Hyperattenins L (1) and M (2), two new benzoylated polyprenylated phloroglucinol derivatives possessing unusual adamantyl and homoadamantyl core structures, were isolated from the aerial parts of Hypericum attenuatum. Their structures were determined by extensive NMR spectroscopic methods. Compound 1 possesses an unusual tetracyclo[6.3.1.1(1,10).0(1,5)]tridecane skeleton, representing the second report of natural product with this carbon skeleton. Compound 2 features an unexpected 2,3,13-trioxapentacyclo [7.5.3.1(7,11).0(5,16).0(12,16)]oc-todecane ring system formed by the fusion of 2,3,8-trioxabicyclo [4.3.1] decane moiety to the tricycle[4.3.1.1(3,8)] undecane core. Both compounds were evaluated for their cytotoxic activities against five human cancer cell lines and compound 1 showed excellent inhibitory activities against HL-60, A-549, and MCF-7 cell lines with IC50 values of 3.86, 4.34, and 5.78 mu M, respectively.