Furan sesquiterpenes

The ( ,Z)-cyclodeeadiene-type furan sesquiterpenes 16 and 18 (neolinderalactone) undergo abnormal Cope rearrangement to give the /rans-divinylcyclohexane derivatives 17 and 19, respectively, in which the abnormality is caused by the five-membered nature of the furan ring rather than by the methyl groups on the 3,4- and 9,10-double bonds 030 103. ... 

Ipomeamarone. Formula see ipomeanin. C,5H220], Mr 250.34, [aJi> +27 (C2H5OH), a phytoalexin of the class of furan sesquiterpenes. I. was first isolated from sweet potatoes (Ipomoea batatas) afflicted by black rot. liie amount of I. reaches ca. 2 weight-% of the plant after a short time, it then acts as a highly active fungicide. ... 

The reasonably good regioselectivities observed in the [2-1-4] cycloadditions of 1-Me onto unsymmetrically disubstituted furans 61d,e can be utilized in a new synthetic approach to the cytotoxic sesquiterpenes illudin M 64 a [47] and... 

The sesquiterpene skeleton has also been assembled by the intramolecular nitrile oxide cycloaddition sequence. Oxime 238 (obtained from epoxy silyl ether 237), on treatment with sodium hypochlorite gave isoxazoline 239, which was sequentially hydrolyzed and then subjected to the reductive hydrolysis conditions-cyclization sequence to give the furan derivative 240 (330) (Scheme 6.93). In three additional steps, compound 240 was converted to 241. This structure contains the C11-C21 segment of the furanoterpene ent-242, that could be obtained after several more steps (330). 

A novel synthesis of the sesquiterpene ( )-cinnamodial (148) utilizes the furan ring as a latent 1,4-dialdehyde synthon (81JA3226). The triol (141) was thus oxidized to the ketofuran (142). Oxidation of the furan moiety with lead tetraacetate afforded a 90% yield of epimeric diacetates (143) which when exposed briefly to DBU gave dienone (144). Epoxidation of (144) and exposure of the epoxide (145) to p-toluenesulfonic acid gave the bis-acetal (146). Reduction of this intermediate to a diol and hydrolysis of the bis-acetal furnished dialdehyde (147). Acetylation of the secondary hydroxyl group completed the synthesis of cinnamodial (Scheme 32). 

Naphtho[6]furans are slightly known spectroscopically. An important one is furocadalene (118), obtained as a dehydrogenation product of certain sesquiterpenes <75jcs(Pl)478). A partial assignment for naphtho[l,2-6]furan (119) has been effected (69TL4479) the shift for 9-H is little more than would result from a peri interaction usual in naphthalenes and does not necessarily imply deshielding by an aromatic (furan) ring current. 

As illustrated in Equation (1), a Lewis acid-promoted intramolecular conjugate addition of a furan to a dienone was used to generate the seven-membered ring of the fused tricyclic framework of sesquiterpene echinofuran <2003T1877>. The use of Et2AlCl provided the best result, minimizing undesired polymerization of the furan substrate 3. 

The secochamigrane sesquiterpene laureacetal-C (215) provides a fitting introduction to the synthetic applications available to the intramolecular furan-carbonyl photocycloaddition. Initial investigations by Schreiber and Hoveyda used 3-substituted furans as substrates, and many of these were available themselves through the intermolecular photoaldol reaction (Section 2.4.5). ° For example, irradiation of... 

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). 

The series of reactions shown in Scheme 12 proved the same absolute configuration at C-3 for all natural lactone and furan secolactarane sesquiterpenes (60). 

Noteworthy among these conversions are the first successful cyclizations of lactone and furan secolactaranes to the corresponding lactarane sesquiterpenes, which were obtained by a Me2AlCl catalysed ene reaction (60). Under these conditions lactaral (19.1) yielded directly the diene 18.1, identical with the dehydration product of furosardonin A (18.8), while smooth cyclization of aldehyde 14.3 gave the lactone 11.8 in which the protons H-8 and H-9 have the "unnatural" cis stereochemistry. This result could be anticipated by examination of the Dreiding models of the two possible transition states 11.8 A and 11.8 B, which showed that unfavourable steric interactions developing between the C-3 methyl group and the bulky >C=0-—AI J complex are minimized in the... 

Structure elucidation of many Lactarius sesquiterpenes often requires interconversions of y-hydroxybutenolide, butenolide, and furan rings for confirming spectroscopic assignments. Examples of DIBAL reductions of y-lactones to the corresponding furans include the conversions of blennin C (14.5) to lactarol (19.3) (Scheme 12) (60) and of compound 11.20 to 3-dcoxy-3-cp/-furandiol (87) (Scheme 10). 

Compounds 58 1 belong to a small group of sesquiterpenes which contain a furan spirofused to bicyclo[4.3.0]nonane and have a rare spirolactol moiety. We, however, could not conclude that compounds 60 and 61 are natural products, because it is possible that compounds 60 and 61 derive from 62 by oxidation with molecular oxygen in the isolation process. 

The total synthesis of (+)-furoscrobiculin B, a lactarane sesquiterpene isolated from basidiomycetes of mushrooms, was accomplished in the laboratory of H. Suemune and K. Kanematsu using a furan ring transfer reaction and a semipinacol rearrangement as key steps. The secondary hydroxyl group of the tricyclic c/s-vicinal diol substrate was converted to the corresponding tosylate that in situ underwent a ring-expansion reaction to afford an azulenofuran in good yield. 

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