Two pyrolysate products from Omani frankincense smoke: First evidence of thermal aromatization of boswellic acids

The frankincense resin of botanically-certified Boswellia sacra was pyrolyzed and the smoke was trapped into water using self-developed assembly. Two compounds, namely 1,2,4a,9-tetramethyl-1,2,3,4,4a,5, 6,14b-octahydropicene (1) and 2,9-dimethylpicene (2) were isolated from n-hexane extract of the smoke-saturated water. Their structures were determined by means of spectroscopic data including ESIMS, 1H NMR, 13C NMR, and 2D NMR (COSY, HSQC, HMBC, and NOESY). Dehydration of C-3 alcohols in boswellic acids (BAs) lead to aromatization of ring A followed by dehydrogenation and demethylation to afford triaromatic derivative viz., 1,2,4a,9-tetramethyl-1,2,3,4,4a,5, 6,14b-octahydropicene (1) and finally penta-aromatic derivative viz., 2,9-dimethylpicene (2). Compounds 1 and 2 were screened for their antiproliferative effects on MDA-MB-231 breast cancer cells. It was found that these pyrolysate products were capable of inhibiting cancer cell growth. However, this growth inhibitory effect was less when compared to their precursor AKBA (3-acetyl-11-keto-β-boswellic acid). The antiproliferative activity is inversely proportional to loss of functional groups.