Velutin

Velutin (flavone) Terpene Lethedon tannaensis (Thymelaeaceae) TOPI (calf thymus) [cytotoxic -human nasopharynx carcinoma KB cells (5)] 9.3Ft... 

Catalytic transesterification of esters 7.14 or 7.15 in EtO /EtOH afforded the free hemiacetal velutinal (7.11) (38), while both velutinal esters gave the methyl acetal 7.13 (38, 40) in HPLC grade methanol. 

Velutinal (7.11) can be synthesized by selective reduction of isovelleral (6.1) to isovellerol (7.2) using KBHj in ethanol at r. t., followed by the Sharpless epoxydation of isovellerol (37). 

Moreover, the treatment of methylvelutinal (7.13) with lithium diisopropylamide afforded, by a P-elimination of the epoxyde, the corresponding 7,13-en-8a-ol derivative 7.9 (36). This product can be easily hydrolysed to isovelleral (6.1) in a THF/H2O mixture containing traces of acid or on prolonged contact with silica gel (36). This conversion may support the biosynthetic pathway proposed for the transformation of velutinals to isovelleral (6.1) in injured mushrooms (36,46). 

The only two known natural glutinopallane sesquiterpenes (8.2,8.3) have been isolated from Lactarius glutinopallens (42) (Table 8). Their structures are strictly related to velutinal esters (7.14, 7.15) for the presence of the cyclopropane ring and the 7a,8a-epoxyde, and to rubrocinctals 7.7 and 7.8 for the carbomethoxy group at C-3. 

Furantriol (18.27), isolated from L. mitissimus (84), is one of the few lactarane sesquiterpenes in which one of the gem-methyl groups at C-11 is oxidized and it was chemically correlated (82) with lactarorufin B (11.71), another example of this kind. The Polish authors suggested that lactone 11.71 was enzymatically formed from furan 18.27, and that a C-15 oxidized sesquiterpene of the velutinal type was the common precursor of both compounds in the mushroom (84). Actually, the possibility for the C-15 methyl group to be oxidized at an early stage of the lactarane biosynthesis seems to be confirmed by the recent finding of C-15 hydroxylated protoilludane sesquiterpenoids (5.1 and 5.2) in L. violascens (23) (Table 5). 

Attempted formation of the bromide from the furanosesquiterpene 18.10 with Ph3P and CBr4 gave furanether A (18.5) and pyrovellerofuran (18.3), as main products (93). The latter compound had previously been obtained by thermal rearrangement of isovelleral (6.1) (90), while furanol 18.10, isolated from L. vellereus (93), was also formed when velutinal derivatives were decomposed on silica gel (85). This experiment correlated the absolute configuration of isovelleral (6.1) with that of velutinal esters and, indirectly, with the stereostructures of many other Lactarius... 

Compared with the pungent Lactarius species containing velutinal derivatives, the enzymatic conversions taking place in injured specimens of other species show both similarities and differences. In fact, the original esters are first quickly hydrolysed by lipases to give free alcohols or phenols,... 

In conclusion, while the first steps of the velutinal ester conversions arc rather certain, the details of further enzymatic transformations are still hypothetical, and their investigation will require more sophisticated experiments, possibly with isolated enzymes and labelled advanced intermediates. 

Flavidulol D was isolated from L. flavidulus, and its structure was identified as the stearate of flavidulol A (22.8) on the basis of spectroscopic data (147). The new marasmane lactone P3 and diketopiperazine P4 were isolated from the most polar fraction of an ethanolic extract of L. vellereus (148). The structures of compounds P3 and P4 were established by extensive NMR studies together with acetylation reactions. The isolation of lactone P3 suggested that the possible oxidation of the 15-methyl group, typical of a few lactarane sesquiterpenes, could take place at the early velutinal stage... 

Conversion of velutinal esters in the fruit bodies of Russula cuprea, M. Cler-icuzio and O. Sterner, Phytochemistry, 1997, 45, 1569. 

Velutinic acid (190) is a new friedelin derivative from Xylosma velutina. " The crystal structure of epifriedelinol (191) has been published. "... 

Introduction to Parts 6 and 7 - Velutinal Esters and Related Sesquiterpenes. 85... 

Velutinal esters are the only observable sesquiterpenes occurring in undamaged fruiting bodies, when worked up under carefully controlled conditions (see introductory remarks). If extraction is not carried out properly, they are rapidly degraded and artifacts are formed (vide infra). 

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