## 3 Chemistry and Biological Activities of Selected Meroterpenoids
The corresponding compound **210** was protected with a silyl group followed by stereoselective \(\alpha\)-hydroxylation, conducted via trapping of the enolate with oxygen to efficiently generate compound **211**. Next, lactone **211** was reduced to the triol system followed by oxidative cleavage of the diol moiety, rapidly furnishing the key aldehyde **209**. Aldehyde **209** was coupled with pyrone **213** via a formal [3+3] cycloaddition/hydrogen atom transfer (HAT) reaction mediated by phosphoric acid **215** to generate cyclized product **217**, which was acetylated in a one-pot procedure with acetic anhydride. Phosphoric acid catalyst promoted in situ alcohol dehydration and formal [3+3] cycloaddition to provide the shown stereocenter as the major product. Then, it was treated with Mn(dpm)\({}_{3}\) in the presence of TBHP (_tert_-butyl hydrogen peroxide) and PhSiH\({}_{3}\) to facilitate a chemoselective HAT reduction reaction to provide the natural product phenylpyropene C (**146**). This Mukaiyama reduction protocol has high stereospecificity as it favored the hydride addition from the bottom face, yielding the product in a 20 to 1 ratio with respect to its diastereoisomer. Alternatively, aldehyde **209** was coupled with pyrone **212** via a formal [3+3] cycloaddition reaction mediated by phosphoric acid **215** to generate cyclized product **216**, which was converted to arisugacin F (**145**) via chemoselective HAT reduction reaction mediated by Mn(dpm)\({}_{3}\) in the presence of TBHP and PhSiH\({}_{3}\). Compound **209** was dehydrated to provided enal **218**, which was coupled with the shown pyrone via formal [3+3] cycloaddition mediated by piperidium acetate to generate compound **219**. The corresponding compound can be then converted to the natural product pyripyropene E (**148**) via chemoselective HAT reduction reaction using their established reaction conditions for these systems in modest yields and good diastereomeric ratio. In addition, compound **216** was treated with Mukaiyama hydration chemoselective HAT reaction mediated by Mn(dpm)\({}_{3}\) in the presence of oxygen gas and PhSiH\({}_{3}\), followed by Swern oxidation to provide 5-deoxyterreulactone C (**144**) in good yields. Finally, the acyl chloride **220** that arises from aldehyde **209** was subjected to Fredel-Crafts acylation reaction with pyrone **213** mediated by BF\({}_{3}\) eiterate followed by cyclization reaction. Then, reduction of the ketone system under Luche reduction conditions provided the natural product phenylpyropene F (**147**). The total syntheses of various members of this drimane merotrepenoid family was achieved through the creative use of strategic enzymatic and chemoselective reactions to provide flexible, modular, and a generalized synthetic route that may facilitate biological studies of these natural products and their derivatives. Another modular synthetic effort is highlighted in Scheme 17 to access these natural products, but mainly to stypodiol [84]. The synthesis commenced with the available natural product sclareol **222**, which was converted to intermediate **223** via a 3-step sequential protocol: acylation with acryloyl chloride, \(\gamma\)-lactone ring formation via intramolecular olefin metathesis mediated by Grubbs second generation catalyst, and finally dehydration reaction mediated by base treatment. Next, gram-scale HAT-based intramolecular Giese type coupling reaction was conducted on compound **223** with Fe(acac)\({}_{3}\) in the presence of PhSiH\({}_{3}\), which goes through the radical transition state shown to preferentially provide the tetracyclic compound **224**. Then, a two-step sequence mediated by lithium iodide resulted in the opening of the lactone ring and generation of the corresponding carboxylic acid. This was subjected to enzymatic hydroxylation using P450\({}_{\text{BM3}}\) variant BM3 L75A, leading to regioselective hydroxylated compound **225**.