Sterpurenes, synthesis

Rearrangement of dienynols to vinylallene sulfoxides. A few years ago, Oka-mura et al. (11, 39) reported the rearrangement of a dienynol to an allenyldiene with transfer of chirality of the propargylic alcohol. This rearrangement has now been used for an enantioselective synthesis of a sesquiterpene, (+ )-sterpurene (3).Thus reaction of the optically active propargylic alcohol 1 with C6H,SC1 at 25° results in a vinylallene (a) that cyclizes to the optically active sulfoxide 2. Nickel-... 

Scheme 2.30 Synthesis ofthe fungal metabolite ( )-sterpurene (88).

The utility of the electrohydrocyclization reaction to assemble natural products remains essentially untapped. It has, however, been used in the synthesis of the tricyclic sesquiterpene 1-sterpurene (3), the presumed causative agent of the so-called silver leaf disease that effects certain species of scrubs and trees in Western North America [15]. As is illustrated in Scheme 1 and in Table 2, the electrohydrocyclization reaction of bisenoate (5) was used to create the five-membered ring of the natural product. 

A relatively recent application to natural product synthesis stems from efforts to synthesize a sesquiterpene called 1-sterpurene 7 [17]. This substance is thought to be the causative agent of the so-called silver leaf disease that affects certain species of shrubs and trees. The strategy focuses on three key steps (a) electrochemical cyclization of the bis unsaturated ester 11 to produce the five-membered ring of 10, (b) a Ruhlman-modified acyloin condensation to... 

A cyclopentane annelation by intramolecular C—H insertion of intermediate cationic iron-carbene complexes has been applied to the synthesis of the fungal metabolite ( )-sterpurene [49]. 

A stereoselective electroreductive cyclization has been used in the synthesis of /-sterpurene, an isolactarane-type sesquiterpene 95). 

Intramolecular C-H insertion reactions of ( -cyclo-pentadienyl)dicarbonyliron carbene complexes can be used to prepare complex polycyclic compounds. Carbon-hydrogen bond insertion using an iron carbene was used as a key step in the synthesis of sterpurene andpentalene (Scheme 81). ... 

An intramolecular version of the hydrodimerization reaction discussed in the preceding section has proved to be especially attractive and efficient for natural product synthesis. A total synthesis of the sesquiterpene sterpurene exploits just such a reaction, at a very early stage of the synthesis, for the closure of the five-membered ring (Scheme 72) [120]. 

Scheme 7.41 Total synthesis of sterpurene via a f4+3)-cvcloaddition/quasi-Favorskii rearrangement sequence.

Sterpurene (188, Scheme 7.41 is one of a relatively small class of fungal sesquiterpenes whose unique 5-6-4 carbocyclic structure made it an interesting target for synthesis in the context of the (4+3)-cycloaddition/quasi-Favorskii methodology. The quaternary carbon... 

Carbene)iron complexes are often only labile intermediates that undergo followup reactions as usually observed in carbene chemistry such as C-H insertion or cyclopropanation. Intramolecular C-H insertion reaction of (rj -cyclopentadienyl)-dicarbonyliron-carbene complexes provides bicyclic cyclopentanes (Scheme 4-60). The reaction has been applied for the synthesis of the fungal metabolite ( )-sterpurene and ( )-pentalenene. ... 

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