Keywords: Calopins / Lactones / Mushrooms / Natural products / Spiro compounds Boletus calopus and closely related mushrooms of the Boletus section Calopodes are characterized by their bitter taste, which is caused mainly by O-acetylcyclocalopin A (6a), a member of a series of unique δ-lactone derivatives. Besides the simple lactone calopin (1b) and its O-acetyl derivative 1a, the more complex cyclocalopins 6−12 have been isolated and their structures elucidated. Cyclocalopin D (10a) is an unpre-

The cover picture shows the mushroom Boletus calopus, which contains a series of unique δ-lactones like calopin and the enolic α-glycoside cyclocalopin G. Some of these compounds are responsible for the bitter taste of this mushroom and related Boletus species. The absolute configuration of calopin was established by stereoselective synthesis of its phenyl analogue and comparison with the natural product by the high-field NMR variant of Mosher's method. Details are discussed in the articles of W. Steglich et al. on p. 2895 ff. and 2905 ff.

Tridentoquinone (1), the main pigment of Suillus tridentinus, is accompanied by the known meroterpenoid bolegrevilol (3) and a dimer, tridentorubin (5). The absolute configuration of 1 was unambiguously established by a single‐crystal X‐ray analysis of the corresponding (–)‐camphanoate. The structure of 5 was elucidated by 2D NMR techniques including a 2D INADEQUATE experiment. Feeding experiments with [1‐13C]‐labeled 4‐hydroxybenzoic acid (6*) and 3,4‐dihydroxybenzoic acid (7*) proved the incorporation of these precursors into all three metabolites. Tridentoquinone (1) was monolabeled at C‐1 suggesting the formation of the ansa ring by oxidative cyclisation of 2‐geranylgeranyl‐6‐hydroxybenzoquinone (10). This was supported by the isolation of the expected intermediate, deoxytridentoquinone (11), from the mushroom extract. Tridentorubin (5) may be formed by addition of precursor 10 to tridentoquinone (1). This hypothesis is backed up by the in vitro formation of an analogous product 13 from 1 and 1,4‐benzoquinone. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)