS-TDCM-C-14 (4c) showed significant IL-6 level enhancement activities.